Planet Mercury as seen from the MESSENGER spacecraft in 2008. Credit: NASA/JPL
Virtually every planet in the Solar System has moons. Earth has The Moon, Mars has Phobos and Deimos, and Jupiter and Saturn have 67 and 62 officially named moons, respectively. Heck, even the recently-demoted dwarf planet Pluto has five confirmed moons – Charon, Nix, Hydra, Kerberos and Styx. And even asteroids like 243 Ida may have satellites orbiting them (in this case, Dactyl). But what about Mercury?
If moons are such a common feature in the Solar System, why is it that Mercury has none? Yes, if one were to ask how many satellites the planet closest to our Sun has, that would be the short answer. But answering it more thoroughly requires that we examine the process through which other planets acquired their moons, and seeing how these apply (or fail to apply) to Mercury.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
The Sun has always been the center of our cosmological systems. But with the advent of modern astronomy, humans have become aware of the fact that the Sun is merely one of countless stars in our Universe. In essence, it is a perfectly normal example of a G-type main-sequence star (G2V, aka. “yellow dwarf”). And like all stars, it has a lifespan, characterized by a formation, main sequence, and eventual death.
This lifespan began roughly 4.6 billion years ago, and will continue for about another 4.5 – 5.5 billion years, when it will deplete its supply of hydrogen, helium, and collapse into a white dwarf. But this is just the abridged version of the Sun’s lifespan. As always, God (or the Devil, depending on who you ask) is in the details!
To break it down, the Sun is about half way through the most stable part of its life. Over the course of the past four billion years, during which time planet Earth and the entire Solar System was born, it has remained relatively unchanged. This will stay the case for another four billion years, at which point, it will have exhausted its supply of hydrogen fuel. When that happens, some pretty drastic things will take place!
The Birth of the Sun:
According to Nebular Theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.
Artist’s concept of a star surrounded by a molecular cloud to form a swirling disk called a “protoplanetary disk.” Credit: NASA/JPL-Caltech
From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it.
The ball at the center would eventually form the Sun, while the disk of material would form the planets. The Sun spent about 100,000 years as a collapsing protostar before temperature and pressures in the interior ignited fusion at its core. The Sun started as a T Tauri star – a wildly active star that blasted out an intense solar wind. And just a few million years later, it settled down into its current form. The life cycle of the Sun had begun.
The Main Sequence:
The Sun, like most stars in the Universe, is on the main sequence stage of its life, during which nuclear fusion reactions in its core fuse hydrogen into helium. Every second, 600 million tons of matter are converted into neutrinos, solar radiation, and roughly 4 x 1027 Watts of energy. For the Sun, this process began 4.57 billion years ago, and it has been generating energy this way every since.
However, this process cannot last forever since there is a finite amount of hydrogen in the core of the Sun. So far, the Sun has converted an estimated 100 times the mass of the Earth into helium and solar energy. As more hydrogen is converted into helium, the core continues to shrink, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force.
The Sun captured by NASA’s Solar Dynamics Observatory Spacecraft.
This places more pressure on the core, which is resisted by a resulting increase in the rate at which fusion occurs. Basically, this means that as the Sun continues to expend hydrogen in its core, the fusion process speeds up and the output of the Sun increases. At present, this is leading to a 1% increase in luminosity every 100 million years, and a 30% increase over the course of the last 4.5 billion years.
In 1.1 billion years from now, the Sun will be 10% brighter than it is today, and this increase in luminosity will also mean an increase in heat energy, which Earth’s atmosphere will absorb. This will trigger a moist greenhouse effect here on Earth that is similar to the runaway warming that turned Venus into the hellish environment we see there today.
In 3.5 billion years from now, the Sun will be 40% brighter than it is right now. This increase will cause the oceans to boil, the ice caps to permanently melt, and all water vapor in the atmosphere to be lost to space. Under these conditions, life as we know it will be unable to survive anywhere on the surface. In short, planet Earth will come to be another hot, dry Venus.
Core Hydrogen Exhaustion:
All things must end. That is true for us, that is true for the Earth, and that is true for the Sun. It’s not going to happen anytime soon, but one day in the distant future, the Sun will run out of hydrogen fuel and slowly slouch towards death. This will begin in approximate 5.4 billion years, at which point the Sun will exit the main sequence of its lifespan.
