Guide to Space Archives

June 21, 2016July 1, 2016 by Matt Williams

What Are The Diameters of the Planets?

Planets in the Solar System. Image credit: NASA/JPL/IAU

The planets of our Solar System vary considerably in size and shape. Some planets are small enough that they are comparable in diameter to some of our larger moons – i.e. Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan. Meanwhile, others like Jupiter are so big that they are larger in diameter than most of the others combined.

In addition, some planets are wider at the equator than they are at the poles. This is due to a combination of the planets composition and their rotational speed. As a result, some planets are almost perfectly spherical while others are oblate spheroids (i.e. experience some flattening at the poles). Let us examine them one by one, shall we?

Mercury:

With a diameter of 4,879 km (3031.67 mi), Mercury is the smallest planet in our Solar System. In fact, Mercury is not much larger than Earth’s own Moon – which has a diameter of 3,474 km (2158.64 mi). At 5,268 km (3,273 mi) in diameter, Jupiter’s moon of Ganymede is also larger, as is Saturn’s moon Titan – which is 5,152 km (3201.34 mi) in diameter.

As with the other planets in the inner Solar System (Venus, Earth, and Mars), Mercury is a terrestrial planet, which means it is composed primarily of metals and silicate rocks that are differentiated into an iron-rich core and a silicate mantle and crust.

Also, due to the fact that Mercury has a very slow sidereal rotational period, taking 58.646 days to complete a single rotation on its axis, Mercury experiences no flattening at the poles. This means that the planet is almost a perfect sphere and has the same diameter whether it is measured from pole to pole or around its equator.

Venus:

Venus is often referred to as Earth’s “sister planet“, and not without good reason. At 12,104 km (7521 mi) in diameter, it is almost the same size as Earth. But unlike Earth, Venus experiences no flattening at the poles, which means that it almost perfectly circular. As with Mercury, this is due to Venus’ slow sidereal rotation period, taking 243.025 days to rotate once on its axis.

Earth:

With a mean diameter of 12,756 km (7926 mi), Earth is the largest terrestrial planet in the Solar System and the fifth largest planet overall. However, due to flattening at its poles (0.00335), Earth is not a perfect sphere, but an oblate spheroid. As a result, its polar diameter differs from its equatorial diameter, but only by about 41 km (25.5 mi)

In short, Earth measures 12713.6 km (7900 mi) in diameter from pole to pole, and 12756.2 km (7926.3 mi) around its equator. Once again, this is due to Earth’s sidereal rotational period, which takes a relatively short 23 hours, 58 minutes and 4.1 seconds to complete a single rotation on its axis.

Mars:

Mars is often referred to as “Earth’s twin”; and again, for good reason. Like Earth, Mars experiences flattening at its poles (0.00589), which is due to its relatively rapid sidereal rotational period (24 hours, 37 minutes and 22 seconds, or 1.025957 Earth days).

As a result, it experiences a bulge at its equator which leads to a variation of 40 km (25 mi) between its polar radius and equatorial radius. This works out to Mars having a mean diameter of 6779 km (4212.275 mi), varying between 6752.4 km (4195.75 mi) between its poles and 6792.4 km (4220.6 mi) at its equator.

Jupiter:

Jupiter is the largest planet in the Solar System, measuring some 142,984 km (88,846 mi) in diameter. Again, this its mean diameter, since Jupiter experiences some rather significant flattening at the poles (0.06487). This is due to its rapid rotational period, with Jupiter taking just 9 hours 55 minutes and 30 seconds to complete a single rotation on its axis.

Combined with the fact that Jupiter is a gas giant, this means the planet experiences significant bulging at its equator. Basically, it varies in diameter from 133,708 km (83,082.3 mi) when measured from pole to pole, and 142,984 km (88,846 mi) when measured around the equator. This is a difference of 9276 km (5763.8 mi), one of the most pronounced in the Solar System.

Saturn:

With a mean diameter of 120,536 km (74897.6 mi), Saturn is the second largest planet in the Solar System. Like Jupiter, it experiences significant flattening at its poles (0.09796) due to its high rotational velocity (10 hours and 33 minutes) and the fact that it is a gas giant. This means that it varies in diameter from 108,728 km (67560.447 mi) when measured at the poles and 120,536 km (74,897.6 mi) when measured at the equator. This is a difference of almost 12,000 km, the greatest of all planets.

Uranus:

Uranus has a mean diameter of 50,724 km (31,518.43 mi), making it the third largest planet in the Solar System. But due to its rapid rotational velocity – the planet takes 17 hours 14 minutes and 24 seconds to complete a single rotation – and its composition, the planet experiences a significant polar flattening (0.0229). This leads to a variation in diameter of 49,946 km (31,035 mi) at the poles and 51,118 km (31763.25 mi) at the equator – a difference of 1172 km (728.25 mi).

Neptune:

Lastly, there is Neptune, which has a mean diameter of 49,244 km (30598.8 mi). But like all the other gas giants, this varies due to its rapid rotational period (16 hours, 6 minutes and 36 seconds) and composition, and subsequent flattening at the poles (0.0171). As a result, the planet experiences a variation of 846 km (525.68 mi), measuring 48,682 km (30249.59 mi) at the poles and 49,528 km (30775.27 mi) at the equator.

In summary, the planets of our Solar System vary in diameter due to differences in their composition and the speed of their rotation. In short, terrestrial planets tend to be smaller than gas giants, and gas giants tend to spin faster than terrestrial worlds. Between these two factors, the worlds we know range between near-perfect spheres and flattened spheres.

We have written many articles about the Solar System here at Universe Today. Here’s Interesting Facts about the Solar System, How Long Is A Day On The Other Planets Of The Solar System?, What Are the Colors of the Planets?, How Long Is A Year On The Other Planets?, What Is The Atmosphere Like On Other Planets?, and How Strong is Gravity on Other Planets?

For more information of the planets, here is a look at the eight planets and some fact sheets about the planets from NASA.

Astronomy Cast has episodes on all the planets. Here is Mercury to start out with.

June 20, 2016April 17, 2019 by Tammy Plotner

Messier 15 (M15) – The Great Pegasus Cluster

The M15 Globular Cluster (aka. Great Hercules Cluster). Astronomers suspect the existence of one or more intermediate-mass black holes at its heart. Credit: NASA/ESA/HST

Welcome back to Messier Monday! Today, in our ongoing tribute to Tammy Plotner, we take a look at the M15 globular cluster, one of the oldest and best known star clusters in the night sky. Enjoy!

In the 18th century, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky while looking for comets. Not wanting other astronomers to make the same mistake, he began compiling a list of these objects into a catalog. In time, this list would include 100 objects, and came to be known by future astronomers as the Messier Catalog.

