Today we’re going to have the most surreal conversation. I’m going to struggle to explain it, and you’re going to struggle to understand it. And only Stephen Hawking is going to really, truly, understand what’s actually going on.
But that’s fine, I’m sure he appreciates our feeble attempts to wrap our brains around this mind bending concept.
All right? Let’s get to it. Black holes again. But this time, we’re going to figure out their temperature.
The very idea that a black hole could have a temperature strains the imagination. I mean, how can something that absorbs all the matter and energy that falls into it have a temperature? When you feel the warmth of a toasty fireplace, you’re really feeling the infrared photons radiating from the fire and surrounding metal or stone.
And black holes absorb all the energy falling into them. There is absolutely no infrared radiation coming from a black hole. No gamma radiation, no radio waves. Nothing gets out.
As with most galaxies, a supermassive black hole lies at the heart of NGC 5548. Credit: ESA/Hubble and NASA. Acknowledgement: Davide de Martin
Now, supermassive black holes can shine with the energy of billions of stars, when they become quasars. When they’re actively feeding on stars and clouds of gas and dust. This material piles up into an accretion disk around the black hole with such density that it acts like the core of a star, undergoing nuclear fusion.
But that’s not the kind of temperature we’re talking about. We’re talking about the temperature of the black hole’s event horizon, when it’s not absorbing any material at all.
The temperature of black holes is connected to this whole concept of Hawking Radiation. The idea that over vast periods of time, black holes will generate virtual particles right at the edge of their event horizons. The most common kind of particles are photons, aka light, aka heat.
Normally these virtual particles are able to recombine and disappear in a puff of annihilation as quickly as they appear. But when a pair of these virtual particles appear right at the event horizon, one half of the pair drops into the black hole, while the other is free to escape into the Universe.
From your perspective as an outside observer, you see these particles escaping from the black hole. You see photons, and therefore, you can measure the temperature of the black hole.
Artist’s concept of the black hole at the center of the Pinwheel Galaxy. Credit: NASA/JPL-Caltech
The temperature of the black hole is inversely proportional to the mass of the black hole and the size of the event horizon. Think of it this way. Imagine the curved surface of a black hole’s event horizon. There are many paths that a photon could try to take to get away from the event horizon, and the vast majority of those are paths that take it back down into the black hole’s gravity well.
But for a few rare paths, when the photon is traveling perfectly perpendicular to the event horizon, then the photon has a chance to escape. The larger the event horizon, the less paths there are that a photon could take.
Since energy is being released into the Universe at the black hole’s event horizon, but energy can neither be created or destroyed, the black hole itself provides the mass that supplies the energy to release these photons.
The black hole evaporates.
The most massive black holes in the Universe, the supermassive black holes with millions of times the math of the Sun will have a temperature of 1.4 x 10^-14 Kelvin. That’s low. Almost absolute zero, but not quite.
Artist’s impression of a feeding stellar-mass black hole. Credit: NASA, ESA, Martin Kornmesser (ESA/Hubble)
A solar mass black hole might have a temperature of only .0.00000006 Kelvin. We’re getting warmer.
Since these temperatures are much lower than the background temperature of the Universe – about 2.7 Kelvin, all the existing black holes will have an overall gain of mass. They’re absorbing energy from the Cosmic Background Radiation faster than they’re evaporating, and will for an incomprehensible amount of time into the future.
Until the background temperature of the Universe goes below the temperature of these black holes, they won’t even start evaporating.
A black hole with the mass of the Earth is still too cold.
Only a black hole with about the mass of the Moon is warm enough to be evaporating faster than it’s absorbing energy from the Universe.
As they get less massive, they get even hotter. A black hole with the mass of the asteroid Ceres would be 122 Kelvin. Still freezing, but getting warmer.
A black hole with half the mass of Vesta would blaze at more than 1,200 Kelvin. Now we’re cooking!
Less massive, higher temperatures.
When black holes have lost most of their mass, they release the final material in a tremendous blast of energy, which should be visible to our telescopes.
Artist’s conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library
Some astronomers are actively searching the night sky for blasts from black holes which were formed shortly after the Big Bang, when the Universe was hot and dense enough that black holes could just form.
It took them billions of years of evaporation to get to the point that they’re starting to explode now.
This is just conjecture, though, no explosions have ever been linked to primordial black holes so far.
It’s pretty crazy to think that an object that absorbs all energy that falls into it can also emit energy. Well, that’s the Universe for you. Thanks for helping us figure it out Dr. Hawking.
Composite satellite image of Antarctica, the location of the largest desert on Earth. Credit: NASA/Dave Pape
When you hear the word desert, what comes to mind? Chances are, you’d think of sun, sand, and very little in the way of rain. Perhaps cacti, vultures, mesas, and scorpions come to mind as well, or possibly camels and oases? But in truth, deserts come in all shapes and sizes, and vary considerably from one part of the world to the next.
Like all of Earth’s climates, it all comes down to some basic characteristics that they share – which in this case, involves being barren, dry, and hostile to life. For this reason, you might be surprised to learn that the largest desert in the world is actually in Antarctica. How’s that for a curveball?
Definition:
To break it down, a desert is a region that is simply very dry because its receives little to no water. To be considered a desert, an area must receive than 250 millimeters of annual precipitation. But precipitation can take the form of rain, snow, mist or fog – literally any form of water being transferred from the atmosphere to the earth.
The Lut Desert of Iran, as observed by NASA’s Earth Observatory. It was here that the hottest temperature ever was recorded between 2003-9. Credit: NASA
Deserts can also be described as areas where more water is lost by evaporation than falls as precipitation. This certainly applies in regions that are subject to “desertification”, where increasing temperatures (i.e. climate change) result in river beds drying up, precipitation patterns changing, and vegetation dying off.
Deserts are often some of the hottest and most inhospitable places on Earth, as exemplified by the Sahara Desert in Africa, the Gobi desert in northern China and Mongolia, and Death Valley in California. But they can also be cold, windswept landscapes where little to no snow ever falls – like in the Antarctic and Arctic.
So in the end, being hot has little to do with it. In fact, it would be more accurate to say that deserts are characterized by little to no moisture and extremes in temperature. All told, deserts make up one-third of the surface of the Earth. But most of that is found in the polar regions.
Antarctica:
In terms of sheer size, the Antarctic Desert is the largest desert on Earth, measuring a total of 13.8 million square kilometers. Antarctica is the coldest, windiest, and most isolated continent on Earth, and is considered a desert because its annual precipitation can be less than 51 mm in the interior.
A Sun halo seen among the the landscape and ice flows of Antarctica. Credit and copyright: Alex Cornell
It’s covered by a permanent ice sheet that contains 90% of the Earth’s fresh water. Only 2% of the continent isn’t covered by ice, and this land is strictly along the coasts, where all the life that is associated with the land mass (i.e. penguins, seals and various species of birds) reside. The other 98% of Antarctica is covered by ice which averages 1.6 km in thickness.
There are no permanent human residents, but anywhere from 1,000 to 5,000 researchers inhabit the research stations scattered across the continent – the largest being McMurdo Station, located on the tip of Ross Island. Beyond a limited range of mammals, only certain cold-adapted species of mites, algaes, and tundra vegetation can survive there.
Despite having very little precipitation, Antarctica still experiences massive windstorms. Much like sandstorms in the desert, the high winds pick up snow and turn into blizzards. These storms can reach speeds of up to 320 km an hour (200 mph) and are one of the reasons the continent is so cold.
In fact, the coldest temperature ever recorded was taken at the Soviet Vostok Station on the Antarctic Plateau. Using ground-based measurements, the temperature reached a historic low of -89.2°C (-129°F) on July 21st, 1983. Analysis of satellite data indicated a probable temperature of around -93.2 °C (-135.8 °F; 180.0 K), also in Antarctica, on August 10th, 2010. However, this reading was not confirmed.
Antarctica’s McMurdo Station at night. Credit: m.earthtripper.com
Other Deserts:
Interestingly, the second-largest desert in the world is also notoriously cold – The Arctic Desert. Located above 75 degrees north latitude, the Arctic Desert covers a total area of about 13.7 million square km (5.29 million square mi). Here, the total amount of precipitation is below 250mm (10 inches), which is predominantly in the form of snow.
The average temperature in the Arctic Desert is -20 °C, reaching as low as -50 °C in the winter. But perhaps the most interesting aspect of the Arctic Desert is its sunshine patterns. During the summer months, the sun doesn’t set for a period of 60 days. These are then followed in the winter by a period of prolonged darkness.