With its hydrogen exhausted in the core, the inert helium ash that has built up there will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size and enter the Red Giant phase of its evolution. It is calculated that the expanding Sun will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth. Even if the Earth survives, the intense heat from the red sun will scorch our planet and make it completely impossible for life to survive.
Final Phase and Death:
Once it reaches the Red-Giant-Branch (RGB) phase, the Sun will haves approximately 120 million years of active life left. But much will happen in this amount of time. First, the core (full of degenerate helium), will ignite violently in a helium flash – where approximately 6% of the core and 40% of the Sun’s mass will be converted into carbon within a matter of minutes.
The Sun will then shrink to around 10 times its current size and 50 times its luminosity, with a temperature a little lower than today. For the next 100 million years, it will continue to burn helium in its core until it is exhausted. By this point, it will be in its Asymptotic-Giant-Branch (AGB) phase, where it will expand again (much faster this time) and become more luminous.
Over the course of the next 20 million years, the Sun will then become unstable and begin losing mass through a series of thermal pulses. These will occur every 100,000 years or so, becoming larger each time and increasing the Sun’s luminosity to 5,000 times its current brightness and its radius to over 1 AU.
At this point, the Sun’s expansion will either encompass the Earth, or leave it entirely inhospitable to life. Planets in the Outer Solar System are likely to change dramatically, as more energy is absorbed from the Sun, causing their water ices to sublimate – perhaps forming dense atmosphere and surface oceans. After 500,000 years or so, only half of the Sun’s current mass will remain and its outer envelope will begin to form a planetary nebula.
The post-AGB evolution will be even faster, as the ejected mass becomes ionized to form a planetary nebula and the exposed core reaches 30,000 K. The final, naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black.
Ultimate Fate of our Sun:
When people think of stars dying, what typically comes to mind are massive supernovas and the creation of black holes. However, this will not be the case with our Sun, due to the simple fact that it is not nearly massive enough. While it might seem huge to us, but the Sun is a relatively low mass star compared to some of the enormous high mass stars out there in the Universe.
As such, when our Sun runs out of hydrogen fuel, it will expand to become a red giant, puff off its outer layers, and then settle down as a compact white dwarf star, then slowly cooling down for trillions of years. If, however, the Sun had about 10 times its current mass, the final phase of its lifespan would be significantly more (ahem) explosive.
When this super-massive Sun ran out of hydrogen fuel in its core, it would switch over to converting atoms of helium, and then atoms of carbon (just like our own). This process would continue, with the Sun consuming heavier and heavier fuel in concentric layers. Each layer would take less time than the last, all the way up to nickel – which could take just a day to burn through.
Then, iron would starts to build up in the core of the star. Since iron doesn’t give off any energy when it undergoes nuclear fusion, the star would have no more outward pressure in its core to prevent it from collapsing inward. When about 1.38 times the mass of the Sun is iron collected at the core, it would catastrophically implode, releasing an enormous amount of energy.
Within eight minutes, the amount of time it takes for light to travel from the Sun to Earth, an incomprehensible amount of energy would sweep past the Earth and destroy everything in the Solar System. The energy released from this might be enough to briefly outshine the galaxy, and a new nebula (like the Crab Nebula) would be visible from nearby star systems, expanding outward for thousands of years.
All that would remain of the Sun would be a rapidly spinning neutron star, or maybe even a stellar black hole. But of course, this is not to be our Sun’s fate. Given its mass, it will eventually collapse into a white star until it burns itself out. And of course, this won’t be happening for another 6 billion years or so. By that point, humanity will either be long dead or have moved on. In the meantime, we have plenty of days of sunshine to look forward to!
Planets and other objects in our Solar System. Credit: NASA.
First the quick facts: Our Solar System has eight “official” planets which orbit the Sun. Here are the planets listed in order of their distance from the Sun:
If you add in the dwarf planets, Ceres is located in the asteroid belt between Mars and Jupiter, while the remaining dwarf planets are in the outer Solar System and in order from the Sun are Pluto, Haumea, Makemake, and Eris. There is, as yet, a bit of indecision about the Trans-Neptunian Objects known as Orcus, Quaoar, 2007 O10, and Sedna and their inclusion in the dwarf planet category.
A mnemonic for this list would be “My Very Educated Mother Could Just Serve Us Noodles, Pie, Ham, Muffins, and Eggs” (and Steak, if Sedna is included.) You can find more tricks for remembering the order of the planets at our detailed article here.