One of these objects is the globular cluster known as M15. Located in the northern constellation Pegasus, it is one of the brightest clusters in the night sky (with a visual brightness that is roughly 360,000 times that of our Sun). It is also one of the finest globular clusters in the northern section of the sky, the best deep-sky object in the constellation of Pegasus, and one of the oldest and best known globular clusters.

Description:

Messier 15 is probably the most dense globular cluster in our entire Milky Way galaxy – having already undergone a process of contraction. What does that mean to what you’re seeing? This ball of stars measures about 210 light years across, yet more than half of the stars you see are packed into the central area in a space just slightly more than ten light years in size.

By looking for single stars within globular clusters, the Hubble Space Telescope was either looking for a massive black hole or evidence of a “core collapse” – the intense gravity of so many stars so close together. Although it was peeking nearly 37,000 light-years away, the Hubble was able to resolve hundreds of stars converging on M15’s core. Like magnetism, their gravity would either cause them to attract or repel one another – and a black hole may have formed at some point in the cluster’s 12-billion-year life.

*The globular cluster known as Messier 15, located some 35 000 light-years away in the Pegasus constellation. Credit: Mount Lemmon SkyCenter/University of Arizona*

The study which addressed this data – which appeared in the January 1996 issue of the Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick Observatory, UC Santa Cruz – asked the question of whether or not the speed of the cluster’s stars could tell us if M15’s dense core was caused by a single huge object, or just mutual attraction. As Guhathakurta stated in the study:

“It is very likely that M15’s stars have concentrated because of their mutual gravity. The stars could be under the influence of one giant central object, although a black hole is not necessarily the best explanation for what we see. But if any globular cluster has a black hole at its center, M15 is the most likely candidate.”

John Bahcall and astrophysicist Jeremiah Ostriker of Princeton University were the first to forward the idea that Messier 15 might be hiding a black hole. While it is distinct from many other globular clusters by having such a dense core, it really isn’t that much different than all the rest of the globular clusters we see. Yet, no where else in our galaxy, except at its core, are the stars that dense!

It is estimated that 30,000 distinct stars exist in the inner 22 light-years of the cluster alone. The closer the Hubble telescope looked, the more stars it found. This increase in stellar density continued all the way to within 0.06 light-years of the center – about 100 times the distance between our Sun and Pluto. “Detecting separate stars that close to the core was at the limit of Hubble’s powers,” says Brian Yanny of the Fermi National Accelerator Laboratory.

*The location of M15, within the Pegasus Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)*

At this point, even the great Hubble could not distinguish individual stars, or locate the exact position of the core. Guhathakurta and is colleagues theorized that the stars crowd even closer inside the radius, so they plotted the distribution of the stars as a function of distance from the core. When the results came back, they had two answers – either a black hole was responsible, or a gravothermal catastrophe called core collapse was the culprit.

“It’s a catastrophe in the sense that once it starts, this process can run away very quickly,” said Guhathakurta. “But other processes could cause the core to bounce back before it collapses all the way.”

At an estimated 13.2 billion years old, it is one of the oldest known globular clusters, but it isn’t done throwing some surprises at us. M15 was the first globular cluster in which a planetary nebula, Pease 1 or K 648 (“K” for “Kuster”), could be identified – and can be seen with larger aperture amateur telescopes. Even stranger is the fact that Messier 15 contains 112 variable stars, and 9 known pulsars – neutron stars which are the leftovers of ancient supernovae. And one of these is a double neutron star system – M15 C.

History of Observation:

M15 was discovered by Jean-Dominique Maraldi on September 7, 1746 while he was looking for a comet. Says he:

“On September 7 I noticed between the stars Epsilon Pegasi and Beta Equulei, a fairly bright nebulous star, which is composed of many stars, of which I have determined the right ascension of 319d 27′ 6″, and its northern declination of 11d 2′ 22”. About 25 years later, Charles Messier would independently rediscover it to add to his own catalog, describing it as: “In the night of June 3 to 4, 1764, I have discovered a nebula between the head of Pegasus and that of Equuleus it is round, its diameter is about 3 minutes of arc, the center is brilliant, I have not distinguished any star; having examined it with a Gregorian telescope which magnifies 104 times, it had little elevated over the horizon, and maybe that observed at a greater elevation one can perceive stars.”

Camera SBIG STX16803 CCD Camera Filters Astrodon Gen II Dates December 2015 Location Mount Lemmon SkyCenter Exposure RGB = 2 : 2 : 2 Hours Acquisition Astronomer Control Panel (ACP), Maxim DL/CCD (Cyanogen), FlatMan XL (Alnitak) Processing CCDStack, Photoshop, PixInsight Credit Line & Copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona — *Deep Broadband (RGB) image of M15, taken from the Mount Lemmon SkyCenter. Credit and Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona*

Sir William Herschel would be the first to resolve some of its stars, but not the core. It would be his son John who would later pick up structure. However, like the dutiful and colorful observer that he was, Admiral Smyth will leave us with this lasting impression:

“Although this noble cluster is rated as globular, it is not exactly round, and under the best circumstances is seen as in the diagram, with stragglers branching from a central blaze. Under a moderate magnifying power, there are many telescopic and several brightish stars in the field; but the accumulated mass is completely insulated, and forcibly strikes the senses as being almost infinitely beyond those apparent comets. Indeed, it may be said to appear evidently aggregated by mutual laws, and part of some stupendous and inscrutable scheme of involution; for there is nothing quiescent throughout the immensity of the vast creation.”

Considering Smyth’s observations were made nearly two centuries before we really began to understand what was going on inside Messier 15, you’ll have to admit he was a very good observer!

Locating Messier 15:

Surprisingly enough, globular cluster M15 is easy to find. Once you’ve located the “Great Square” of Pegasus, simply choose its brightest and southwesternmost star – Alpha. Now identify the small, kite shape of the constellation of Delphinus. Roughly halfway between these two (and slightly south), you’ll spy a slightly reddish star – Epsilon Peg (Enif).

By placing Enif in your binoculars or image correct finderscope at the 7:00 position, you can’t miss this bright, compact beauty. Even the smallest of optics will reveal the round glow and telescopes starting at 4″ will begin resolution – while large telescopes will simply amaze you. However, don’t expect to open this globular up to the core region. As already noted, its pretty dense in there!

And here are the quick facts for Messier 15, for your convenience:

Object Name: Messier 15
Alternative Designations: M15, NGC 7078
Object Type: Class IV Globular Cluster
Constellation: Pegasus
Right Ascension: 21 : 30.0 (h:m)
Declination: +12 : 10 (deg:m)
Distance: 33.6 (kly)
Visual Brightness: 6.2 (mag)
Apparent Dimension: 18.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the

June 17, 2016February 22, 2017 by Fraser Cain

What Are Virtual Particles?

Sometimes I figure out the weak spot in my articles based on the emails and comments they receive.

