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7.36 Is Pluto a Planet?

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 to the status of a dwarf planet. 


A?best-fit color image/map of?Pluto?generated with the?Hubble Space Telescope?and advanced computers. It is unknown if the brightness differences are mountains, craters, or polar caps. Click here for original source URL

A comparison of the size of Pluto to Earth. Click here for original source URL.

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

Pluto's orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17?) and highly eccentric (elliptical). Click here for original source URL.

• 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, which is 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.

Dwarf planet Eris and its moon Dysnomia. Click here for original source URL.

• 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 moon Triton as seen by Voyager 2. Triton is approximately the same size as Pluto. Click here for original source URL.

• 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. 

The Pluto system, including all three satellites, Charon, Nix, and Hydra. Click here for original source URL.

• 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.


Pluto was discovered in 1930 by Clyde Tombaugh, the first person in nearly a century to discover a planet (or so he thought). 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 orbit the Sun, they must be large enough that gravity causes them to be spherical, and they must clear out their orbit of other debris. It's on this third criterion that Pluto was demoted to a dwarf planet. The demotion caused some backlash among members of the pulbic, many of whom were emotionally attached to the idea of Pluto as a planet. Dwarf planet hunter Michael Brown laid out the case against Pluto in his 2010 popular book "How I Killed Pluto and Why It Had It Coming." NASA's New Horizons spacecraft made its closest approach to Pluto in 2015, carrying some of Clyde Tombaugh's ashes.

", 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 planet hood since it shares it's orbit with numerous other objects