Now, let’s look at a few details including the definition of a planet and a dwarf planet, as well as details about each of the planets in our Solar System.
Artistic impression of the Solar System, with all known terrestrial planets, as giants, and dwarf planets. Credit: NASA
What is a Planet?
In 2006, the International Astronomical Union (IAU) decided on the definition of a planet. The definition states that in our Solar System, a planet is a celestial body which:
is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape),
has “cleared the neighborhood” around its orbit.
is not a moon.
This means that Pluto, which was considered to be the farthest planet since its discovery in 1930, now is classified as a dwarf planet. The change in the definition came after the discovery three bodies that were all similar to Pluto in terms of size and orbit, (Quaoar in 2002, Sedna in 2003, and Eris in 2005).
With advances in equipment and techniques, astronomers knew that more objects like Pluto would very likely be discovered, and so the number of planets in our Solar System would start growing quickly. It soon became clear that either they all had to be called planets or Pluto and bodies like it would have to be reclassified.
With much controversy then and since, Pluto was reclassified as a dwarf planet in 2006. This also reclassified the asteroid Ceres as a dwarf planet, too, and so the first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens more dwarf planets awaiting discovery.
Later, in 2008, the IAU announced the subcategory of dwarf planets with trans-Neptunian orbits would be known as “plutoids.” Said the IAU, “Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit.”
This subcategory includes Ceres, Pluto, Haumea, Makemake, and Eris.
The Planets in our Solar System:
Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.
Mercury: Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is not the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)
Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:
Diameter: 4,879 km (3,032 miles)
Mass: 3.3011 x 1023 kg (0.055 Earths)
Length of Year (Orbit): 87.97 Earth days
Length of Day: 59 Earth days.
Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
Mercury does not have any moons.
Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
Venus:
Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal (231.9 °C)!
Diameter: 7,521 miles (12,104 km)
Mass: 4.867 x 1024 kg (0.815 Earth mass)
Length of Year (Orbit): 225 days
Length of day: 243 Earth days
Surface temperature: 462 degrees C (864 degrees F)
Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
Venus has no moons.
Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
Earth: Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.
Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA
Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of
Diameter: 12,760 km (7,926 miles)
Mass: 5.97 x 1024 kg
Length of Year (Orbit): 365 days
Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
Earth has one moon.
Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
Mars: Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.
Global image of the planet Mars. Credit: NASA
The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.
Diameter: 6,787 km, (4,217 miles)
Mass: 6.4171 x 1023 kg (0.107 Earths)
Length of Year (Orbit): 687 Earth days.
Length of day: 24 hours 37 minutes.
Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
Mars has two small moons, Phobos and Deimos.
Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
Jupiter: Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.
Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).
Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.
Diameter: 428,400 km (88,730 miles)
Mass: 1.8986 × 1027 kg (317.8 Earths)
Length of Year (Orbit): 11.9 Earth years
Length of day: 9.8 Earth hours
Temperature: -148 C, (-234 F)
Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Saturn: Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.
Diameter: 120,500 km (74,900 miles)
Mass: 5.6836 x 1026 kg (95.159 Earths)
Length of Year (Orbit): 29.5 Earth years
Length of day: 10.7 Earth hours
Temperature: -178 C (-288 F)
Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
Uranus: Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.
Uranus as seen by NASA’s Voyager 2 space probe. Credit: NASA/JPL
Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.
Diameter: 51,120 km (31,763 miles)
Mass:
Length of Year (Orbit): 84 Earth years
Length of day: 18 Earth hours
Temperature: -216 C (-357 F)
Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
Uranus has 27 moons.
Uranus has faint rings; the inner rings are narrow and dark and the outer rings are brightly colored.
Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
Neptune: Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.
Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL
Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.
Diameter: 49,530 km (30,775 miles)
Mass: 1.0243 x 1026 kg (17 Earths)
Length of Year (Orbit): 165 Earth years
Length of day: 16 Earth hours
Temperature: -214 C (-353 F)
Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
Neptune has six rings.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune.
Neptune was the first planet to be predicted to exist by using math.
Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.
Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!
A newly found object named V774104 was found using the Subaru Telescope. Credit: Scott Sheppard, Chad Trujillo, and David Tholen.