The third largest desert in the world is the more familiar Sahara, with a total size of 9.4 million square km. The average annual rainfall ranges from very low (in the northern and southern fringes of the desert) to nearly non-existent over the central and the eastern part. All told, most of the Saraha receives less than 20 mm (0.79 in).
However, in northern fringe of the desert, low pressure systems from the Mediterranean Sea result in an annual rainfall of between 100 to 250 mm (3.93 – 9.84 in). The southern fringe of the desert – which extends from coastal Mauritania to the Sudan and Eritrea – receives the same amount of rainfall from the south. The central core of the desert, which is extremely arid, experiences an annual rainfall of less than 1 mm (0.04 in).
Temperatures are also quite intense in the Sahara, and can rise to more than 50 °C. Interestingly, this is not the hottest desert on the planet though. The hottest temperature ever recorded on Earth was 70.7 °C (159 °F), which was taken in the Lut Desert of Iran. These measurements were part of a global temperature survey conducted by scientists at NASA’s Earth Observatory during the summers of 2003 to 2009.
In short, deserts are not just sand dunes and places where you might come across Bedouins and Berbers, or a place you have to drive through to get to Napa Valley. They are common to every continent of the world, and can take the form of sandy deserts or icy deserts. In the end, the defining characteristic is their pronounced lack of moisture.
In that respect, the polar regions are the largest deserts in the world, with Antarctica narrowly beating out the Arctic for first place. And going by this definition – i.e. cold, arid, and with little to no precipitation – we’re sure to find some particularly big deserts elsewhere in Solar System. After all, what is Mars if not one big, cold, arid, and extremely dry climate?
Artist's impression of a spaceship making the jump to "light speed". Credit: NASA/Glenn Research Center
Since ancient times, philosophers and scholars have sought to understand light. In addition to trying to discern its basic properties (i.e. what is it made of – particle or wave, etc.) they have also sought to make finite measurements of how fast it travels. Since the late-17th century, scientists have been doing just that, and with increasing accuracy.
In so doing, they have gained a better understanding of light’s mechanics and the important role it plays in physics, astronomy and cosmology. Put simply, light moves at incredible speeds and is the fastest moving thing in the Universe. Its speed is considered a constant and an unbreakable barrier, and is used as a means of measuring distance. But just how fast does it travel?
Speed of Light (c):
Light travels at a constant speed of 1,079,252,848.8 (1.07 billion) km per hour. That works out to 299,792,458 m/s, or about 670,616,629 mph (miles per hour). To put that in perspective, if you could travel at the speed of light, you would be able to circumnavigate the globe approximately seven and a half times in one second. Meanwhile, a person flying at an average speed of about 800 km/h (500 mph), would take over 50 hours to circle the planet just once.
Illustration showing the distance light travels between the Earth and the Sun. Credit: LucasVB/Public Domain
To put that into an astronomical perspective, the average distance from the Earth to the Moon is 384,398.25 km (238,854 miles ). So light crosses that distance in about a second. Meanwhile, the average distance from the Sun to the Earth is ~149,597,886 km (92,955,817 miles), which means that light only takes about 8 minutes to make that journey.
Little wonder then why the speed of light is the metric used to determine astronomical distances. When we say a star like Proxima Centauri is 4.25 light years away, we are saying that it would take – traveling at a constant speed of 1.07 billion km per hour (670,616,629 mph) – about 4 years and 3 months to get there. But just how did we arrive at this highly specific measurement for “light-speed”?
History of Study:
Until the 17th century, scholars were unsure whether light traveled at a finite speed or instantaneously. From the days of the ancient Greeks to medieval Islamic scholars and scientists of the early modern period, the debate went back and forth. It was not until the work of Danish astronomer Øle Rømer (1644-1710) that the first quantitative measurement was made.
In 1676, Rømer observed that the periods of Jupiter’s innermost moon Io appeared to be shorter when the Earth was approaching Jupiter than when it was receding from it. From this, he concluded that light travels at a finite speed, and estimated that it takes about 22 minutes to cross the diameter of Earth’s orbit.
Prof. Albert Einstein delivering the 11th Josiah Willard Gibbs lecture at the Carnegie Institute of Technology on Dec. 28th, 1934, where he expounded on his theory of how matter and energy are the same thing in different forms. Credit: AP Photo
Christiaan Huygens used this estimate and combined it with an estimate of the diameter of the Earth’s orbit to obtain an estimate of 220,000 km/s. Isaac Newton also spoke about Rømer’s calculations in his seminal work Opticks (1706). Adjusting for the distance between the Earth and the Sun, he calculated that it would take light seven or eight minutes to travel from one to the other. In both cases, they were off by a relatively small margin.
Later measurements made by French physicists Hippolyte Fizeau (1819 – 1896) and Léon Foucault (1819 – 1868) refined these measurements further – resulting in a value of 315,000 km/s (192,625 mi/s). And by the latter half of the 19th century, scientists became aware of the connection between light and electromagnetism.
This was accomplished by physicists measuring electromagnetic and electrostatic charges, who then found that the numerical value was very close to the speed of light (as measured by Fizeau). Based on his own work, which showed that electromagnetic waves propagate in empty space, German physicist Wilhelm Eduard Weber proposed that light was an electromagnetic wave.
The next great breakthrough came during the early 20th century/ In his 1905 paper, titled “On the Electrodynamics of Moving Bodies”, Albert Einstein asserted that the speed of light in a vacuum, measured by a non-accelerating observer, is the same in all inertial reference frames and independent of the motion of the source or observer.
A laser shining through a glass of water demonstrates how many changes in speed (in mph) it undergoes as it passes from air, to glass, to water, and back again. Credit: Bob King
Using this and Galileo’s principle of relativity as a basis, Einstein derived the Theory of Special Relativity, in which the speed of light in vacuum (c) was a fundamental constant. Prior to this, the working consensus among scientists held that space was filled with a “luminiferous aether” that was responsible for its propagation – i.e. that light traveling through a moving medium would be dragged along by the medium.
This in turn meant that the measured speed of the light would be a simple sum of its speed through the medium plus the speed of that medium. However, Einstein’s theory effectively made the concept of the stationary aether useless and revolutionized the concepts of space and time.
Not only did it advance the idea that the speed of light is the same in all inertial reference frames, it also introduced the idea that major changes occur when things move close the speed of light. These include the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer (i.e. time dilation, where time slows as the speed of light approaches).
His observations also reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations by doing away with extraneous explanations used by other scientists, and accorded with the directly observed speed of light.
During the second half of the 20th century, increasingly accurate measurements using laser inferometers and cavity resonance techniques would further refine estimates of the speed of light. By 1972, a group at the US National Bureau of Standards in Boulder, Colorado, used the laser inferometer technique to get the currently-recognized value of 299,792,458 m/s.
Role in Modern Astrophysics:
Einstein’s theory that the speed of light in vacuum is independent of the motion of the source and the inertial reference frame of the observer has since been consistently confirmed by many experiments. It also sets an upper limit on the speeds at which all massless particles and waves (which includes light) can travel in a vacuum.
One of the outgrowths of this is that cosmologists now treat space and time as a single, unified structure known as spacetime – in which the speed of light can be used to define values for both (i.e. “lightyears”, “light minutes”, and “light seconds”). The measurement of the speed of light has also become a major factor when determining the rate of cosmic expansion.
Beginning in the 1920’s with observations of Lemaitre and Hubble, scientists and astronomers became aware that the Universe is expanding from a point of origin. Hubble also observed that the farther away a galaxy is, the faster it appears to be moving. In what is now referred to as the Hubble Parameter, the speed at which the Universe is expanding is calculated to 68 km/s per megaparsec.
This phenomena, which has been theorized to mean that some galaxies could actually be moving faster than the speed of light, may place a limit on what is observable in our Universe. Essentially, galaxies traveling faster than the speed of light would cross a “cosmological event horizon”, where they are no longer visible to us.
Also, by the 1990’s, redshift measurements of distant galaxies showed that the expansion of the Universe has been accelerating for the past few billion years. This has led to theories like “Dark Energy“, where an unseen force is driving the expansion of space itself instead of objects moving through it (thus not placing constraints on the speed of light or violating relativity).
Along with special and general relativity, the modern value of the speed of light in a vacuum has gone on to inform cosmology, quantum physics, and the Standard Model of particle physics. It remains a constant when talking about the upper limit at which massless particles can travel, and remains an unachievable barrier for particles that have mass.