One popular article we did was all about Stephen Hawking’s realization that black holes must evaporate over vast periods of time. We talked about the mechanism, and mentioned how there are these virtual particles that pop in and out of existence.

Normally these particles self annihilate, but at the edge of a black hole’s event horizon, one particle falls in, while another is free to wander the cosmos. Since you can’t create particles from nothing, the black hole needs to sacrifice a little bit of itself to buy this newly formed particle’s freedom.

But my short article wasn’t enough to clarify exactly what virtual particles are. Clearly, you all wanted more information. What are they? How are they detected? What does this mean for black holes?

In situations like this, when I know the actual Physics Police are watching, I like to call in a ringer. Once again, I’m going to go back and talk to my good friend, and actual working astrophysicist, Dr. Paul Matt Sutter. He has written papers on subjects like the Bayesian Analysis of Cosmic Dawn and MHD Simulations of Magnetic Outflows. He really knows his stuff.

Fraser Cain:
Hey Paul, first question: What are virtual particles?

Paul Matt Sutter:
Alright. No pressure, Fraser. Okay, okay.

To get the concept of virtual particles you actually have to take a step back and think about the field, especially the electromagnetic field. In our current view of how the universe works all of space and time is filled up with this kind of background field. And this field can wibble and wabble around, and sometimes these wibbles and wabbles are like waves that propagate forward, and we call these waves photons or electromagnetic radiation, but sometimes it can just sit there and you know bloop bloop bloop, just you know pop fizzle in and out, or up and down, and kind of boil a little all on its own.

In fact all the time space is kind of wibbling/wabbling around this field even in a vacuum. A vacuum isn’t the absence of everything. The vacuum is just where this field is in its lowest energy state. But even though it’s in that lowest energy state, even though maybe on average there is nothing there. There’s nothing stopping it from just bloop bloop bloop you know bubbling around.

Credit: NASA, ESA, Q.D. Wang (University of Massachusetts, Amherst), and S. Stolovy (Caltech)

So actually the vacuum is kind of boiling with these fields. In particular the electromagnetic field which is what we are talking about right now.

And we know that photons, that light, can turn into particle, anti-particle pairs. It can turn into say an electron and a positron. It can just do this. It can happen to normal photons, and it can happen to these kind of temporary wibbly wobbly photons.

So sometimes a photon or sometimes the electromagnetic field can propagate from one place to another, and we call it a photon. And that photon can split off into a positron and an electron, and other times it can just wibble wobble kind of in place and then wibble wobble POP POP. It pops into a positron and an electron and then they crash into each other or whatever, and they just simmer back down. So, wibble wobble, pop pop, fizz fizz is kind of what’s going on in the vacuum all they time, and that’s the name we give these virtual particles are just the normal kind of background fuzz or background static to the vacuum.

Fraser:
Okay. So how do we see evidence for virtual particles?

Paul:
Yeah, great question. We know that the vacuum has an energy associated with it. We know that these virtual particles are always fizzing in and out of existence for a few reasons.

One is the transition of the electron in different states of the atom. If you excite the atom the electron pops up to a higher energy state. There is kind of no reason for that electron to pop back down to a lower energy state. It’s already there. It’s actually a stable state. There is no reason for it to leave unless there is little wibble wobbles in the electromagnetic field and it can giggle around that electron and knock it out of that higher energy state and send it crashing down into a lower state

Another thing is called the Lamb Shift, and this is when the wibbly wobbly electromagnetic field or the virtual particles interact again with electrons in say a hydrogen atom. It can gently nudge them around, and this shift effects some states of the electron and not other states. And there are actually states that you would say have the exact same say energy properties, they are just kind of identical, but because the Lamb Shift, because of this wibbly wobbly electromagnetic field interacts with one of those states and not the other, it actually subtly changes the energy levels of those states even though you’d expect them to be completely the same.

And another piece of evidence is in photon photon scattering usually two photons just, phweeet, fly by each other. They are electrically neutral, so they have no reason to interact, but sometimes the photons can wibble wobble into say electron/positron pairs, and that electron/positron pair can interact with the other photons. So sometimes they bounce off each other. It’s super rare because you have to wait for the wibble wobble to happen at just the right time, but it can happen.

Credit: NASA/Dana Berry/SkyWorks Digital

Fraser:
So how do they interact with black holes?

Paul:
Alright, this is the heart of the matter. What do all of these virtual particles or wibbly wobbly electromagnetic fields have to do with black holes, and specifically Hawking radiation? But check this out. Hawkings original formulation of this idea that black holes can radiate and lose mass actually has nothing to do with virtual particles. Or it doesn’t speak directly about virtual particle pairs, and in fact no other formulations or more modern conceptions of this process talk about virtual particle pairs.

Instead, they talk more about the field itself and specifically what’s happening to the field before the black hole is there, what’s happening to it as the black hole forms, and then what happens to the field after it’s formed. And it kind of asks a question: What happens to these wibbly wobbly bits of the field, these like transient kind of boiling nature of the vacuum of the electromagnetic field? What happens to it as that black hole is forming?

Well what happens is that some of the wibbly wobbly bits just get caught near the black hole, near the event horizon as it is forming, and they spend a long time there, and eventually they do escape. So it takes awhile, but when they escape because of the intense curvature there, the intense curvature of space-time, they can get boosted or promoted. So instead of being temporarily wibbly wobbly’s, in the field they get boosted to become “real” particles or “real” photons. So it’s really like an interaction of the formation of the black hole itself with the wibbly wobbly background field, that eventually escapes because it’s not quite trapped by the black hole.

Eventually it escapes and gets turned into real particles, and you can calculate like what happens with say the expected number of particles near the event horizon of the black hole. The answer is the negative number, which means the black hole is losing mass and spitting out particles.

Now this popular conception of virtual particle pairs popping into existence and one getting caught inside the event horizon. That’s is not exactly tied to the mathematics of Hawking radiation but it’s not exactly wrong either. Remember the wibbly wobbly’s in the electromagnetic field are related to these pairs of particles and anti-particles that are constantly popping in and out of existence. They kind of go hand in hand. So by talking about wibbly wobbly’s in the field you’re also kind of talking about the production of virtual particles. And it’s not exactly the math, but you know close enough.

An artist's conception of a supermassive black hole's jets. Image Credit: NASA / Dana Berry / SkyWorks Digital — An artist’s conception of a supermassive black hole’s jets. Image Credit: NASA / Dana Berry / SkyWorks Digital

Fraser:
Okay, and finally, Paul. I need you to just randomly blow the minds of the viewers. Something about virtual particles that is just amazing!

Paul:
Alright. So you want to bend people’s minds? All right. I was saving this for the last. Something juicy, just for you, Fraser.