It has been estimated that there may be hundreds of dwarf planets in the Kuiper belt and Oort Cloud of the outer Solar System. So far we’ve found – and actually seen – just a few. This past week, one more dwarf planet was added to the list and comes in at the most distant object ever seen in the Solar System.
This newly found world, initially named V774104, is about 15.4 billion kilometers from the Sun. At 103 AU, it is three times further from the Sun than Pluto, and is more distant than the previous record holder, Eris, which lies at 97 AU.
The discovery of V774104 was announced by one of the astronomers who found the object, Scott Sheppard, from the Carnegie Institution for Science, at the American Astronomical Society’s Division for Planetary Sciences fall meeting last week. Sheppard, along with Chad Trujillo and David Tholen used Japan’s 8-meter Subaru Telescope in Hawaii to make the find.
Astronomers say this newly spotted dwarf planet shows the depths of our Solar System.
“The discovery of V774104 is more proof that the Solar System is bigger than we thought,” said astronomer Joseph Burns from Cornell University, who was not associated with the discovery. “We need a little more time to pin down the orbit and determine the object’s exact size, but it must be big to see it at this distance.”
The size of V774104 is currently estimated to be between 500 and 1000 kilometers in diameter, which is less than half Pluto’s size.
While the size of the object is of some interest to astronomers who are searching for KBOs, even more interesting is pinning down its orbit. With its recent discovery, the orbit of V774104 has yet to be tracked for long periods of time.
If the orbit of V774104 comes closer to the Sun, such as between 30 to 50 AU, then it would be considered an icy Kuiper Belt objects which are more common among bodies like this found so far. Their orbits are more elongated because they fall under the gravitational influence of Neptune.
Of even more interest are what Sheppard called “inner Oort Cloud objects,” (also called “sednoids”). Theses bodies exist in a part of the Solar System that astronomers used to think was fairy empty. Of the two previously observed objects in this class — Sedna and 2012 VP113. — their orbits never come closer to the Sun than 50 AU, and they have a semi-major axis greater than 150 AU. The eccentric orbits of these objects have yet to be explained.
This means at their closest to the Sun they are still beyond the Kuiper Belt which lies 30-50 au from the Sun. Only two other objects in this category are known: 90377 Sedna and 2012 VP113.
They intrigue astronomers as they inhabit what was expected to be a largely empty region between the Kuiper Belt and the Oort Cloud, the Solar System’s yet to observed reservoir of comets. As well, the current highly elliptical orbits of Sednoids cannot be their original orbits, the chance of smaller bodies in such eccentric paths accreting into objects hundreds of kilometres across is fantastically low. Sednoids must have originally formed in relatively circular orbits, possibly in the Oort Cloud.
“Non-eccentric orbits seem to be the anomaly here,” Burns told Universe Today.
So, this likely means that something other than the Sun is responsible for influencing the erratic orbits of such small objects like V774104. One theory is that there might be a large planet at the outer reaches of the Solar System influencing the orbits of these distant objects.
Of course, among some crowds that brings up the hypothetical Planet X. But Burns was quick to dismiss that idea.
“While we certainly don’t understand well these objects, we may want to scatter off an object like Planet X,” he said via email.
At the AAS meeting last week, Sheppard said the likely alternative is that the orbits of these objects might reflect the primordial conditions of the Solar System, which formed more than 4.5 billion years ago. This makes them even more enticing for study, and Sheppard and his team will be keeping a close eye on V774104 to try and learn more. Nature News reported that the team plans to look for it again this week using the Magellan Telescopes in Chile, and then again in a year, to calculate its orbit and determine whether if it is an inner Oort cloud resident or an icy Kuiper Belt object.
Europa is probably the best place in the Solar System to go searching for life. But before they’re launched, any spacecraft we send will need to be squeaky clean so don’t contaminate the place with our filthy Earth bacteria. Continue reading “Will We Contaminate Europa?”
Saturn's moon Tethys, imaged by Cassini on April 14, 2012.
Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.
This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.
Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.
Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain
Size, Mass and Orbit: With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.
Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.
Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding
Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.
Composition and Surface Features: Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.
It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).
This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.
The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI
The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.
The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.
Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.
Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.
Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.
Exploration: Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.
Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.
On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.
Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute
The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.
At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.
We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.
Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.
Like me, you’re probably a little ego-geocentric about the importance of Earth. It’s where you were born, it’s where you keep all your stuff. It’s even where you’re going to die – I know, I know, not you Elon Musk, you’re going to “retire” on Mars, right after you nuke the snot out of it.