Perhaps, someday, we will find a way to exceed the speed of light. While we have no practical ideas for how this might happen, the smart money seems to be on technologies that will allow us to circumvent the laws of spacetime, either by creating warp bubbles (aka. the Alcubierre Warp Drive), or tunneling through it (aka. wormholes).
Until that time, we will just have to be satisfied with the Universe we can see, and to stick to exploring the part of it that is reachable using conventional methods.
Being stuck here on Earth, at the bottom of this enormous gravity well really sucks. The amount of energy it takes to escape into the black would make even Captain Reynolds curse up a gorram storm.
But gravity has a funny way of evening the score, giving and taking in equal measure.
There are special places in the Universe, where the forces of gravity nicely balance out. Places that a clever and ambitious Solar System spanning civilization could use to get a toehold on the exploration of the Universe.
The five Sun-Earth Lagrange points. Credit: NOAA
These are known as the Lagrange Points, or Lagrangian Points, or libration points, or just L-Points. They’re named after the French mathematician Joseph-Louis Lagrange, who wrote an “Essay on the Three Body Problem” in 1772. He was actually extending the mathematics of Leonhard Euler.
Euler discovered the first three Lagrangian Points, even though they’re not named after him, and then Lagrange turned up the next two.
But what are they?
When you consider the gravitational interaction between two massive objects, like the Earth and the Sun, or the Earth and the Moon, or the Death Star and Alderaan. Actually, strike that last example…
As I was saying, when you’ve got two massive objects, their gravitational forces balance out perfectly in 5 places. In each of these 5 places you could position a relatively low mass satellite, and maintain its position with very little effort.
For example, you could park a space telescope or an orbital colony, and you’d need very little, or even zero energy to maintain its position.
The most famous and obvious of these is L1. This is the point that’s balanced between the gravitational pull of the two objects. For example, you could position a satellite a little above the surface of the Moon. The Earth’s gravity is pulling it towards the Moon, but the Moon’s gravity is counteracting the pull of the Earth, and the satellite doesn’t need to use much fuel to maintain position.
There’s an L1 point between the Earth and the Moon, and a different spot between the Earth and the Sun, and a different spot between the Sun and Jupiter, etc. There are L1 points everywhere.
L2 is located on the same line as the mass but on the far side. So, you’d get Sun, Earth, L2 point. At this point, you’re probably wondering why the combined gravity of the two massive objects doesn’t just pull that poor satellite down to Earth.
It’s important to think about orbital trajectories. The satellite at that L2 point will be in a higher orbit and would be expected to fall behind the Earth, as it’s moving more slowly around the Sun. But the gravitational pull of the Earth pulls it forward, helping to keep it in this stable position.
Animation showing the relationship between the Lagrangian points (red) of a planet (blue) orbiting a star (yellow), and the gravitational potential in the plane containing the orbit (grey surface with purple contours of equal potential). Credit: cmglee (CC-SA 3.0)
You’ll want to play a lot of Kerbal Space Program to really wrap your head around it. Sadly, your No Man’s Sky time isn’t helping you at all, except to teach you that hyperdrives are notoriously finicky and you’ll never have enough inventory space.
L3 is located on the direct opposite side of the system. Again, the forces of gravity between the two masses balance out so that the third object maintains the same orbital velocity. For example, a satellite in the L3 point would always remain exactly hidden by the Sun.
Hold on, hold on, I know there are a million thoughts going through your brain right now, but bear with me.
There are two more points, the L4 and L5 points. These are located ahead and behind the lower mass object in orbit. You form an equilateral triangle between the two masses, and the third point of the triangle is the L4 point, flip the triangle upside down and there’s L5.
Now, it’s important to note that the first 3 Lagrange points are gravitationally unstable. Any satellite positioned there will eventually drift away from stability. So they need some kind of thrusters to maintain this position.
Imagine a tall smooth mountain, with a sharp peak. Put a bowling ball at the very top and you’re not going to need a lot of energy to keep it in that location. But the blowing wind will eventually knock it out of place, and down the mountain. That’s L1, L2 and L3, and it’s why we don’t see any natural objects located in those places.
But L4 and L5 are actually stable. It’s the opposite situation, a deep valley where a bowling ball will tend to fall down into. And we find asteroids in the natural L4 and L5 positions in the larger planets, like Jupiter. These are the Trojan asteroids, trapped in these natural gravity wells though the gravitational interaction of Jupiter and the Sun.
Artist’s diagram of Jupiter and some Trojan asteroids nearby the gas giant. Credit: NASA/JPL-Caltech
So what can we use Lagrange points for? There are all kinds of space exploration applications, and there are already a handful of satellites in the various Earth-Sun and Earth-Moon points.
Sun-Earth L1 is a great place to station a solar telescope, where it’s a little closer to the Sun, but can always communicate with us back on Earth.
The James Webb Space Telescope is destined for Sun-Earth L2, located about 1.5 million km from Earth. From here, the bright Sun, Earth and Moon are huddled up in a tiny location in the sky, leaving the rest of the Universe free for observation.
NASA’s James Webb Telescope, shown in this artist’s conception, will provide more information about previously detected exoplanets. It will be at Sun-Earth L2.
Earth-Moon L1 is a perfect place to put a lunar refueling station, a place that can get to either the Earth or the Moon with minimal fuel.
Perhaps the most science fictiony idea is to put huge rotating O’Neill Cylinder space stations at the L4 and L5 points. They’d be perfectly stable in orbit, and relatively easy to get to. They’d be the perfect places to begin the colonization of the Solar System.
Thanks gravity. Thanks for interacting in all the strange ways that you do, and creating these stepping stones that we can use as we reach up and out from our planet to become a true Solar System spanning civilization.
An illustration showing the 8 planets of the Solar System to scale Credit: NASA
At one time, humans believed that the Earth was the center of the Universe; that the Sun, Moon, planets and stars all revolved around us. It was only after centuries of ongoing observations and improved instrumentation that astronomers came to understand that we are in fact part a larger system of planets that revolve around the Sun. And it has only been within the last century that we’ve come to understand just how big our Solar System is.
And even now, we are still learning. In the past few decades, the total number of celestial bodies and moons that are known to orbit the Sun has expanded. We have also come to debate the definition of “planet” (a controversial topic indeed!) and introduced additional classifications – like dwarf planet, minor planet, plutoid, etc. – to account for new finds. So just how many planets are there and what is special about them? Let’s run through them one by one, shall we?
Mercury:
As you travel outward from the Sun, Mercury is the closest planet. It orbits the Sun at an average distance of 58 million km (36 million mi). Mercury is airless, and so without any significant atmosphere to hold in the heat, it has dramatic temperature differences. The side that faces the Sun experiences temperatures as high as 420 °C (788 °F), and then the side in shadow goes down to -173 °C (-279.4 °F).
MESSENGER image of Mercury from its third flyby. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Like Venus, Earth and Mars, Mercury is a terrestrial planet, which means it is composed largely of refractory minerals such as the silicates and metals such as iron and nickel. These elements are also differentiated between a metallic core and a silicate mantle and crust, with Mercury possessing a larger-than-average core. Multiple theories have been proposed to explain this, the most widely accepted being that the impact from a planetesimal in the past blew off much of its mantle material.
Mercury is the smallest planet in the Solar System, measuring just 4879 km across at its equator. However, it is second densest planet in the Solar System, with a density of 5.427 g/cm3 – which is the second only to Earth. Because of this, Mercury experiences a gravitational pull that is roughly 38% that of Earth’s (0.38 g).
Mercury also has the most eccentric orbit of any planet in the Solar System (0.205), which means its distance from the Sun ranges from 46 to 70 million km (29-43 million mi). The planet also takes 87.969 Earth days to complete an orbit. But with an average orbital speed of 47.362 km/s, Mercury also takes 58.646 days to complete a single rotation.
Combined with its eccentric orbit, this means that it takes 176 Earth days for the Sun to return to the same place in the sky (i.e. a solar day) on Mercury, which is twice as long as a single Hermian year. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees – compared to Jupiter’s 3.1 degrees, which is the second smallest.
The MESSENGER spacecraft has been in orbit around Mercury since March 2011 – but its days are numbered. Credit: NASA/JHUAPL/Carnegie Institution of Washington
Mercury has only been visited two times by spacecraft, the first being the Mariner 10 probe, which conducted a flyby of the planet back in the mid-1970s. It wasn’t until 2008 that another spacecraft from Earth made a close flyby of Mercury (the MESSENGER probe) which took new images of its surface, shed light on its geological history, and confirmed the presence of water ice and organic molecules in its northern polar region.