Check this out, it’s one other big piece of evidence we have for the existence of these background fluctuations and the existence of virtual particles, and that’s something we call the Casimir Effect, or Casimir Force.

You take two neutral metal plates, and what happens is this field that permeates all of space-time is inside the plates and it’s outside the plates. Inside the plates, you can only have certain wavelengths of modes. Almost like the inside of a trumpet can only have certain modes that make sound. The ends of the wavelengths must connect to the plates, because that’s what metal plates do to electromagnetic fields.

Outside the plates you can have any wavelength you want. It doesn’t matter.

So it means outside the plates you have an infinite number of possible wavelengths of modes. Every kind of possible kind of fluctuation, wibble wabble in the electromagnetic field is there, but inside the plates it’s only certain wavelengths that can fit inside the plates.

Now, outside there’s an infinite number of modes. Inside, there is still an infinite number of modes, just slightly fewer infinite number of modes. And you can take the infinity on the outside, and subtract the infinite infinity on the inside, and actually get a finite number, and what you end up with is a pressure or a force that brings the plates together. And we have actually measured this. This is a real thing, and yes, I am not kidding around, you can take infinity minus a different infinity, and get a finite number. It’s possible. One example is the Euler Mascheroni Constant. I dare you to look it up!

So there you go, now I hope you understand what these virtual particles are, how they’re detected, and how they contribute to the evaporation of a black hole.

And if you haven’t already, make sure you click here and go to his channel. You’ll find dozens of videos answering equally mind-bending questions. In fact, send your questions and he might just make a video and answer them.

June 9, 2016February 22, 2017 by Fraser Cain

Are There Antimatter Galaxies?

One of the biggest mysteries in astronomy is the question, where did all the antimatter go? Shortly after the Big Bang, there were almost equal amounts of matter and antimatter. I say almost, because there was a tiny bit more matter, really. And after the matter and antimatter crashed into each other and annihilated, we were left with all the matter we see in the Universe.

You, and everything you know is just a mathematical remainder, left over from the great division of the Universe’s first day.

We did a whole article on this mystery, so I won’t get into it too deeply.

But is it possible that the antimatter didn’t actually go anywhere? That it’s all still there in the Universe, floating in galaxies of antimatter, made up of antimatter stars, surrounded by antimatter planets, filled with antimatter aliens?

Aliens who are friendly and wonderful in every way, except if we hugged, we’d annihilate and detonate with the energy of gigatons of TNT. It’s sort of tragic, really.

If those antimatter galaxies are out there, could we detect them and communicate with those aliens?

First, a quick recap on antimatter.

Antimatter is just like matter in almost every way. Atoms have same atomic mass and the exact same properties, it’s just that all the charges are reversed. Antielectrons have a positive charge, antihydrogen is made up of an antiproton and a positron (instead of a proton and an electron).

It turns out this reversal of charge causes regular matter and antimatter to annihilate when they make contact, converting all their mass into pure energy when they come together.

We can make antimatter in the laboratory with particle accelerators, and there are natural sources of the stuff. For example, when a neutron star or black hole consumes a star, it can spew out particles of antimatter.

In fact, astronomers have detected vast clouds of antimatter in our own Milky Way, generated largely by black holes and neutron stars grinding up their binary companions.

Wyoming Milky Way set. Credit and copyright: Randy Halverson.

But our galaxy is mostly made up of regular matter. This antimatter is detectable because it’s constantly crashing into the gas, dust, planets and stars that make up the Milky Way. This stuff can’t get very far without hitting anything and detonating.

Now, back to the original question, could you have an entire galaxy made up of antimatter? In theory, yes, it would behave just like a regular galaxy. As long as there wasn’t any matter to interact with.

And that’s the problem. If these galaxies were out there, we’d see them interacting with the regular matter surrounding them. They would be blasting out radiation from all the annihilations from all the regular matter gas, dust, stars and planets wandering into an antimatter minefield.

Astronomers don’t see this as far as they look, just the regular, quiet and calm matter out to the edge of the observable Universe.

That doesn’t make it completely impossible, though, there could be galaxies of antimatter as long as they’re completely cut off from regular matter.

But even those would be detectable by the supernova explosions within them. A normally matter supernova generates fast moving neutrinos, while an antimatter supernova would generate a different collection of particles. This would be a dead giveaway.

There’s one open question about antimatter that might make this a deeper mystery. Scientists think that antimatter, like regular matter, has regular gravity. Matter and antimatter galaxies would be attracted to each other, encouraging annihilation.

But scientists don’t actually know this definitively yet. It’s possible that antimatter has antigravity. An atom of antihydrogen might actually fall upwards, accelerating away from the center of the Earth.

alpha_image_resized_for_web — The ALPHA experiment, one of five experiments that are studying antimatter at CERN Credit: Maximilien Brice/CERN

Physicists at CERN have been generating antimatter particles, and trying to detect if they’re falling downward or up.

If that was the case, then antimatter galaxies might be able to repel particles of regular matter, preventing the annihilation, and the detection.

If you were hoping there are antimatter lurking out there, hoarding all that precious future energy, I’m sorry to say, but astronomers have looked and they haven’t found it. Just like the socks in your dryer, we may never discover where it all went.

June 2, 2016February 22, 2017 by Fraser Cain

What Are Planetary Transits?

Thanks to Ptolemy and his cronies, everyone used to think that the Earth was the center of the Solar System, with the Sun, planets and even the stars orbiting around it on a series of concentric crystal spheres. It was a clever idea, and explained the motions of the planets… sort of.

Then Copernicus figured out in 1543, that the Earth isn’t the centre of the Solar System. In fact, it’s just one planet in a vast Solar System, with objects whirling and whirling around the Sun.

With the structure of the Solar System figured out, and the crystal sphere idea in the garbage, astronomers still had a big unknown: how big is the Solar System?

Was it a few million kilometers across, or hundreds of millions. How big is the Sun? How far away is Venus?

Astronomers needed some kind of cosmic yardstick to measure everything against. Figure out one piece of the puzzle, and then you could measure everything else in relation.

In 1627, Johannes Kepler figured out that the motion of Venus was predictable, and that Venus would pass in front of the Sun in 1631, probably in the afternoon.

A timelapse of Mercury transiting across the face of the Sun. Credit: NASA

This is known as a “transit” of Venus.

The first crude measurements of Venus’ motion across the Sun were made in 1639 by Jeremiah Horrocks and William Crabtree from two different spots in England. And with these two observations, they were able to calculate the geometry between the Earth, Venus and the Sun.

If you recall all those memories you’re repressing from your high school geometry, once you’ve got an angle and a side of a triangle, you can work out all the other parts of the triangle. Horrocks and Crabtree worked out the distance from the Earth to the Sun within about 2/3rd accuracy. Not bad, considering the fact that astronomers literally had no idea before this point.

