Reading: "Is Pluto a Planet?" by Chris Impey

In the 1990s, scientists started to discover objects in a region of the outer solar system called the Kuiper Belt. They turned out to be so similar to Pluto that a debate began over whether Pluto should properly be called a planet. Some scientists now feel that it would be more accurate to view Pluto as merely the largest of this group of interplanetary bodies, instead of a planet in its own right. There are several arguments for downgrading Pluto. 

* Size: Pluto is less than half as big as Mercury. Seven other planetary satellites, including Earth’s own Moon, are larger than Pluto. 

* Orbit: Pluto’s orbit is far more elliptical than any other planet’s, so much so that it crosses the orbit of Neptune. It’s also a highly inclined orbit: Pluto has more than twice the orbital inclination of any other planet.

* Rotation: Pluto spins in a retrograde direction - the opposite of almost every other planet in the solar system. The spin axis is also tilted to be almost in the plane of the solar system - although Uranus is in a similar position, it is unusual among the other planets.

* Atmosphere: Pluto has a very tenuous atmosphere, if any at all. It may only exist near perihelion, when Pluto is closest to the Sun. For the rest of Pluto’s long orbit, the gases may freeze out onto the surface.

* Pluto is not alone: Other bodies have been found in the same region of the outer solar system. Most are smaller, but a few are nearly the size of Pluto, and one, Eris, is actually larger. This means either Eris is also a planet or Pluto is not.

* Neptune's satellite, Triton: It’s about the size of Pluto, and appears to have a similar composition. It may be a captured, Pluto-class body - again indicating that Pluto is not a unique planet. 

* Pluto’s satellites, Charon, Nix, and Hydra: Although Pluto has three satellites, at least a few interplanetary bodies have also been found to have at least one satellite - the asteroid 243 Ida, Eris, and Haumea. So having a satellite does not make a body a planet.

In 2006 the International Astronomical Union took into consideration all these arguments and determined Pluto is not a planet. It has been reclassified as a "dwarf planet". Their modern definition of a planet states that planets must 1) Be large enough that gravity causes them to be spherical, 2) They orbit the Sun, and 3) They've cleared their orbit of other stuff. It is on this third count that Pluto loses it's planethood since it shares it's orbit with numerous other objects.

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Author: Chris Impey
Editor/Contributor: Ingrid Daubar-Spitale
Editor/Contributor: Pamela Gay

Taken from: http://www.teachastronomy.com/astropedia/article.aspx?qid=183