For the rest of us, Earth is the place. But in reality, when it comes to planets, this is somebody else’s racket. This is Jupiter’s Solar System, and we all sleep on its couch.
Jupiter accounts for 75% of the mass of the planets of the Solar System, nearly 318 times more massive than Earth, and isn’t just the name of everyone’s favorite secret princess. It’s the 1.9 × 10^27 kilogram gorilla in the room. Whatever Jupiter wants, Jupiter gets. Jupiter hungry? JUPITER HUNGRY.
What Jupiter apparently wants is to throw our stuff around the Solar System. Thanks to its immense gravity, Jupiter yanks material around in the asteroid belt, preventing the poor space rocks from ever forming up into anything larger than Ceres.
Jupiter gobbles up asteroids, comets, and spacecraft, and hurtles others on wayward trajectories. Who knows how much mayhem and destruction Jupiter has gotten into over the course of its 4.5 billion years in the Solar System.
Some scientists think we owe our existence to Jupiter’s protective gravity. It greedily vacuums up dangerous asteroids and comets in the Solar System.
Other scientists totally disagree and think that Jupiter is a bully, perturbing perfectly safe comets and asteroids into dangerous trajectories and flushing earth’s head in the toilet during recess.
Which is it? Is Jupiter our friend and protector, or evil enemy. We’ve already figured out how to dismantle you Jupiter, don’t make us put our plans into action.
Some of the most dangerous objects in the Solar System are long-period comets. These balls of rock and ice come from the deepest depths of the Oort cloud. Some event nudges these death missiles into trajectories that bring them into the inner Solar System, to shoot past the Sun and maybe, just maybe, smash into a planet and kill 99.99999% of the life on it.
The Solar System. Credit: NASA
There’s a pretty good chance some of the biggest extinctions in the history of the Earth were caused by impacts by long period comets.
As these comets make their way through the Solar System, they interact with Jupiter’s massive gravity, and get pushed this way and that. As we saw with Comet Shoemaker-Levy, some just get consumed entirely, like a tasty ice-rock sandwich.
The theory goes that Jupiter pushes these dangerous comets out of their murder orbits so they don’t smash into Earth and kill us all.
But a competing theory says that Jupiter actually diverts comets that would have completely missed our planet into deadly, Earth-killing trajectories.
Will the Sailor Scouts provide us any clues? Who can say?
At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)
Here’s friend of the show, Dr. Kevin Grazier, a planetary scientist and scientific advisor for many of your favorite sci-fi TV shows and movies.
… [ see video for Interview with Dr. Grazier about Jupiter]
So which is it? Is Jupiter our friend or enemy? We’ll need to run more simulations and figure this out with more accuracy. And until then, it’s probably best if we just tremble in fear and worship Jupiter as a dark and capricious god until the evidence proves otherwise. It’s what Pascal would wager.
What are some other theories you’ve heard about and you’d like us to dig in further? Make some suggestions in the comments below.
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If you’re into other facts about our Solar system here’s a link to our Solar system playlist. Thanks to Ben Johnson and Tal Ghengis, and the members of the Guide to Space community who keep these shows rolling. Love space science? Want to see episodes before anyone else? Get extras, contests, and shenanigans with Jay, myself and the rest of the team. Get in on the action. Click here.
Is our 13.8 billion year old universe actually in its death throes?
Poor Universe, its demise announced right in it’s prime. At only 13.8 billion years old, when you peer across the multiverse it’s barely middle age. And yet, it sadly dwindles here in hospice.
Is it a Galactus infestation? The Unicronabetes? Time to let go, move on and find a new Universe, because this one is all but dead and gone and but a shell of its former self.
The news of imminent demise was recently broadcast in mid 2015. Based on research looking at the light coming from over 200,000 galaxies, they found that the galaxies are putting out half as much light as they were 2 billion years ago. So if our math is right, less light equals more death.
So tell it to me straight, Doctor Spaceman(SPAH-CHEM-AN), how long have we got? Astronomers have known for a long time that the Universe was much more active in the distant past, when everything was closer and denser, and better. Back then, more of it was the primordial hydrogen left over from the Big Bang, supplying galaxies for star formation. Currently, there are only 1 to 3 new stars formed in the Milky Way every year. Which is pretty slow by Milky Way standards.