Following on from this observation, astronomers returned to their telescopes with each transit of Venus, better refining their calculations, and eventually settling on the current distance of about 150 million kilometers.

From here on Earth, we can see a few objects pass in front of the Sun: Venus, Mercury and the Moon.

Venus transits are the most rare, happening two times every 108 years or so. Mercury transits happen more often, about a dozen times a century. And a transit of the Moon, also known as a solar eclipse, happens a few times a year, on average.

It’s all a matter of perspective. If you’re standing on the Moon, you might see the Earth pass in front of the Sun. We’d call that a lunar eclipse, while the lunatics would call it an Earth transit.

We can also see transits in other parts of the Solar System, like when moons pass in front of planets. For example, if you have a small telescope, you can see when Jupiter’s larger moons pass in front of the planet from our perspective.

One of the questions you might have, though, is why don’t these transits happen more often. Why don’t we see a Mercury or Venus transit every time they line up with us and the Sun.

This is because the planets aren’t exactly lined up at the same angle towards the Sun. All of the planets are inclined at an angle that takes them above or below the Sun at various points of their orbit.

For example, Venus’ orbit is inclined 3 degrees off the Sun’s equator, while the Earth is inclined 7 degrees. This means that most of the time that Venus and Earth are lined up, Venus is either above or below the Sun.

Are you an ageless vampire, or planning to live a long time in multiple robot bodies, then you’re in luck. In the year 69,163, there’ll be a double transit on the surface of Sun with both Mercury and Venus at the same time. Enjoy that while you contemplate the horror of your existence.

Once we become a true Solar System civilization, there will be even more opportunities for transits. People living on Mars will be able to see Mercury, Venus and even transits of Earth passing in front of the Sun. Neptunians will be bored they can see them so often.

The transit method is one of the ways that astronomers discover planets orbiting other stars. Using a space telescope like Kepler, they survey a portion of the night sky, watching the brightness of thousands of stars. When a planet perfectly passes directly in between us and a star, Kepler detects a drop in brightness.

Since its deployment in 2007, Kepler has confirmed the existence of over 2000 extra-solar planets. Credit: NASA

When you think of the geometries involved, it’s amazing this even happens at all. But the Universe is a vast place. Even if only a tiny percentage of star systems are perfectly lined up with us, there are enough to help us discover thousands and thousands of planets.

Kepler has turned up Earth-sized worlds orbiting other stars, some of which are even orbiting in their planet’s habitable zone.

Watching planetary transits is more than just a fun astronomy event, they’re how astronomers figured out the size of the Solar System itself. And now they help us find other planets orbiting other stars.

So, let’s agree to meet up in 2117 to catch the next transit of Venus, and celebrate this amazing event.

June 1, 2016July 19, 2016 by Matt Williams

How Was the Solar System Formed? – The Nebular Hypothesis

Solar System Themed Products — Solar System Montage. Credit: science.nationalgeographic.com

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis. In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this 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.

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. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc.

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt, Kuiper Belt, and Oort Cloud.

Artist's impression of the early Solar System, where collision between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech — *Artist’s impression of the early Solar System, where collision between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech*

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

This image from the NASA/ESA Hubble Space Telescope shows Sh 2-106, or S106 for short. This is a compact star forming region in the constellation Cygnus (The Swan). A newly-formed star called S106 IR is shrouded in dust at the centre of the image, and is responsible for the surrounding gas cloud’s hourglass-like shape and the turbulence visible within. Light from glowing hydrogen is coloured blue in this image. Credit: NASA/ESA — *The Sh 2-106 Nebula (or S106 for short), a compact star forming region in the constellation Cygnus (The Swan). Credit: NASA/ESA*

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972). In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Problems:

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

The latest list of potentially habitable exoplanets, courtesy of The Planetary Habitability Laboratory. Credit: phl.upr.edu — *A list of potentially habitable exoplanets, courtesy of The Planetary Habitability Laboratory. Credit: phl.upr.edu*

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System, Did our Solar System Start with a Little Bang?, and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed.

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

May 28, 2016April 16, 2017 by Matt Williams

The Orbit of Mars. How Long is a Year on Mars?

Mars and Earth have quite a few things in common. Both are terrestrial planets, both are located within the Sun’s habitable zone, both have polar ice caps, similarly tilted axes, and similar variations in temperature. And according to some of the latest scientific data obtained by rovers and atmospheric probes, it is now known that Mars once had a dense atmosphere and was covered with warm, flowing water.

But when it comes to things like the length of a year, and the length of seasons, Mars and Earth are quite different. Compared to Earth, a year on Mars lasts almost twice as long – 686.98 Earth days. This is due to the fact that Mars is significantly farther from the Sun and its orbital period (the time it takes to orbit the Sun) is significantly greater than that of Earth’s.

Orbital Period:

Mars average distance (semi-major axis) from the Sun is 227,939,200 km (141,634,852.46 mi) which is roughly one and half times the distance between the Earth and the Sun (1.52 AU). Compared to Earth, its orbit is also rather eccentric (0.0934 vs. 0.0167), ranging from 206.7 million km (128,437,425.435 mi; 1.3814 AU) at perihelion to 249.2 million km (154,845,701 mi; 1.666 AU) at aphelion. At this distance, and with an orbital speed of 24.077 km/s, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a orbit around the Sun.

The eccentricity in Mars' orbit means that it is . Credit: NASA — *The eccentricity in Mars’ orbit means that it is . Credit: NASA*

This eccentricity is one of the most pronounced in the Solar System, with only Mercury having a greater one (0.205). However, this wasn’t always the case. Roughly 1.35 million years ago, Mars had an eccentricity of just 0.002, making its orbit nearly circular. It reached a minimum eccentricity of 0.079 some 19,000 years ago, and will peak at about 0.105 in about 24,000 years from now.

But for the last 35,000 years, the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other planets. The closest distance between Earth and Mars will continue to mildly decrease for the next 25,000 years. And in about 1,000,000 years from now, its eccentricity will once again be close to what it is now – with an estimated eccentricity of 0.01.

Earth Days vs. Martian “Sols”:

Whereas a year on Mars is significantly longer than a year on Earth, the difference between an day on Earth and a Martian day (aka. “Sol”) is not significant. For starters, Mars takes 24 hours 37 minutes and 22 seconds to complete a single rotation on its axis (aka. a sidereal day), where Earth takes just slightly less (23 hours, 56 minutes and 4.1 seconds).

On the other hand, it takes 24 hours, 39 minutes, and 35 seconds for the Sun to appear in the same spot in the sky above Mars (aka. a solar day), compared to the 24 hour solar day we experience here on Earth. This means that, based on the length of a Martian day, a Martian year works out to 668.5991 Sols.