In summary, Mercury is made special by the fact it is small, eccentric, and varies between extremes of hot and cold. It’s also very mineral rich, and quite dense!
Venus:
Venus is the second planet in the Solar System, and is Earth’s virtual twin in terms of size and mass. With a mass of 4.8676×1024 kg and a mean radius of about 6,052 km, it is approximately 81.5% as massive as Earth and 95% as large. Like Earth (and Mercury and Mars), it is a terrestrial planet, composed of rocks and minerals that are differentiated.
But apart from these similarities, Venus is very different from Earth. Its atmosphere is composed primarily of carbon dioxide (96%), along with nitrogen and a few other gases. This dense cloud cloaks the planet, making surface observation very difficult, and helps heat it up to 460 °C (860 °F). The atmospheric pressure is also 92 times that of Earth’s atmosphere, and poisonous clouds of carbon dioxide and sulfuric acid rain are commonplace.
Venus’ similarity in size and mass has led to it being called “Earth’s sister planet’. Credit: NASA/JPL/Magellan
Venus orbits the Sun at an average distance of about 0.72 AU (108 million km; 67 million mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
When Venus lies between Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet, at an average distance of 41 million km. This takes place, on average, once every 584 days, and is the reason why Venus is the closest planet to Earth. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a single day on Venus lasts longer than a Venusian year.
Venus’ atmosphere is also known to experience lightning storms. Since Venus does not experience rainfall (except in the form of sulfuric acid), it has been theorized that the lightning is being caused by volcanic eruptions. Several spacecraft have visited Venus, and a few landers have even made it to the surface to send back images of its hellish landscape. Even though there were made of metal, these landers only survived a few hours at best.
Venus is made special by the fact that it is very much like Earth, but also radically different. It’s thick atmosphere could crush a living being, its heat could melt lead, and its acid rain could dissolve flesh, bone and metal alike! It also rotates very slowly, and backwards relative to the other plants.
Earth:
Earth is our home, and the third planet from the Sun. With a mean radius of 6371 km and a mass of 5.97×1024 kg, it is the fifth largest and fifth most-massive planet in the Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System. Like Mercury, Venus and Mars, Earth is a terrestrial planet.
But unlike these other planets, Earth’s core is differentiated between a solid inner core and liquid outer core. The outer core also spins in the opposite direction as the planet, which is believed to create a dynamo effect that gives Earth its protective magnetosphere. Combined with a atmosphere that is neither too thin nor too thick, Earth is the only planet in the Solar System known to support life.
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun between 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU)
The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though a single solar day on Earth is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).
Earth’s axis is also tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Earth has only a single moon: the Moon. Thanks to examinations of Moon rocks that were brought back to Earth by the Apollo missions, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.
A picture of Earth taken by Apollo 11 astronauts. Credit: NASA
What makes Earth special, you know, aside from the fact that it is our home and where we originated? It is the only planet in the Solar System where liquid, flowing water exists in abundance on its surface, has a viable atmosphere, and a protective magnetosphere. In other words, it is the only planet (or Solar body) that we know of where life can exist on the surface.
In addition, no planet in the Solar System has been studied as well as Earth, whether it be from the surface or from space. Thousands of spacecraft have been launched to study the planet, measuring its atmosphere, land masses, vegetation, water, and human impact. Our understanding of what makes our planet unique in our Solar System has helped in the search for Earth-like planets in other systems.
Mars:
The fourth planet from the Sun is Mars, which is also the second smallest planet in the Solar System. It has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s density is also lower than Earths, which leads to it experiencing about 1/3rd Earth’s gravity (0.376 g).
It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons. Mars has almost no atmosphere to help trap heat from the Sun, and so temperatures can plunge to a low of -140 °C (-220 °F) in the Martian winter. However, at the height of summer, temperatures can get up to 20 °C (68 °F) during midday at the equator.
However, recent data obtained by the Curiosity rover and numerous orbiters have concluded that Mars once had a denser atmosphere. Its loss, according to data obtained by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), the atmosphere was stripped away by solar wind over the course of a 500 million year period, beginning 4.2 billion years ago.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.
Like Mercury, Venus, and Earth, Mars is a terrestrial planet, composed mainly of silicate rock and metals that are differentiated between a core, mantle and crust. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.
Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
MSL Curiosity selfie on the surface of Mars. Image: NASA/JPL/Cal-Tech
Mars has two tiny asteroid-sized moons: Phobos and Deimos. Because of their size and shape, the predominant theory is that Mars acquired these two moons after they were kicked out of the Asteroid Belt by Jupiter’s gravity.
Mars has been heavily studied by spacecraft. There are multiple rovers and landers currently on the surface and a small fleet of orbiters flying overhead. Recent missions include the Curiosity Rover, which gathered ample evidence on Mars’ water past, and the groundbreaking discovery of finding organic molecules on the surface. Upcoming missions include NASA’s InSight lander and the Exomars rover.
Hence, Mars’ special nature lies in the fact that it also is terrestrial and lies within the outer edge of the Sun’s habitable zone. And whereas it is a cold, dry place today, it once had an thicker atmosphere and plentiful water on its surface.
Jupiter:
Mighty Jupiter is the fouth planet for our Sun and the biggest planet in our Solar System. Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 1027 kg, 1.43128 x 1015 km3, 6.1419 x 1010 km2, and 4.39264 x 105 km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 times the mass of all the other planets in the Solar System combined.
Jupiter has spectacular aurora, such as this view captured by the Hubble Space Telescope. Credit: NASA, ESA, and J. Nichols (University of Leicester)
But, being a gas giant, it has a relatively low density – 1.326 g/cm3 – which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).
Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.
However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact). Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.
Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.
The Juno spacecraft isn’t the first one to visit Jupiter. Galileo went there in the mid 90’s, and Voyager 1 snapped a nice picture of the clouds on its mission. Credit: NASA
Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.
Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.
The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.
Jupiter has been visited by several spacecraft, including NASA’s Pioneer 10 and Voyager spacecraft in 1973 and 1980, respectively; and by the Cassini and New Horizons spacecraft more recently. Until the recent arrival of Juno, only the Galileo spacecraft has ever gone into orbit around Jupiter, and it was crashed into the planet in 2003 to prevent it from contaminating one of Jupiter’s icy moons.
Illustration of Jupiter and the Galilean satellites. Credit: NASA
In short, Jupiter is massive and has massive storms. But compared to the planets of the inner Solar System, is it significantly less dense. Jupiter also has the most moons in the Solar System, with 67 confirmed and named moons orbiting it. But it is estimated that as many as 200 natural satellites may exist around the planet. Little wonder why this planet is named after the king of the gods.
Saturn:
Saturn is the second largest planet in the Solar System. With a mean radius of 58232±6 km, it is approximately 9.13 times the size of Earth. And at 5.6846×1026 kg, it is roughly 95.15 as massive. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10th, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
As a gas giant, the outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. Like Jupiter, it also has a banded appearance, but Saturn’s bands are much fainter and wider near the equator.
On occasion, Saturn’s atmosphere exhibits long-lived ovals that are thousands of km wide, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Of course, the most amazing feature of Saturn is its rings. These are made of particles of ice ranging in size from a grains of sand to the size of a car. Some scientists think the rings are only a few hundred million years old, while others think they could be as old as the Solar System itself.
Saturn has been visited by spacecraft 4 times: Pioneer 11, Voyager 1 and 2 were just flybys, but Cassini has actually gone into orbit around Saturn and has captured thousands of images of the planet and its moons. And speaking of moons, Saturn has a total of 62 moons discovered (so far), though estimates indicate that it might have as many as 150.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
So like Jupiter, Saturn is a massive gas giant that experiences some very interesting weather patterns. It also has lots of moons and has a very low density. But what really makes Saturn stand out is its impressive ring system. Whereas every gas and ice giant has one, Saturn’s is visible to the naked eye and very beautiful to behold!
Uranus:
Next comes Uranus, the seventh planet from the Sun. With a mean radius of approximately 25,360 km and a mass of 8.68 × 1025 kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm3) is significantly lower; hence, it is only 14.5 as massive as Earth.
The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.
The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).
Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL)
The third most abundant element is methane ice (CH4), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).
In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.
The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. Hence its weather systems are also broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere.
