NO – Pluto Is Not a Planet

Five bodies in the solar system are currently accorded dwarf planet status. Earth and the Moon are shown for comparison purposes.

In 2006, the International Astronomical Union (IAU) stated that a planet in our solar system must:

Pluto fits the first two criteria, however, fails the last condition about having cleared the neighborhood around its orbit. Pluto does not have sufficient gravitational attraction to sweep out the debris in its orbit around the Sun. The failure of this third condition resulted in Pluto being stripped of its planet status by the IAU. According to the IAU, a dwarf planet such as Pluto is defined as a celestial body of large mass that does not meet all of the three IAU conditions for planet status.

Pluto has a highly elliptical and inclined orbit.

Pluto has long been known to have some anomalous orbital characteristics that seemed to set it apart from other planets. Pluto, for example, has an extremely large orbital inclination to the ecliptic of 17.16°. This parameter alone seems to indicate that Pluto is somehow different. The planet with the next highest orbital inclination is Mercury with a value of 7.01°; Earth has an orbital inclination of 0.00°. Pluto’s orbit is also more eccentric than that of any other planet in the solar system. It has an eccentricity of 0.25; this highly elliptical orbit carries it on a path in which Pluto’s distance from the Sun varies between 4.4 and 7.4 × 109 km (2.7–4.6 × 109 miles). Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit as a result of this extreme eccentricity; the last time this occurred was between 1979 and 1999. Mercury has the next highest orbital eccentricity at 0.21; Earth has an orbital eccentricity of 0.02.

These unusual orbital characteristics did not generate much controversy as long as Pluto seemed to be in a class by itself. The discovery of several other Pluto-sized objects and the IAU’s new definition of what constitutes a planet have cast a new light on Pluto and its orbital characteristics. Mike Brown, one of the discoverers of Eris, agrees with the IAU’s new planet definition, and does not believe that Pluto is a planet.

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