The Opportunity rover captured this analemma showing the Sun's movements over one Martian year. Images taken every third sol (Martian day) between July, 16, 2006 and June 2, 2008. Credit: NASA/JPL/Cornell/ASU/TAMU — The Opportunity rover captured this analemma showing the Sun’s movements over one Martian year. Images taken every third sol (Martian day) between July, 16, 2006 and June 2, 2008. Credit: NASA/JPL/Cornell/ASU/TAMU

Seasonal Variations:

Mars also has a seasonal cycle that is similar to that of Earth’s. This is due in part to the fact that Mars also has a tilted axis, which is inclined 25.19° to its orbital plane (compared to Earth’s axial tilt of approx. 23.44°). It’s also due to Mars orbital eccentricity, which means it will periodically receive less in the way of the Sun’s radiance during at one time of the year than another. This change in distance causes significant variations in temperature.

While the planet’s average temperature is -46 °C (51 °F), this ranges from a low of -143 °C (-225.4 °F) during the winter at the poles to a high of 35 °C (95 °F) during summer and midday at the equator. This works out to a variation in average surface temperature that is quite similar to Earth’s – a difference of 178 °C (320.4 °F) versus 145.9 °C (262.5 °F). This high in temperatures is also what allows for liquid water to still flow (albeit intermittently) on the surface of Mars.

In addition, Mars’ eccentricity means that it travels more slowly in its orbit when it is further from the Sun, and more quickly when it is closer (as stated in Kepler’s Three Laws of Planetary Motion). Mars’ aphelion coincides with Spring in its northern hemisphere, which makes it the longest season on the planet – lasting roughly 7 Earth months. Summer is second longest, lasting six months, while Fall and Winter last 5.3 and just over 4 months, respectively.

Artist's impression of the seasons on Mars. Credit: britannica.com — *Artist’s impression of the seasons on Mars. Credit: britannica.com*

In the south, the length of the seasons is only slightly different. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder. The summer temperatures in the south can be up to 30 K (30 °C; 54 °F) warmer than the equivalent summer temperatures in the north.

Weather Patterns:

These seasonal variations allow Mars to experience some extremes in weather. Most notably, Mars has the largest dust storms in the Solar System. These can vary from a storm over a small area to gigantic storms (thousands of km in diameter) that cover the entire planet and obscure the surface from view. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.

The first mission to notice this was the Mariner 9 orbiter, which was the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.

And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.

These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, leading to Mars’ experiencing its own greenhouse effect.

Given the differences in seasons and day length, one is left to wonder if a standard Martian calendar could ever be developed. In truth, it could, but it would be a bit of a challenge. For one, a Martian calendar would have to account for Mars’ peculiar astronomical cycles, and our own non-astronomical cycles like the 7-day week work with them.

Another consideration in designing a calendar is accounting for the fractional number of days in a year. Earth’s year is 365.24219 days long, and so calendar years contain either 365 or 366 days accordingly. Such a formula would need to be developed to account for the 668.5921-sol Martian year. All of this will certainly become an issue as human beings become more and more committed to exploring (and perhaps colonizing) the Red Planet.

We have written many interesting articles about Mars here at Universe Today. Here’s How Long is a Year on the Other Planets?, Which Planet has the Longest Day?, How Long is a Year on Mercury, How Long is a Year on Earth?, How Long is a Year on Venus?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Uranus?, How Long is a Year on Neptune?, How Long is a Year on Pluto?

For more information, check out NASA’s Solar System Exploration page on Mars.

Astronomy Cast also has several interesting episodes on the subject. Like Episode 52: Mars, and Episode 91: The Search for Water on Mars.

May 27, 2016May 27, 2016 by Matt Williams

What is the Coldest Planet of Our Solar System?

Neptune photographed by Voyage. Image credit: NASA/JPL — Neptune photographed by Voyager 2. Image credit: NASA/JPL

The Solar System is pretty huge place, extending from our Sun at the center all the way out to the Kuiper Cliff – a boundary within the Kuiper Belt that is located 50 AU from the Sun. As a rule, the farther one ventures from the Sun, the colder and more mysterious things get. Whereas temperatures in the inner Solar System are enough to burn you alive or melt lead, beyond the “Frost Line“, they get cold enough to freeze volatiles like ammonia and methane.

So what is the coldest planet of our Solar System? In the past, the title for “most frigid body” went to Pluto, as it was the farthest then-designated planet from the Sun. However, due to the IAU’s decision in 2006 to reclassify Pluto as a “dwarf planet”, the title has since passed to Neptune. As the eight planet from our Sun, it is now the outermost planet in the Solar System, and hence the coldest.

Orbit and Distance:

With an average distance (semi-major axis) of 4,504,450,000 km (2,798,935,466.87 mi or 30.11 AU), Neptune is the farthest planet from the Sun. The planet has a very minor eccentricity of 0.0086, which means that its orbit around the Sun varies from a distance of 29.81 AU (4.459 x 10⁹ km) at perihelion to 30.33 AU (4.537 x 10⁹ km) at aphelion.

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 is on Earth.

Average Temperature:

When it comes to ascertaining the average temperature of a planet, scientists rely on temperature variations measured from the surface. As a gas/ice giant, Neptune has no surface, per se. As a result, scientists rely on temperature readings from where the atmospheric pressure is equal to 1 bar (100 kPa), the equivalent to atmospheric pressure at sea level here on Earth.

On Neptune, this area of the atmosphere is just below the upper level clouds. Pressures in this region range between 1 and 5 bars (100 – 500 kPa), and temperature reach a high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).

Farther into space, where pressures drop to about 0.1 bars (10 kPa), temperatures decrease to their low of around 55 K (-218 °C; -360 °F). Further into the planet, pressures increase dramatically, which also leads to a dramatic increase in temperature. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun.

*Neptune Great Dark Spot in High Resolution. Credit: NASA/JPL*

The huge temperature differences between Neptune’s center and its surface (along with its differential rotation) create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System. The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter.

Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter lifespan than Jupiter’s. The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot.

This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot. The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.

Temperature Anomalies:

Despite being 50% further from the Sun than Uranus – which orbits the Sun at an average distance of 2,875,040,000 km (1,786,467,032.5 mi or 19.2184 AU) – Neptune receives only 40% of the solar radiation that Uranus does. In spite of that, the two planets’ surface temperatures are surprisingly close, with Uranus experiencing an average “surface” temperature of 76 K (-197.2 °C)

*Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)*

And while temperatures similarly increase the further one ventures into the core, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System.

One would expect Neptune to be much colder than Uranus, and the mechanism for this remains unknown. However, astronomers have theorized that Neptune’s higher internal temperature (and the exchange of heat between the core and outer layers) might be the reason for why Neptune isn’t significantly colder than Uranus.