As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).
Huge storms on Uranus were spotted by the Keck Observatory on Aug. 5 and Aug. 6, 2014. Credit: Imke de Pater (UC Berkeley), Pat Fry (University of Wisconsin), Keck Observatory
One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.
Uranus was the first planet to be discovered with a telescope; it was first recognized as a planet in 1781 by William Herschel. Beyond Earth-based observations, only one spacecraft (Voyager 2) has ever studied Uranus up close. It passed by the planet in 1986, and captured the first close images. Uranus has 27 known moons.
Uranus’ special nature comes through in its natural beauty, its intense weather, its ring system and its impressive array of moons. And it’s compositions, being an “ice” giant, is what gives its aquamarine color. But perhaps mist interesting is its sideways rotation, which is unique among the Solar planets.
Neptune:
Neptune is the 8th and final planet in the Solar System, orbiting the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion. With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus.
Neptune’s system of moons and rings visualized. Credit: SETI
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour, which is believed to be due to Neptune’s cold temperatures – which may decrease the friction in the system, During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune.
Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.
The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.
Neptune is the second planet discovered in modern times. It was discovered at the same time by both Urbain Le Verrier and John Couch Adams. Neptune has only ever been visited by one spacecraft, Voyager 2, which made a fly by in August, 1989. Neptune has 13 known moons. Th largest and most famous of these is Triton, which is believed to be a former KBO that was captured by Neptune’s gravity.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
So much like Uranus, Neptune has a ring system, some intense weather patterns, and an impressive array of moons. Also like Uranus, Neptune’s composition allows for its aquamarine color; except that in Neptune’s case, this color is more intense and vivid. In addition, Neptune experiences some temperature anomalies that are yet to be explained. And let’s not forgt the raining diamonds!
And those are the planets in the Solar System thank you for joining the tour! Unfortunately, Pluto isn’t a planet any more, hence why it was not listed. We know, we know, take it up with the IAU!
Want to hear something cool? There’s a black hole at the center of the Milky Way. And not just any black hole, it’s a supermassive black hole with more than 4.1 million times the mass of the Sun.
It’s right over there, in the direction of the Sagittarius constellation. Located just 26,000 light-years away. And as we speak, it’s in the process of tearing apart entire stars and star systems, occasionally consuming them, adding to its mass like a voracious shark.
Sagittarius A*. Image credit: Chandra
Wait, that doesn’t sound cool, that sort of sounds a little scary. Right?
Don’t worry, you have absolutely nothing to worry about, unless you plan to live for quadrillions of years, which I do, thanks to my future robot body. I’m ready for my singularity, Dr. Kurzweil.
Is the supermassive black hole going to consume the Milky Way? If not, why not? If so, why so?
The discovery of a supermassive black hole at the heart of the Milky Way, and really almost all galaxies, is one of my favorite discoveries in the field of astronomy. It’s one of those insights that simultaneously answered some questions, and opened up even more.
Back in the 1970s, the astronomers Bruce Balick and Robert Brown realized that there was an intense source of radio emissions coming from the very center of the Milky Way, in the constellation Sagittarius.
They designated it Sgr A*. The asterisk stands for exciting. You think I’m joking, but I’m not. For once, I’m not joking.
An illustration of Saggitarius A*. Credit: NASA/CXC/M.Weiss
In 2002, astronomers observed that there were stars zipping past this object, like comets on elliptical paths going around the Sun. Imagine the mass of our Sun, and the tremendous power it would take to wrench a star like that around.
The only objects with that much density and gravity are black holes, but in this case, a black hole with millions of times the mass of our own Sun: a supermassive black hole.
With the discovery of the Milky Way’s supermassive black hole, astronomers found evidence that there are black holes at the heart of every galaxy.
At the same time, the discovery of supermassive black holes helped answer one of the big questions in astronomy: what are quasars? We did a whole article on them, but they’re intensely bright objects, generating enough light they can be seen billions of light-years away. Giving off more energy than the rest of their own galaxy combined.
The quasar SDSS J1106+1939 has the most energetic outflows ever seen, at least five times more powerful than any that have been observed to date. Credit: ESO/L. Calçada
It turns out that quasars and supermassive black holes are the same thing. Quasars are just black holes in the process of actively feeding; gobbling up so much material it piles up in an accretion disk around it. Once again, these do sound terrifying. But are we in any danger?
In the short term, no. The black hole at the center of the Milky Way is 26,000 light-years away. Even if it turned into a quasar and started eating stars, you wouldn’t even be able to notice it from this distance.
A black hole is just a concentration of mass in a very small region, which things orbit around. To give you an example, you could replace the Sun with a black hole with the exact same mass, and nothing would change. I mean, we’d all freeze because there wasn’t a Sun in the sky anymore, but the Earth would continue to orbit this black hole in exactly the same orbit, for billions of years.
Same goes with the black hole at the center of the Milky Way. It’s not pulling material in like a vacuum cleaner, it serves as a gravitational anchor for a group of stars to orbit around, for billions of years.
In order for a black hole to actually consume a star, it needs to make a direct hit. To get within the event horizon, which is only about 17 times bigger than the Sun. If a star gets close, without hitting, it’ll get torn apart, but still, it doesn’t happen very often.
A black hole, with an accretion disk, consuming a star. Credit: ESO/L. Calçada
The problem happens when these stars interact with one another through their own gravity, and mess with each other’s orbits. A star that would have been orbiting happily for billions of years might get deflected into a collision course with the black hole. But this happens very rarely.
Over the short term, that supermassive black hole is totally harmless. Especially from out here in the galactic suburbs.
But there are a few situations that might cause some problems over vast periods of time.
The first panic will happen when the Milky Way collides with Andromeda in about 4 billion years – let’s call this mess Milkdromeda. Suddenly, you’ll have two whole clouds of stars interacting in all kinds of ways, like an unstable blended family. Stars that would have been safe will careen past other stars and be deflected down into the maw of either of the two supermassive black holes on hand. Andromeda’s black hole could be 100 million times the mass of the Sun, so it’s a bigger target for stars with a death wish.
View of Milkdromeda from Earth “shortly” after the merger, around 3.85-3.9 billion years from now Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Over the coming billions, trillions and quadrillions of years, more and more galaxies will collide with Milkdromeda, bringing new supermassive black holes and more stars to the chaos.
So many opportunities for mayhem.
Of course, the Sun will die in about 5 billion years, so this future won’t be our problem. Well, fine, with my eternal robot body, it might still be my problem.
After our neighborhood is completely out of galaxies to consume, then there will just be countless eons of time for stars to interact for orbit after orbit. Some will get flung out of Milkdromeda, some will be hurled down into the black hole.
And others will be safe, assuming they can avoid this fate over the Googol years it’ll take for the supermassive black hole to finally evaporate. That’s a 1 followed by 100 zeroes years. That’s a really really long time, so now I don’t like those odds.
For our purposes, the black hole at the heart of the Milky Way is completely and totally safe. In the lifetime of the Sun, it won’t interact with us in any way, or consume more than a handful of stars.
But over the vast eons, it could be a different story. I hope we can be around to find out the answer.
The Moon is great and all, but I wish it was closer. Close enough that I could see all kinds of detail on its surface without a telescope or a pair of binoculars. Close enough that I could just reach up and grab enough cheese for a lifetime of grilled cheese sandwiches.
Sure, there would be all kinds of horrible problems with having the Moon that much closer. Intense tides, a total lack of good dark nights for stargazing, and something else… Oh right, the total destruction of life on Earth. On second thought the Moon can stay right where it is, thank you very much.
The Earth’s Moon is located an average distance of 384,400 kilometers away. I say average because the Moon actually follows an elliptical orbit. At its closest point, it’s only 362,600 km, and at its furthest point, it’s 405,400 kilometers.
Still, that’s so far that it takes light a little over a second to reach the Moon, traveling almost 300,000 km/s. The Moon is far.
But what if the Moon was much closer? How close could it get and still be the Moon?
The Moon isn’t actually getting closer. It just looks that way because it’s on your computer screen. Credit: NASA
Once again, I need to remind you that this is purely theoretical. The Moon isn’t getting closer to us, in fact, it’s getting further. The Moon is slowly drifting away from us at a distance of almost 4 centimeters per year.
Let’s go back to the beginning, when the young Earth collided with a Mars-sized planet billions of years ago. This catastrophic encounter completely resurfaced planet Earth, and kicked up a massive amount of debris into orbit. Well, a Moon’s worth of debris, which collected together by mutual gravity into the roughly spherical Moon we recognize today.
