1. Pluto has an atmosphere
Even though Pluto's average temperature averages a mere 44 degrees above absolute zero, the dwarf planet has an atmosphere. Not an atmosphere as we know it, but an atmosphere, none the less.
It was first discovered back in 1985, when astronomers watched as Pluto passed in front of a star. They were able to calculate a slight dimming as its atmosphere passed in front of the star, before Pluto itself blocked the star entirely. From those observations, they were able to calculate that it has a thin envelope of nitrogen, methane and carbon dioxide.
As Pluto moves away from the Sun, this atmosphere gets so cold that it freezes onto the surface. And then as the dwarf planet warms again, the atmosphere evaporates again, forming a gas around it.
2. Pluto has 3 moons
You might have heard that Pluto has a large moon called Charon (more on that later), but did you know that it actually has 3 moons in total. Charon is the large one, with a mass of roughly half that of Pluto's.
Two additional moons, Nix and Hydra, were discovered by astronomers using the Hubble Space Telescope on May 15, 2005. They were originally called S/2005 P1 and S/2005 P2, and then given their final names on June 21, 2006.
They took a long time to discover because they're so tiny. Nix is only 46 km across, while Hydra is 61 km across.
3. Pluto hasn't cleared out its orbit
Although Pluto orbits the Sun and it's round, it's not a planet. And that's because Pluto hasn't cleared out its orbit of material. This was the reason that the International Astronomical Union chose to demote it from planet to dwarf planet in 2006.
Just to give you an idea, if you added up the mass of all the other objects in Pluto's orbit, Pluto's mass would only be a tiny fraction of that total. In fact, it would only be 0.07 times as massive as everything else. For comparison, if you did the same thing with all the other material in the Earth's orbit, our planet would be 1.5 million times as massive.
And that's why Pluto's not a planet.
4. Pluto is actually a binary system
You'd think that Charon orbits Pluto, but actually, Pluto and Charon orbit a common point in space. In the case of the Earth and the Moon, we actually orbit a common point, but that spot exists inside the Earth. In the case of Pluto and Charon, however, that common point is above the surface of Pluto.
Before Pluto was downgraded to a dwarf planet, astronomers were thinking of classifying it as a binary planet system. And then as a binary dwarf planet system. Perhaps that will help it recover some of its lost glory.
5. Pluto is named after a god, not a dog
If you think Pluto is named after a Disney character, you're wrong. It's actually named after the Roman god of the underworld. And Charon is the ferryman who carries souls across the river Styx.
When it was first discovered, Pluto was just given the name Planet X, but then the discoverers needed to come up with something better and more permanent. The name Pluto was suggested by Venetia Burney, an 11-year old school girl in oxford, England. She thought it was a good name for such a cold, dark world.It was passed along to the discoverers and they liked it enough to make it official.
6. Pluto can be closer than Neptune.
For most of its orbit, Pluto is more distant than Neptune, reaching out as far as 49 astronomical units (49 times the distance from the Earth to the Sun). But it has such an eccentric, elliptical orbit that it gets much closer, reaching a mere 29 AU. And during that time, it's actually orbiting within the orbit of Neptune. The last time Pluto and Neptune made this switch was between February 7, 1979 and February 11, 1999. And give it another couple of hundred years and it'll happen again.
7. Pluto is smaller than any planet, and even 7 moons
Pluto is small. How small? Astronomers recently calculated that its mass is 1.31 x 1022 kg (less than 0.24% the mass of Earth). And its diameter is only 2,390 km across.
At this point, it's smaller than Mercury, and seven other moons including: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton.
And now astronomers know that it's even smaller than the recently discovered dwarf planet Eris. Here's more information about how big Pluto is.
8. If it were closer to the Sun, Pluto would be a comet
Although this isn't officially a reason for losing its planet status, Pluto wouldn't last long if it got much closer to the Sun. It's comprised of about half rock and half ice. This is a similar ratio to many rocky comets in the Solar System.
If you could somehow bring Pluto closer to the Sun, it would sprout a tail, becoming a spectacular comet. And over millions of years, the solar wind would blast away its icy structure, causing it to lose mass.
It's lucky Pluto lives in such a cold, dark part of the Solar System.
9. Charon might have geysers
In the last few years, astronomers have discovered that several objects in the Solar System have ice geysers, including Saturn's moon Enceladus, and maybe several others as well. But Pluto's moon Charon could have this happening too.
Astronomers using the Gemini Observatory in Mauna Kea in Hawaii recently turned up evidence that geysers on Charon are spreading ammonia hydrates and water crystals across the surface of the moon.
Is this really happening? We'll know soon, because… here's the last Pluto fact.
10. There's a spacecraft going to Pluto right now
NASA's New Horizons spacecraft is making its way to Pluto right now. The spacecraft launched in 2005, and its expected to reach the dwarf planet in 2015. It will pass right through the system, imaging the surface of Pluto and its moons, and finally answering questions that have puzzled astronomers for nearly a hundred years.
The Dwarf Planet Pluto
Why Isn't Pluto a Planet Any More?
Why Isn't Pluto a Planet Any More?