As already noted, Pluto’s surface temperatures do get to being lower than Neptune’s. Between its greater distance from the Sun, and the fact that it is not a gas/ice giant (so therefore doesn’t have extreme temperatures at its core) means that it experiences temperatures between a high of 55 K (-218 °C; -360 °F)and a low of 33 K (-240 °C; -400 °F). However, since it is no longer classified as a planet (but a dwarf planet, TNO, KBO, plutoid, etc.) it is no longer in the running. Sorry, Pluto!

We’ve written many articles about Neptune here at Universe Today. Here’s Who Discovered Neptune?, What is the Surface Temperature of Neptune?, What is the Surface of Neptune Like?, 10 Interesting Facts about Neptune, The Rings of Neptune, How Many Moons Does Neptune Have?

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We’ve also recorded an entire episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.

May 27, 2016December 12, 2019 by Matt Williams

10 Interesting Facts About Earth

This view of Earth comes from NASA's Moderate Resolution Imaging Spectroradiometer aboard the Terra satellite.

Planet Earth. That shiny blue marble that has fascinated humanity since they first began to walk across its surface. And why shouldn’t it fascinate us? In addition to being our home and the place where life as we know it originated, it remains the only planet we know of where life thrives. And over the course of the past few centuries, we have learned much about Earth, which has only deepened our fascination with it.

But how much does the average person really know about the planet Earth? You’ve lived on Planet Earth all of your life, but how much do you really know about the ground underneath your feet? You probably have lots of interesting facts rattling around in your brain, but here are 10 more interesting facts about Earth that you may, or may not know.

1. Plate Tectonics Keep the Planet Comfortable:

Earth is the only planet in the Solar System with plate tectonics. Basically, the outer crust of the Earth is broken up into regions known as tectonic plates. These are floating on top of the magma interior of the Earth and can move against one another. When two plates collide, one plate will subduct (go underneath another), and where they pull apart, they will allow fresh crust to form.

The Earth's Tectonic Plates. Credit: msnucleus.org — *The Earth’s Tectonic Plates. Credit: msnucleus.org*

This process is very important, and for a number of reasons. Not only does it lead to tectonic resurfacing and geological activity (i.e. earthquakes, volcanic eruptions, mountain-building, and oceanic trench formation), it is also intrinsic to the carbon cycle. When microscopic plants in the ocean die, they fall to the bottom of the ocean.

Over long periods of time, the remnants of this life, rich in carbon, are carried back into the interior of the Earth and recycled. This pulls carbon out of the atmosphere, which makes sure we don’t suffer a runaway greenhouse effect, which is what happened on Venus. Without the action of plate tectonics, there would be no way to recycle this carbon, and the Earth would become an overheated, hellish place.

2. Earth is Almost a Sphere:

Many people tend to think that the Earth is a sphere. In fact, between the 6th cenury BCE and the modern era, this remained the scientific consensus. But thanks to modern astronomy and space travel, scientists have since come to understand that the Earth is actually shaped like a flattened sphere (aka. an oblate spheroid).

This shape is similar to a sphere, but where the poles are flattened and the equator bulges. In the case of the Earth, this bulge is due to our planet’s rotation. This means that the measurement from pole to pole is about 43 km less than the diameter of Earth across the equator. Even though the tallest mountain on Earth is Mount Everest, the feature that’s furthest from the center of the Earth is actually Mount Chimborazo in Ecuador.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com — *The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com*

3. Earth is Mostly Iron, Oxygen and Silicon:

If you could separate the Earth out into piles of material, you’d get 32.1 % iron, 30.1% oxygen, 15.1% silicon, and 13.9% magnesium. Of course, most of this iron is actually located at the core of the Earth. If you could actually get down and sample the core, it would be 88% iron. And if you sampled the Earth’s crust, you’d find that 47% of it is oxygen.

4. 70% of the Earth’s Surface is Covered in Water:

When astronauts first went into the space, they looked back at the Earth with human eyes for the first time. Based on their observations, the Earth acquired the nickname the “Blue Planet:. And it’s no surprise, seeing as how 70% of our planet is covered with oceans. The remaining 30% is the solid crust that is located above sea level, hence why it is called the “continental crust”.

5. The Earth’s Atmosphere Extends to a Distance of 10,000 km:

Earth’s atmosphere is thickest within the first 50 km from the surface or so, but it actually reaches out to about 10,000 km into space. It is made up of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface.

Winter Solstice — *Earth, as viewed from the cabin of the Apollo 11 spacecraft. Credit: NASA*

The bulk of the Earth’s atmosphere is down near the Earth itself. In fact, 75% of the Earth’s atmosphere is contained within the first 11 km above the planet’s surface. However, the outermost layer (the Exosphere) is the largest, extending from the exobase – located at the top of the thermosphere at an altitude of about 700 km above sea level – to about 10,000 km (6,200 mi). The exosphere merges with the emptiness of outer space, where there is no atmosphere.

The exosphere is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules – including nitrogen, oxygen and carbon dioxide. The atoms and molecules are so far apart that the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or with the solar wind.

Want more planet Earth facts? We’re halfway through. Here come 5 more!

6. The Earth’s Molten Iron Core Creates a Magnetic Field:

The Earth is like a great big magnet, with poles at the top and bottom near to the actual geographic poles. The magnetic field it creates extends thousands of kilometers out from the surface of the Earth – forming a region called the “magnetosphere“. Scientists think that this magnetic field is generated by the molten outer core of the Earth, where heat creates convection motions of conducting materials to generate electric currents.

The magnetic field and electric currents in and around Earth generate complex forces that have immeasurable impact on every day life. The field can be thought of as a huge bubble, protecting us from cosmic radiation and charged particles that bombard Earth in solar winds. It's shaped by winds of particles blowing from the sun called the solar wind, the reason it's flattened on the "sun-side" and swept out into a long tail on the opposite side of the Earth. Credit: ESA/ATG medialab — *Artist’s impression of the Earth’s protective magnetic field and the dynamo effect in its core that gives rise to it. Credit: ESA/ATG medialab*

Be grateful for the magnetosphere. Without it, particles from the Sun’s solar wind would hit the Earth directly, exposing the surface of the planet to significant amounts of radiation. Instead, the magnetosphere channels the solar wind around the Earth, protecting us from harm. Scientists have also theorized that Mars’ thin atmosphere is due to it having a weak magnetosphere compared to Earth’s, which allowed solar wind to slowly strip it away.

7. Earth Doesn’t Take 24 Hours to Rotate on its Axis:

It actually takes 23 hours, 56 minutes and 4 seconds for the Earth to rotate once completely on its axis, which astronomers refer to as a Sidereal Day. Now wait a second, doesn’t that mean that a day is 4 minutes shorter than we think it is? You’d think that this time would add up, day by day, and within a few months, day would be night, and night would be day.