Shortly after its formation, the Moon was much closer, and the Earth was spinning more rapidly. A day on Earth was only 6 hours long, and the Moon took just 17 days to orbit the Earth.
The Earth’s gravity stopped the Moon’s relative rotation, and the Moon’s gravity has been slowing the Earth’s rotation. To maintain the overall angular momentum of the system, the Moon has been drifting away to compensate.
This conservation of momentum is very important because it works both ways. As long as a moon takes longer than a day to orbit its planet, you’re going to see this same effect. The planet’s rotation slows, and the moon drifts further to compensate.
But if you have a scenario where the moon orbits faster than the planet rotates, you have the exact opposite situation. The moon makes the planet rotate more quickly, and it drifts closer to compensate. This can’t end well.
Once you get close enough, gravity becomes a harsh mistress.
Reaching the Roche limit can ruin your day. Credit: Hazmat2. Original Image Credit: Theresa Knott. CC-SA 3.0
There’s a point in all gravitational interactions called the Roche Limit. This is the point at which an object held together by gravity (like the Moon), gets close enough to another celestial body that it gets torn apart.
The exact point depends on the mass, size and density of the two objects. For example, the Roche Limit between the Earth and the Moon is about 9,500 kilometers, assuming the Moon is a solid ball. In other words, if the Moon gets within 9,500 kilometers or so, of the Earth, the gravity of the Earth overwhelms the gravity holding the Moon together.
The Moon would be torn apart, and turned into a ring. And then the pieces of the ring would continue to orbit the Earth until they all came crashing down. When that happened, it would be a series of very bad days for anyone living on Earth.
Get too close to the sun and a comet could be torn apart. Credit: NASA/JPL-Caltech
If an average comet got within about 18,000 km of Earth, it would get torn to pieces. While the Sun can, and does, tear apart comets from about 1.3 million km away.
This sounds purely theoretical, but this is actually going to happen over at Mars. Its largest moon Phobos orbits more quickly than a Martian day, which means that it’s drifting closer and closer to the planet. In a few million years, it’ll cross the Roche Limit, tear into a ring, and then all the pieces of the former Phobos will crash down onto Mars. We did a whole article on this.
Phobos will eventually break apart from reaching the Roche limit, which will leave Deimos as Mars’ only moon. Credit: HiRISE, MRO, LPL (U. Arizona), NASA
Now you might be wondering, wait a second. I’m a separate object from the Earth, why don’t I get torn apart since I’m definitely within the Earth’s Roche Limit.
You do have gravity holding you together, but it’s insignificant compared to the chemical bonds holding you together. This is why physicists consider gravity to actually be a pretty weak force compared to all the other forces of the Universe.
You’d need to go somewhere with really intense gravity, like a black hole, for the Roche Limit to overcome the forces holding you together.
So that’s it. Bring the Moon within 9,500 kilometers or so and it would no longer be a Moon. It would be torn apart into a ring, a Halo ring, if you will, capable of wiping out all life on a planet infected by the flood. All the moons we see in the Solar System are are least at the Roche Limit or beyond, otherwise they would have broken up long ago… and probably did.
The moons of solar system, showed to scale with Earth's Moon. Credit: NASA
For millennia, human beings stared up at the night sky and were held in awe by the Moon. To many ancient cultures, it represented a deity, and its cycles were accorded divine significance. By the time of Classical Antiquity and the Middle Ages, the Moon was considered to be a heavenly body that orbited Earth, much like the other known planets of the day (Mercury, Venus, Mars, Jupiter, and Saturn).
However, our understanding of moons was revolutionized when in 1610, astronomer Galileo Galilei pointed his telescope to Jupiter and noticed ” four wandering stars” around Jupiter. From this point onward, astronomers have come to understand that planets other than Earth can have their own moons – in some cases, several dozen or more. So just how many moons are there in the Solar System?
In truth, answering that question requires a bit of clarification first. If we are talking about confirmed moons that orbit any of the planets of the Solar System (i.e. those that are consistent with the definition adopted by the IAU in 2006), then we can say that there are currently 207 known moons. If however, we open the floor to “dwarf planets” that have confirmed satellites, the number reached 220.
The moons, several minor planets, and comets of the Solar System shown to scale. Credit: Antonio Ciccolella
However, 479 minor-planet moons have also been observed in the Solar System (as of Dec. 2022). This includes the 229 known objects in the asteroid belt with satellites, six Jupiter Trojans, 91 near-Earth objects (two with two satellites each), 31 Mars-crossers, and 84 natural satellites of Trans-Neptunian Objects. And some 150 additional small bodies have been observed within the rings of Saturn. If we include all these, then we can say that the Solar System has 849 known satellites.
Inner Solar System:
The planets of the Inner Solar system – Mercury, Venus, Earth, and Mars – are all terrestrial planets, which means that they are composed of silicate rock and minerals that are differentiated between a metallic core and a silicate mantle and crust. For a number of reasons, few satellites exist within this region of the Solar System.
All told, only three natural satellites exist orbiting planetary bodies in the Inner Solar System – Earth and Mars. While scientists theorize that there were moons around Mercury and Venus in the past, it is believed that these moons impacted the surface a long time ago. The reason for this sparseness of satellites has a lot to do with the gravitational influence of the Sun.
Both Mercury and Venus are too close to the Sun to have grabbed onto a passing object or held onto rings of debris in orbit that could have coalesced to form a satellite over time. In Mercury’s case, it is also too weak in terms of its own gravitational pull to grab a satellite in its orbit. Earth and Mars were able to retain satellites, but mainly because they are the outermost of the Inner planets.
Earth has only one natural satellite, which we are familiar with – the Moon. With a mean radius of 1737 km (1,080 mi) and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive, which is quite large for a satellite. It is also the second densest moon in our Solar System (after Io), with a mean density of 3.3464 g/cm³.
Several theories have been proposed for the formation of the Moon. The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
Mars, meanwhile, has two moons – Phobos and Deimos. Like our own Moon, both of the Martian moons are tidally locked to Mars, so they always present the same face to the planet. Compared to our Moon, they are rough and asteroid-like in appearance and also much smaller. Hence the prevailing theory is that they were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity and were then acquired by Mars.
The larger moon is Phobos, whose name comes from the Greek word which means “fear” (i.e. phobia). Phobos measures just 22.7 km (14 mi) across and has an orbit that places it closer to Mars than Deimos. Compared to Earth’s own Moon — which orbits at a distance of 384,403 km (238,857 mi) away from our planet — Phobos orbits at an average distance of only 9,377 km (5,826.5 mi) above Mars.
Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter. Credit: NASA
Mars’ second moon is Deimos, which takes its name from the Greek word for panic. It is even smaller, measuring just 12.6 km (7.83 mi) across, and is also less irregular in shape. Its orbit places it much farther away from Mars, at a distance of 23,460 km (14,577 mi), which means that Deimos takes 30.35 hours to complete an orbit around Mars.
These three moons are the sum total of moons to be found within the Inner Solar System (at least, by the conventional definition). But looking further abroad, we see that this is really just the tip of the iceberg. To think we once believed that the Moon was the only one of its kind!
Outer Solar System:
Beyond the Asteroid Belt (and Frost Line), things become quite different. In this region of the Solar System, every planet has a substantial system of Moons; in the case of Jupiter and Saturn, reaching perhaps even into the hundreds. So far, a total of 213 moons have been confirmed orbiting the Outer Planets, while several hundred more orbit minor bodies and asteroids.
Due to its immense size, mass, and gravitational pull, Jupiter has the most satellites of any planet in the Solar System. At present, the Jovian system includes 80 known moons, though it is estimated that it may have over 200 moons and moonlets (the majority of which are yet to be confirmed and classified).
The four largest Jovian moons are known as the Galilean Moons (named after their discoverer, Galileo Galilei). They include Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.
Illustration of Jupiter and the Galilean satellites. Credit: NASA
Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km (124 mi), orbit at radii less than 200,000 km (124,275 mi), and have orbital inclinations of less than half a degree. This group includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.
Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.
Similar to Jupiter, it is estimated that Saturn has at least 150 moons and moonlets, but only 83 of these moons have been given official names or designations. Of these, 57 are less than 10 km (6.2 mi) in diameter, and another 13 are between 10 and 50 km (6.2 to 31 mi) in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km (155 to 3100 mi)
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
The Inner Large Moons, which orbit within the E Ring, include the larger satellites MimasEnceladus, Tethys, and Dione. These moons are all composed primarily of water ice and are believed to be differentiated into a rocky core and an icy mantle and crust. The Large Outer Moons, which orbit outside of Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice, and rock.
