Pluto, Not Quite a Planet

Poor, poor Pluto. The heavenly object that once stood tall with the eight planets that orbit our sun is now just another anomalous piece of rock, demoted, soon to be left off those colorful solar system diagrams that filled the walls of our grade school classrooms. Pluto may not be a planet, but it is still interesting.

Sort of Like a Planet, but not Really

Pluto was discovered in 1930, and named after the Roman god of the underworld for its expected cold, dark nature so far from the sun. The name also referenced the initials of Percival Lowell, who spent the final 20 years of his life searching, unsuccessfully, for a planet outside Neptune. Pluto orbits at 40 times the Earth's distance from the Sun, orbiting every 248 Earth years. There the sunlight is less than one thousandth as strong as on Earth. Pluto is so cold its surface is covered in frozen nitrogen. For about 20 year of every orbit, it crosses Neptune's path and is closer to the Sun than the eighth planet, though they cannot collide. This last happened between 1979 and 1999.

Sizes to scale (NASA)

Pluto is about two-thirds the size of the moon, with a radius of 1153 km. A large object, but small compared to planets. There are seven moons in our solar system larger than Pluto, as are all of the planets. Pluto was demoted to the status of "Dwarf Planet" in 2006, ending its 76 year run as the little planet that could. Sadly, it does not even get to be the king of its new classification, being smaller than the dwarf planet Eris, discovered in 2005. Dwarf planets are objects orbiting the Sun, large enough that their gravity makes them round, but not large enough to vacuum up all of the other objects near their orbit. Pluto has a rocky core making up about half its mass with a thick layer of ice and a thin layer of frozen nitrogen on top. This is similar to many moons of Saturn, Uranus, and Neptune as well as other objects floating in the far reaches of our solar system and comets. If Pluto were moved to Earth's location, it would develop a comet tail. 

That's No Moon!

Pluto has three moons: Charon, Nix, and Hydra. Nix and Hydra are small, less than 100 km across and are fairly boring little chunks of rock. But Charon is special. In addition to sharing a name with this kick ass heavy metal band, Charon is big enough (603 km radius) that is does not simply orbit Pluto. They both orbit a point somewhere in between them, known as the barycenter. This is true of all bodies in space. If the Moon was the same size as Earth, they would revolve together around a point halfway in between the two. The Earth is so much more massive than the moon that their barycenter lies deep within the Earth's core, so it is easily said that the Moon orbits the Earth. But Charon and Pluto orbit a point well above either's surface, though it is much closer to Pluto. This has led to some astronomers labeling Charon not a moon, but a second dwarf planet in the binary Pluto-Charon system.

The Unexplored

Perhaps it is fitting that Pluto is not considered a planet. Humankind, in our quest for knowledge, has reached into space and sent a spacecraft to each planet in our solar system, but we have not arrived at Pluto. At least not yet. So far the best picture we have of Pluto's surface is this:

Pluto is just too far away and too small to be seen more clearly, or be studied in depth. But in 2015 the New Horizons probe will fly by Pluto and its moons. It will give us a better idea of what Pluto is made of and what the surface is like. Many similar objects are floating past Neptune, and studying Pluto will tell us what we may find on the thousands of object further away. After that, New Horizons will search for other objects to explore at the edge of our solar system, and eventually cross into and study the heliosphere, leaving our solar system like the Voyager probes before it.

Additional Links

Pluto at Wikipedia

Pluto at NASA