Pluto: Planet?

Is it or isn't it?  Many people, comfortable and familiar with Pluto's status as the 9th rock from the Sun, have expressed dismay at the International Astronomical Union (IAU)'s ruling that Pluto would be demoted. The 2006 vote that determined what exactly constitutes a planet banished Pluto to the role of dwarf planet.

But what do we really know about our poor friend Pluto, other than that it's really far away?
(By the way, it is really far away: Pluto orbits between 30 and 49 astronomical units (AU), which is 30-49 times the distance between the Earth and the Sun!)

Quick Facts about Pluto

• Orbital period: 248 years
• Apehelion (farthest from the Sun): 48.871AU
• Perhelion (closest to the Sun): 29.657AU
• Size: 
• Radius: 1153 +/- 10km (0.18 Earths)
• Mass: 1.305x10^22 kg (0.00218 Earths, 0.178 Moons)
• Temperature in Kelvin:
• Minimum: 33K (-400F)
• Maximum: 55K (-360F)
• Moons:
• Charon
• Nix
• Hydra
• P4
• P5

Pluto was discovered in 1929 by Clyde Tombaugh, an astronomer at the Lowell Observatory.  He was actually looking for the mysterious "Planet X", which was supposedly to blame for discrepancies in the orbits of Uranus and Neptune.  To do so, he would compare photographs of the night sky taken two weeks apart. To his surprise, after a year of comparisons he found movement in one of these pairs!  The discovery was big news, and the planet was named Pluto, after the Roman god of the underworld.

Over the next decades, further observations made it less and less likely that Pluto was Planet X.  For starters, they couldn't see a disk and the planet was faint, meaning it was probably small. Its albedo (or brightness) was calculated in 1976 and the researchers determined that with an albedo matching methane ice, Pluto could not be more than 1% the mass of Earth, When Pluto's moon Charon was discovered, they were able to calculate its mass and found that it was only 0.2% of the Earth's, which made it way too small to cause the gravitational disturbances of two gas giants.  When Voyager 2 passed by Neptune in 1989, scientists were able to revise the mass of the planet, which eliminated the discrepancies and nixed Planet X forever.

What makes Pluto different?

Pluto is slightly out of place compared to the 8 other planets. The biggest inconsistency is its orbit. While the other planets orbit on the same plane, Pluto's orbit is tilted so that it crosses the plane and orbits above and below it.

It also has an exaggerated elliptical orbit, compared to the almost-circular orbits of the dominant 8 planets.  

Pluto seems to be made up of methane and water ice, with a possible rocky core, much like a comet.

However, it does have quite a bit in common with the other objects in its neighborhood, such as the highly inclined orbit, size and orbital period.

Why can't it be a planet?

The IAU has 3 criteria that a body must meet to be considered a planet.
1. It must orbit the sun
2. It must be pulled into a spherical shape by gravity
3. It must clear the neighborhood around its orbit

Pluto meets the first two, as it does orbit the sun (however strangely) and is roughly spherical. However, the other planets have relatively clear orbits. Most of them have some small asteroids like Trojans or Centaurs that are scattered through their orbits, but the planet itself takes up the majority of the mass of its orbit.  Pluto, however, orbits through this cloud of rocky bodies called the Kuiper Belt  (pronounced like Piper).  You can see in the diagram below that Pluto (the outermost red ring) most definitely doesn't clear its orbit.  (Forgive the non-perfect orbits, paint can only do so much).

What happens to Pluto now?

Never fear, our dwarf planet hasn't been forgotten. It actually holds the honor of being the first of the Plutoids, or Trans-Neptunian bodies (meaning they orbit beyond Neptune) that orbit the sun in a similar fashion to Pluto and are large enough to be rounded. This family has expanded in recent years to include not only Pluto, but Eris, Haumea, Makemake, Quaoar, Orcus and Sedna. 

It's estimated that hundreds of objects like Pluto are orbiting in the Kuiper Belt and beyond; however, their distance and size make them difficult to even find, let alone observe.  

As for Pluto, I have exciting news! In 2015, the New Horizons mission will arrive at Pluto and perform a flyby, giving us our first actual images of that cold, dark world.  It will perform a number of experiments, such as determining the makeup of Pluto's surface, discovering if it has a thin atmosphere as theorized, and perhaps even finding that it has much in common with Triton, Neptune's largest moon and a possible capture from the Kuiper Belt.   It will then continue on through the debris that rings our Solar System to see if there are more bodies like the Plutoids out there, silently swinging through space around our Sun.

If you have any other questions about Pluto, please ask in the comment section below!