At 5,150 km (3,200 mi) in diameter and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System aside from Earth to have bodies of liquid on its surface. These take the form of methane–ethane lakes in Titan’s north and south polar regions.
Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon, and Titania. These moons range in diameter and mass from 472 km (293 mi) and 6.7×1019 kg for Miranda to 1578 km (980.5 mi) and 3.5×1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest, while Umbriel is the darkest.
A montage of Uranus’s moons (from left to right) – Ariel, Credit: NASA
All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda, which is made primarily of ice.
The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.
Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (except for S/2004 N 1, which is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso, and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
With a mean diameter of around 2,700 km (1,678 mi) and a mass of 21,4080 ± 520×1017 kg, Triton is the largest of Neptune’s moons and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km (220,437 mi) from Neptune, it also sits between the planet’s inner and outer moons.
These moons make up the lion’s share of natural satellites found in the Solar System. However, thanks to ongoing exploration and improvements made in our instrumentation, satellites are being discovered in orbit around minor bodies as well.
Dwarf Planets and Other Bodies:
As already noted, there are several dwarf planets, TNOs, and other bodies in the Solar System that also have their own moons. These consist mainly of the natural satellites that have been confirmed orbiting Pluto, Eris, Haumea, and Makemake. With five orbiting satellites, Pluto has the most confirmed moons (though that may change with further observation).
The largest and closest in orbit to Pluto is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.
A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11th, 2015. Credit: NASA-JHUAPL-SWRI.
Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.
Charon, Styx, and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.
In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other, and from any position on either body, the other is always at the same position in the sky or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem to indicate that Pluto has a warm subsurface ocean and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.
Comparison of Pluto with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
Coming in second is Haumea, which has two known moons – Hi’iaka and Namaka – which are named after the daughters of the Hawaiian goddess. Both were discovered in 2005 by Brown’s team while conducting observations of Haumea at the W.M. Keck Observatory. Hi’iaka, which was initially nicknamed “Rudolph” by the Caltech team, was discovered on January 26th, 2005.
It is the outer, the larger (at roughly 310 km (mi) in diameter), and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Infrared observations indicate that its surface is almost entirely covered by pure crystalline water ice. Because of this, Brown and his team have speculated that the moon is a fragment of Haumea that broke off during a collision.
Namaka, the smaller and innermost of the two, was discovered on June 30th, 2005, and nicknamed “Blitzen”. It is a tenth the mass of Hi‘iaka and orbits Haumea in 18 days in a highly elliptical orbit. Both moons circle Haumea is highly eccentric orbits. No estimates have been made yet as to their mass.
Eris has one moon called Dysnomia, named after the daughter of Eris in Greek mythology, and was first observed on September 10th, 2005 – a few months after the discovery of Eris. The moon was spotted by a team using the Keck telescopes in Hawaii, who were busy carrying out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris) at the time.
Artist’s concept of the dwarf planet Eris and its only natural satellite, Dysnomia. Credit: NASA, ESA, Adolph Schaller (for STScI)
In April 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.
Largest and Smallest Moons:
The title of “largest moon in the Solar System” goes to Ganymede, which measures 5,262.4 kilometers (3,270 mi) in diameter. This not only makes it larger than Earth’s Moon but larger even than the planet Mercury – though it has only half of Mercury’s mass. As for the smallest satellite, that is a tie between S/2003 J 9 and S/2003 J 12. These two satellites, both of which orbit Jupiter, measure about 1 km (0.6 mi) in diameter.
An important thing to note when discussing the number of known moons in the Solar System is that the key word here is “known”. With every passing year, more satellites are being confirmed, and the vast majority of those we now know about were only discovered in the past few decades. As our exploration efforts continue and our instruments improve, we may find that there are hundreds more lurking around out there!
In a previous article, we crushed that idea that the Universe is perfect for life. It’s not. Almost the entire Universe is a horrible and hostile place, apart from a fraction of a mostly harmless planet in a backwater corner of the Milky Way.
While living here on Earth takes about 80 years to kill you, there are other places in the Universe at the very other end of the spectrum. Places that would kill you in a fraction of a fraction of a second. And nothing is more lethal than supernovae and remnants they leave behind: neutron stars.
We’ve done a few articles about neutron stars and their different flavours, so there should be some familiar terrain here.
Artist concept of a neutron star. Credit: NASA
As you know, neutron stars are formed when stars more massive than our Sun explode as supernovae. When these stars die, they no longer have the light pressure pushing outward to counteract the massive gravity pulling inward.
This enormous inward force is so strong that it overcomes the repulsive force that keeps atoms from collapsing. Protons and electrons are forced into the same space, becoming neutrons. The whole thing is just made of neutrons. Did the star have hydrogen, helium, carbon and iron before? That’s too bad, because now it’s all neutrons.
You get pulsars when neutron stars first form. When all that former star is compressed into a teeny tiny package. The conservation of angular motion spins the star up to tremendous velocities, sometimes hundreds of times a second.
But when neutron stars form, about one in ten does something really really strange, becoming one of the most mysterious and terrifying objects in the Universe. They become magnetars. You’ve probably heard the name, but what are they?
As I said, magnetars are neutron stars, formed from supernovae. But something unusual happens as they form, spinning up their magnetic field to an intense level. In fact, astronomers aren’t exactly sure what happens to make them so strong.
This artist’s impression shows the magnetar in the very rich and young star cluster Westerlund 1. Credit: ESO/L. Calçada
One idea is that if you get the spin, temperature and magnetic field of a neutron star into a perfect sweet spot, it sets off a dynamo mechanism that amplifies the magnetic field by a factor of a thousand.
But a more recent discovery gives a tantalizing clue for how they form. Astronomers discovered a rogue magnetar on an escape trajectory out of the Milky Way. We’ve seen stars like this, and they’re ejected when one star in a binary system detonates as a supernova. In other words, this magnetar used to be part of a binary pair.
And while they were partners, the two stars orbited one another closer than the Earth orbits the Sun. This close, they could transfer material back and forth. The larger star began to die first, puffing out and transferring material to the smaller star. This increased mass spun the smaller star up to the point that it grew larger and spewed material back at the first star.
The initially smaller star detonated as a supernova first, ejecting the other star into this escape trajectory, and then the second went off, but instead of forming a regular neutron star, all these binary interactions turned it into a magnetar. There you go, mystery maybe solved?
The strength of the magnetic field around a magnetar completely boggles the imagination. The magnetic field of the Earth’s core is about 25 gauss, and here on the surface, we experience less than half a gauss. A regular bar magnet is about 100 gauss. Just a regular neutron star has a magnetic field of a trillion gauss. Magnetars are 1,000 times more powerful than that, with a magnetic field of a quadrillion gauss.
What if you could get close to a magnetar? Well, within about 1,000 kilometers of a magnetar, the magnetic field is so strong it messes with the electrons in your atoms. You would literally be torn apart at an atomic level. Even the atoms themselves are deformed into rod-like shapes, no longer usable by your precious life’s chemistry.
But you wouldn’t notice because you’d already be dead from the intense radiation streaming from the magnetar, and all the lethal particles orbiting the star and trapped in its magnetic field.
Artist’s conception of a starquake cracking the surface of a neutron star. Credit: Darlene McElroy of LANL
One of the most fascinating aspects of magnetars is how they can have starquakes. You know, earthquakes, but on stars… starquakes. When neutron stars form, they can have a delicious murder crust on the outside, surrounding the degenerate death matter inside. This crust of neutrons can crack, like the tectonic plates on Earth. As this happens, the magnetar releases a blast of radiation that we can see clear across the Milky Way.
In fact, the most powerful starquake ever recorded came from a magnetar called SGR 1806-20, located about 50,000 light years away. In a tenth of a second, one of these starquakes released more energy than the Sun gives off in 100,000 years. And this wasn’t even a supernova, it was merely a crack on the magnetar’s surface.
Magnetars are awesome, and provide the absolute opposite end of the spectrum for a safe and habitable Universe. Fortunately, they’re really far away and you won’t have to worry about them ever getting close.
The Sagittarius Cluster, aka. Messier 22. Credit: Wikipedia Commons/Hewholooks
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Sagittarius Cluster (aka. Messier 22). Enjoy!
Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, this “Messier Catalog” would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.
One of these objects is the Sagittarius Cluster, otherwise known as Messier 22 (and NGC 6656). This elliptical globular cluster, is located in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky, and was therefore one of the first of its kind to be discovered and later studied.
Description:
Located around 10,400 light years from our Solar System, in the direction of Sagittarius, M22 occupied a volume of space that is 200 light years in diameter and is receding away from us at 149 kilometers per second. M22 has a lot in common with many other clusters of its type, which includes being a gravitationally bound sphere of stars, and that most of its stars are all about the same age – about 12 billion years old.
Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky
It is part of our galactic halo, and may once have been part of a galaxy that our Milky Way cannibalized. But it’s there that the similarities end. For example, it consists of at least 70,000 individual stars, only 32 of which are variable stars. It also spans an incredible 32 arc minutes in the sky and ranks as the fourth brightness of all the known globular clusters in our galaxy.
And four must be its lucky number, because it is also one of only four globular clusters known to contain a planetary nebula. Recent Hubble Space Telescope investigations of Messier 22 have led to the discovery of an astonishing discovery. For starters, in 1999, astronomers discovered six planet-sized objects floating around inside the cluster that were about 80 times the mass of Earth!
Using a technique known as microlensing, which measures the way gravity bends the light of the background stars, the Hubble Space Telescope was able to determine the existence of the gas giant. Even though the Hubble can’t resolve them because the angle at which the light bends is about 100 times smaller than the telescope’s angular resolution, scientist know they are there because the gravity “powers up” the starlight, making it brighter each time a body passes in front of it.
Because a microlensing event is very rare and totally unpredictable, the Hubble team needed to monitor 83,000 stars every three days for nearly four months. Luckily, a sharp peak in brightness was all the proof they needed that they were on the right track.
The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA
Said Kailash Sahu, of the Space Telescope Science Institute, Baltimore, MD, of the discovery in 2007: “Hubble’s excellent sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects. This holds tremendous potential for further searches for dark, low-mass objects.”
During their study time, the Hubble team caught six microlensing events that lasted less than 20 hours and one which endured for 18 days. By calculating the times of the eclipses and the spikes in brightness, astronomers could then estimate the mass of the object passing in front of the star. These wandering rogues might be planets torn away from their parent stars by the huge amounts of gravitational influence from so many closely packed suns – or (in the case of the long event) simply a smaller mass star passing in front of another.
They could be brown dwarfs, or even a totally new type of object. As co-investigator Nino Panagia of the European Space Agency and Space Telescope Science Institute said: “Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early universe,”
Two black holes were also discovered in M22 and confirmed by the Chandra X-ray telescope in 2012. The objects have between 10 and 20 solar masses, and their discovery suggests that there may be 5 to 100 black holes within the cluster (and maybe some multiple black holes as well). The presence of black holes and their interaction with the stars of M22 could explain the cluster’s unusually large central region.
Optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA
Other objects of interesting include two black holes – M22-VLA1 and M22-VLA2 – both of which are part of binary star systems. Each has a companion star and is pulling matter from it. This gas and dust, in turn, forms an accretion disk around each black hole, creating emissions that scientists used to confirm their existence.
Messier 22 is one of only four known globular clusters that contain a planetary nebula. This nebula – catalogued as GJJC1 or IRAS 18333-2357 – is rather small and young, being only 3 arcseconds in diameter and 6,000 years old. It was discovered in 1986 using the infrared satellite IRAS, and identified as a planetary nebula in 1989.
History of Observation:
Chances are, magnificent Messier 22 was probably the first globular cluster to ever be recorded in the history of astronomy, most likely by Abraham Ihle in 1665. Over the years it has been included in many historic observations, including Edmund Halley’s list of 6 objects published 1715, and observed by De Chéseaux (his Number 17) and Le Gentil, as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects (as Lacaille I.12).
Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS). Credit:UoM/IPAC/Caltech/NASA/NSF
However, it was Charles Messier who made it famous when he cataloged it as M22 on June 5th, 1764. As he said of the object at the time:
“I have observed a nebula situated a bit below the ecliptic, between the head and the bow of Sagittarius, near the star of seventh magnitude, the twenty-fifth of that constellation, according to the catalog of Flamsteed. That nebula didn’t appear to me to contain any star, although I have examined it with a good Gregorian telescope which magnified 104 times: it is round, and one sees it very well with an ordinary refractor of 3 feet and a half; its diameter is about 6 minutes of arc. I have determined its position by comparing with the star Lambda Sagittarii: its right ascension has been concluded as 275d 28′ 39″, and its declination as 24d 6′ 11”. It was Abraham Ihle, a German, who discovered this nebula in 1665, when observing Saturn. M. le Gentil has examined it also, and he has made an engraving of the configuration in the volume of the Memoirs of the Academy, for the year 1759, page 470. He observed it on August 29, 1747, under good weather, with a refractor of 18 feet length: He also observed it on July 17, and on other days. “It always appeared to me,” he says, “very irregular in its figure, hair and distributing in space of rays of light all over its diameter.”
While Messier’s description is a wonder, let us remember that he was a comet hunter by profession. Once more, it was the observer Admiral Smythto whom we are indebted for the most detailed and vivid description of the cluster:
“A fine globular cluster, outlying that astral stream, the Via Lactea [Milky Way], in the space between the Archer’s head and bow, not far from the point of the winter solstice, and midway between Mu and Sigma Sagittarii. It consists of very minute and thickly condensed particles of light, with a group of small stars preceding by 3m, somewhat in a crucial form. Halley ascribes the discovery of this in 1665, to Abraham Ihle, the German; but it has been thought this name should have been Abraham Hill, who was one of the first council of the Royal Society, and was wont to dabble with astronomy. Hevelius, however, appears to have noticed it previous to 1665, so that neither Ihle nor Hill can be supported.
“In August, 1747, it was carefully drawn by Le Gentil, as seen with an 18-foot telescope, which drawing appears in the Mémoires de l’Académie for 1759. In this figure three stars accompany the cluster, and he remarks that two years afterwards he did not see the preceding and central one: I, however, saw it very plainly in 1835. In the description he says, “Elle m’a toujours parue tres-irrégulière dans sa figure, chevelue, et rependant des espèces de rayons de lumière tout autout de son diamètre.” This passage, I quote, “as in duty bound;” but from familiarity with the object itself, I cannot say that I clearly understand how or why his telescope exhibited these “espèces de rayons.” Messier, who registered it in 1764, says nothing about them, merely observing that it is a nebula without a star, of a round form; and Sir William Herschel, who first resolved it, merely describes it as a circular cluster, with an estimated profundity of the 344th order. Sir John Herschel recommends it as a capital test for trying the space-penetrating power of a telescope.
“This object is a fine specimen of the compression on which the nebula-theory is built. The globular systems of stars appear thicker in the middle than they would do if these stars were all at equal distances from each other; they must, therefore, be condensed toward the centre. That the stars should be accidentally disposed is too improbable a supposition to be admitted; whence Sir William Herschel supposes that they are thus brought together by their mutual attractions, and that the gradual condensation towards the centre must be received as proof of a central power of such kind.”
The location of Messier 22 in the night sky. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
Locating Messier 22:
From its position almost on the ecliptic plane, bright globular cluster M22 is easy to find in optics of all sizes. The most important clue is simply identifying the Sagittarius “teapot” shape. Once you’ve located it, just choose the “lid” star, Lambda (Kaus Borealis) and look about a fingerwidth (2 degrees) due northeast. In binoculars, if you center on Lambda, M22 will appear in the 10:00 region of your field of view.
In a finderscope, you will need to hop from Lambda northeast to 24 Sagittari and you’ll see it as a faint fuzzy nearby also to the northeast. From a dark sky location, Messier Object 22 can also sometimes be spotted with the unaided eye! No matter what size optics you use, this large, very luminous ball of stars is quite appealing. A joy to binocular users and an exercise in resolution to telescopes.
And here are the quick facts to help you get started:
Object Name: Messier 22 Alternative Designations: M22, NGC 6656 Object Type: Class VII Globular Star Cluster Constellation: Sagittarius Right Ascension: 18 : 36.4 (h:m) Declination: -23 : 54 (deg:m) Distance: 10.4 (kly) Visual Brightness: 5.1 (mag) Apparent Dimension: 32.0 (arc min)
Go on… Magnificent Messier 22 is waiting for you to appreciate it!