But remember that the Earth orbits around the Sun. Every day, the Sun moves compared to the background stars by about 1° – about the size of the Moon in the sky. And so, if you add up that little motion from the Sun that we see because the Earth is orbiting around it, as well as the rotation on its axis, you get a total of 24 hours.

This is what is known as a Solar Day, which – contrary to a Sidereal Day – is the amount of time it takes the Sun to return to the same place in the sky. Knowing the difference between the two is to know the difference between how long it takes the stars to show up in the same spot in the sky, and the it takes for the sun to rise and set once.

8. A year on Earth isn’t 365 days:

It’s actually 365.2564 days. It’s this extra .2564 days that creates the need for a Leap Year once ever four years. That’s why we tack on an extra day in February every four years – 2004, 2008, 2012, etc. The exceptions to this rule is if the year in question is divisible by 100 (1900, 2100, etc), unless it divisible by 400 (1600, 2000, etc).

9. Earth has 1 Moon and 2 Co-Orbital Satellites:

As you’re probably aware, Earth has 1 moon (aka. The Moon). Plenty is known about this body and we have written many articles about it, so we won’t go into much detail there. But did you know there are 2 additional asteroids locked into a co-orbital orbits with Earth? They’re called 3753 Cruithne and 2002 AA₂₉, which are part of a larger population of asteroids known as Near-Earth Objects (NEOs).

The asteroid known as 3753 Cruithne measures 5 km across, and is sometimes called “Earth’s second moon”. It doesn’t actually orbit the Earth, but has a synchronized orbit with our home planet. It also has an orbit that makes it look like it’s following the Earth in orbit, but it’s actually following its own, distinct path around the Sun.

Meanwhile, 2002 AA₂₉ is only 60 meters across and makes a horseshoe orbit around the Earth that brings it close to the planet every 95 years. In about 600 years, it will appear to circle Earth in a quasi-satellite orbit. Scientists have suggested that it might make a good target for a space exploration mission.

10. Earth is the Only Planet Known to Have Life:

We’ve discovered past evidence of water and organic molecules on Mars, and the building blocks of life on Saturn’s moon Titan. We can see amino acids in nebulae in deep space. And scientists have speculated about the possible existence of life beneath the icy crust of Jupiter’s moon Europa and Saturn’s moon Titan. But Earth is the only place life has actually been discovered.

But if there is life on other planets, scientists are building the experiments that will help find it. For instance, NASA just announced the creation of the Nexus for Exoplanet System Science (NExSS), which will spend the coming years going through the data sent back by the Kepler space telescope (and other missions that have yet to be launched) for signs of life on extra-solar planets.

Europa's cracked, icy surface imaged by NASA's Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute. — *Europa’s cracked, icy surface imaged by NASA’s Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute.*

Giant radio dishes are currently scan distant stars, listening for the characteristic signals of intelligent life reaching out across interstellar space. And newer space telescopes, such as NASA’s James Webb Telescope, the Transiting Exoplanet Survey Satellite (TESS), and the European Space Agency’s Darwin mission might just be powerful enough to sense the presence of life on other worlds.

But for now, Earth remains the only place we know of where there’s life. Now that is an interesting fact!

We have written many interesting articles about planet Earth here on Universe Today. Here’s What is the Highest Place on Earth?, What is the Diameter of the Earth?, What is the Closest Planet to Earth?, What is the Surface Temperature of Earth? and The Rotation of the Earth?

Other articles include how fast the Earth rotates, and here’s an article about the closest star to Earth. If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

And there’s even an Astronomy Cast episode on the subject of planet Earth.

May 26, 2016February 22, 2017 by Fraser Cain

Can Stars Be Cold?

If you’ve heard me say “oot and aboot”, you know I’m a Canadian. And we Canadians are accustomed to a little cold. Okay, a LOT of cold. It’s not so bad here on the West Coast, but folks from Winnepeg can endure temperatures colder than the surface of Mars. Seriously, who lives like that?

And on one of those cold days, even on a clear sunny day, the Sun is pointless and worthless. As the bone chilling cold numbs your fingers and toes, it’s as if the Sun itself has gone cold, sapping away all the joy and happiness in the world. And don’t get me started about the rain. Clearly, I need to take more tropical vacations.

But we know the Sun isn’t cold at all, it’s just that the atmosphere around you feels cold. The surface of the Sun is always the same balmy 5,500 degrees Celsius. Just to give you perspective, that’s hot enough to melt iron, nickel. Even carbon melts at 2500 C. So, no question, the Sun is hot.

The Sun – It’s pretty hot. Credit: NASA/SDO.

And you know that the Sun is hot because it’s bright. There are actually photons streaming from the Sun at various wavelengths, from radio, infrared, through the visible spectrum, and into the ultraviolet. There are even X-ray photons blasting off the Sun.

If the Sun was cooler, it would look redder, just like a cooler red dwarf star, and if the Sun was hotter, it would appear more blue. But could you have a star that’s cooler, or even downright cold?

The answer is yes, you just have to be willing to expand your definition of what a star is.

Under the normal definition, a star is a collection of hydrogen, helium and other elements that came together by mutual gravity. The intense gravitational pressure of all that mass raised temperatures at the core of the star to the point that hydrogen could be fused into helium. This reaction releases more energy than it takes, which causes the Sun to emit energy.

The coolest possible red dwarf star, one with only 7.5% the mass of the Sun, will still have a temperature of about 2,300 C, a little less than the melting point of carbon.

But if a star doesn’t have enough mass to ignite fusion, it becomes a brown dwarf. It’s heated by the mechanical action of all that mass compressing inward, but it’s cooler. Average brown dwarfs will be about 1,700 C, which actually, is still really hot. Like, molten rock hot.

This artist’s conception illustrates the brown dwarf named 2MASSJ22282889-431026. Credit: NASA/JPL-Caltech

Brown dwarfs can actually get a lot cooler, a new class of these “stars” were discovered by the WISE Space Observatory that start at 300 degrees, and go all the way down to about 27 degrees, or room temperature. This means there are stars out there that you could touch.

Except you couldn’t, because they’d still have more than a dozen times the mass of Jupiter, and would tear your arm off with their intense gravity. And anyway, they don’t a solid surface. No, you can’t actually touch them.

That’s about as cold as stars get, today, in the Universe.

But if you’re willing to be very very patient, then it’s a different story. Our own Sun will eventually run out of fuel, die and become a white dwarf. It’ll start out hot, but over the eons, it’ll cool down, eventually becoming the same temperature as the background level of the Universe – just a few degrees above absolute zero. Astronomers call these black dwarfs.

We’re talking a long long time, though, in fact, in the 13.8 billion years that the Universe has been around, no white dwarfs have had enough time to cool down significantly. In fact, it would take about a quadrillion years to get within a few degrees of the cosmic microwave background radiation temperature.