For a long time, the previous ninth planet was to us here on Earth nothing more than a tiny speck in the sky or a blob of slightly colored pixels on an image sent down from the Hubble Space Telescope. Yesterday I wrote about New Horizons, the first and so far only mission to Pluto. Today I'd like to follow up on that post and go into detail on why Pluto is awesome and what was learned about this world from the New Horizons mission.

Thanks to New Horizons, we now have high-resolution images of parts of Pluto and its fascinating moons. These images will feature heavily in this post.

True-color photograph of Pluto taken by New Horizons
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Pluto's Orbit

Pluto lies on the edge of the solar system, just past the last gas giant Neptune. It's orbit takes it as close as 30 AU and as far as 50 AU from the sun (1 AU = 1 Astronomical Unit = 1 Earth Distance). This gives it substantial seasons, as the amount of sunlight will vary dramatically over a single Plutonian year. Speaking of years, it takes almost 250 years for Pluto to go around the sun just once - that means in all of the time since Pluto's discovery, it hasn't even gone around the sun one single time.

Pluto's orbit is labelled on the above image. Eris is another Pluto-sized Kuiper Belt Object that I'll be featuring at a later date. Unfortunately, far less is known about it than Pluto.
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But this has been known since just after Pluto was discovered by Clyde Tombaugh in 1930. Pluto's size and vast distance away from Earth makes it very difficult to image from Earth-based telescopes. Even the best images will end up showing Pluto as a glowing mass of pixels with little to no detail. New Horizons changed this during its 2015 flyby of Pluto, and as such every image of Pluto and its moons in this post came from that spacecraft.

The world of Pluto

Pluto may have been the smallest planet, but it is not a small place by any means. It's total land area of over 17 million square kilometers gives it more land area than the entire country of Russia, the largest country on Earth (this is still, however, significantly less than the land area of our own Moon). At 3.5% of the mass of the Earth and over 2,300 kilometers across, you would feel about 16 times lighter if you stood on the surface of Pluto.

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Unlike most objects in the solar system, Pluto actually has an atmosphere, although it's very different from our own. The atmosphere here is made up of molecules outgassing from the various ices (Nitrogen, Methane, etc) on the surface of Pluto, which are able to stay frozen due to the massive distance between the sun and Pluto. The atmospheric pressure is about 1 Pascal. For reference, if you stand at sea level on Earth you are experiencing an atmospheric pressure of about 101300 Pascals. This makes Pluto's atmosphere quite tiny compared to Earth's, and if you stood on the surface without a space suit you wouldn't be able to tell much difference between it and a pure vacuum. As Pluto travels far from the sun to the edges of its elliptical orbit, this atmosphere will likely condense/freeze down back onto the surface ice, reducing the pressure even further, before re-expanding when Pluto approaches its perihelion closer to the sun.

What you are looking at is an image of Pluto's atmosphere taken with the New Horizons LORRI instrument, silhouetted against the sun.
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As such, Pluto is almost entirely covered in ice. However, this ice is almost all (98%) Nitrogen ice. Nitrogen makes up most of the gas in our atmosphere - now imagine a place so cold that the air around you literally freezes to the ground. And cold it is: The average temperature on the surface of Pluto is over 200 degrees Celsius below zero.

Image of Pluto's Sputnik Planum, a vast plain of nitrogen ice that covers a large swathe of the dwarf planet's surface.
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Here you can see mountains lining the edge of the ice plain.
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This image of Pluto once again shows the atmosphere along with some surface features. The dark side of Pluto was not observed during the New Horizons flyby, and we will not know what it looks like or what lies there until another mission is sent here.
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Here, islands of icy mountains stick up out of the nitrogen ice plain. Notice how the plain is subdivided into "cells" - these are believed to be geologically active.
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This images shows just how varied the landscape of Pluto is. Mountains, hills, and plains of various composites cover the skyline.
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Pluto's Moons: Charon, Hydra, Nix, Kerberos, Styx

Pluto has a lot of moons for its size. The (by far) largest moon of Pluto is known as Charon, and at around half the size of Pluto itself is much closer to Pluto in size than the moon is to Earth. The other four moons look more like asterioids than mini planets and are significantly smaller than both Charon and Pluto.

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While Pluto can't be imaged well from Earth before New Horizons, many of these moons could barely be seen at all: Styx, for example, wasn't discovered until 2012, 6 years after New Horizons launched for Pluto.

Charon: Giant Moon

Charon is notable because it is closer to the size of Pluto than any other planet-moon system in the solar system. Charon is about half the size of Pluto and is so big that it doesn't actually orbit Pluto. When two massive bodies orbit each other, they actually both orbit the common center of mass, known as the barycenter. Usually the barycenter lies inside the central object (such as with the Earth and the Moon, or the Earth and the sun). In the case of Pluto and Charon, this barycenter actually lies outside of Pluto due to the size of Charon. This means that Pluto and Charon orbit each other, as shown below.

This strange orbit has led some to classify Pluto and Charon as two different dwarf planets bound together. Although Charon would be its own dwarf planet if it was not orbiting Pluto, this classification is not commonly used.
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Unlike Pluto, Charon's surface is mostly made up of water ice, not nitrogen ice. Longterm interaction with emissions from the sun formed the reddish region on Charon's pole due to compounds called tholins. Charon's surface is scarred by giant canyons and large mountains, including one strange mountain that rises out of a depression.

Charon
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"Mountain in a moat" on Charon
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Nix

Nix is an approximately 50 kilometer by 30 kilometer tumbling rock that orbits Pluto outside of Charon's orbit. Not much is known about this moon due to the distance between it and New Horizons, but several impact craters are visible in images.

Nix, Pluto's 2nd moon
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Hydra

Hydra is slightly bigger than Nix, with a maximum length of over 60 kilometers. It is an oddly shaped moon and is made of mostly water ice. Like Nix, Hydra is not nearly massive enough to become spherical due to its own gravity like Charon and Pluto have.

The best images of Hydra
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Styx and Kerberos

Styx and Kerberos are Pluto's last two moons, and are much smaller than the other three.

Kerberos is the bigger of the two and takes the form of a lumpy peanut. Approximately 20 kilometers by 10 kilometers in size, very little is known about it. It's composition is similar to the other small moons of Pluto.

Kerberos
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Styx is the smallest moon of Pluto, and unfortunately the best image of it is still terrible due to New Horizon's distance from Styx. Styx is also mostly made of water ice, and doesn't have the irregular shape of Hydra and Kerberos.

Styx
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Conclusion

There's still a lot more to learn about Pluto and its moons, more than I could have covered in this post. Hopefully this provided a decent if brief overview of some of what there is to know about Pluto.

Remember that beyond Pluto many, many other object exist past the orbit of Neptune. This includes another dwarf planet that is even more massive than Pluto known as Eris (with its own moon!) along with dozens of smaller dwarf planets. Unfortunately do to their distance from us, for now we only have detailed images of Pluto.

If you have any questions, comments, or corrections, don't hesitate to let me know.

Craters on Pluto - One of my favorite images of Pluto
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Thanks for reading!

Sources for additional reading
NASA New Horizons Images
New Horizons LORRI Images
Pluto Wikipedia Entry
Charon Wikipedia Entry
Nix Wikipedia Entry
Hydra Wikipedia Entry
Kerberos Wikipedia Entry
Styx Wikipedia Entry
ScienceDaily - How a moon slows the decay of an atmosphere
ScienceDaily - New data from flyby of Pluto
ScienceMag - The atmosphere of Pluto as observed by New Horizons