Not even at the busiest time of star formation, our Sun formed 5 billion years ago. 5 billion years before that, just a short 4 billion after the Big Bang, star formation peaked out. There were 30 times more stars forming then, than we see today.
When stars were formed actually makes a difference. For example, the fact that it took so long for our Sun to form is a good thing. The heavier elements in the Solar System, really anything higher up the periodic table from hydrogen and helium, had to be formed inside other stars. Main sequence stars like our own Sun spew out heavier elements from their solar winds, while supernovae created the heaviest elements in a moment of catastrophic collapse. Astronomers are pretty sure we needed a few generations of stars to build up enough of the heavier elements that life depends on, and probably wouldn’t be here without it.
Even if life did form here on Earth billions of years ago, when the Universe was really cranking, it would wish it was never born. With 30 times as much star formation going on, there would be intense radiation blasting away from all these newly forming stars and their subsequent supernovae detonations. So be glad life formed when it did. Sometimes a little quiet is better.
So, how long has the Universe got? It appears that it’s not going to crash together in the future, it’s just going to keep on expanding, and expanding, forever and ever.
Our eyes would never see the Crab Nebula as this Hubble image shows it. Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
In a few billion years, star formation will be a fraction of what it is today. In a few trillion, only the longest lived, lowest mass red dwarfs will still be pushing out their feeble light. Then, one by one, galaxies will see their last star flicker and fade away into the darkness. Then there’ll only be dead stars and dead planets, cooling down to the background temperature of the Universe as their galaxies accelerate from one another into the expanding void.
Eventually everything will be black holes, or milling about waiting to be trapped in black holes. And these black holes themselves will take an incomprehensible mighty pile of years to evaporate away to nothing.
So yes, our Universe is dying. Just like in a cheery Sartre play, it started dying the moment it began its existence. According to astronomers, the Universe will never truly die. It’ll just reach a distant future when there’s so little usable energy, it’ll be mostly dead. Dead enough? Dead inside.
As Miracle Max knows, mostly dead is still slightly alive. Who knows what future civilizations will figure out in the googol years between then and now.
Too sad? Let’s wildly speculate on futuristic technologies advanced civilizations will use to outlast the heat death of the Universe or flat out cheat death and re-spark it into a whole new cycle of Universal renewal.
We’ve all seen illustrations of the Solar System. They’re in our school textbooks, on posters, on websites, on t-shirts… in some cases they’re used to represent the word “science” itself (and for good reason.) But, for the most part, they’re all wrong. At least where scale is concerned.
Sure, you can show the Sun and planets in relative size to each other accurately. But then the actual distances between them will probably be way off.* And OK, you can outline the planets’ concentric orbits around the Sun to scale pretty easily. But then there’s no convenient way to make sure that the planets themselves would actually be visible. In order to achieve both, you have to leave the realm of convenience behind entirely and make a physical model that, were you to start with an Earth the size of a marble, would stretch for several miles (and that’s not even taking Pluto into consideration.)
This is exactly what filmmaker Wylie Overstreet and four of his friends did in 2014, spending a day and a half on a dry lake bed in Nevada where they measured out and set up a scale model of the Sun and planets (not including Pluto, don’t tell Alan Stern) including their respective circular orbits. They then shot time-lapse images of their illuminated cars driving around the orbits. The resulting video is educational, mesmerizing, beautiful, and overall a wonderful demonstration of the staggering scale of space in the Solar System.
For some reason whenever I think about the sheer amount of space there actually is in space, it gets me a like choked up. These guys get an “A+” for effort, execution, and entertainment!
In addition to being the birthplace of humanity and the cradle of human civilization, Earth is the only known planet in our Solar System that is capable of sustaining life. As a terrestrial planet, Earth is located within the Inner Solar System between between Venus and Mars (which are also terrestrial planets). This place Earth in a prime location with regards to our Sun’s Habitable Zone.
Earth has a number of nicknames, including the Blue Planet, Gaia, Terra, and “the world” – which reflects its centrality to the creation stories of every single human culture that has ever existed. But the most remarkable thing about our planet is its diversity. Not only are there an endless array of plants, animals, avians, insects and mammals, but they exist in every terrestrial environment. So how exactly did Earth come to be the fertile, life-giving place we all know and love?
