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What Characteristics of Pluto Made It a Planet?

Pluto was discovered in 1930 by 22-year-old scientist Clyde Tombaugh. Until 2006, Pluto was the largest known object in the solar system past Neptune, but it was smaller than any of the planets. It was originally classified as a planet, but a coalescence in the definition of what constitutes a planet led to a change in its status. Pluto is now a dwarf planet and trans-Neptunian object, a grouping of celestial objects beyond Neptune. There are, however, several characteristics of Pluto that caused scientists to classify it as a planet for 75 years.

Pluto Orbits the Sun
- One of the primary qualifications for planetary status is that the object must orbit the sun. Pluto fits this qualification, but its orbit is different than the other planetary orbits. All of the other planets orbit the sun in a thin disc on the same plane. Pluto's orbit is tilted, causing it to orbit well above and below the orbital plane of the other planets. Its orbit is also excessively elongated, which puts it closer to the sun than Neptune for part of its cycle.
Gravitational Pull
- Another characteristic of planets is that they have enough mass to have a gravitational pull strong enough to pull themselves into a spherical shape. With a kilogram mass of 1.31 x 10 to the 22nd power, less than 0.24 percent of the Earth's mass, Pluto does meet this qualification. Many other celestial objects not considered planets -- including moons and other dwarf planets -- also do this, so it alone is not a qualification for planet classification.
The Existence of an Atmosphere
- Another argument for its inclusion as a planet was the presence of a small atmosphere on Pluto. Though all of the planets do have an atmosphere of some kind, it is not a unique feature to planets. Several moons of planets in the solar system also have atmospheres. Pluto, however, is the only dwarf planet with an atmosphere. Its atmosphere reaches nearly a quarter of the way to its largest moon, Charon.
The Existence of Satellites
- Though not officially a criteria for planetary status, some scientists have argued that the existence of satellites should be a reason to continue calling Pluto a planet. Pluto has three moons, Charon, Nix and Hydra. However, many trans-Neptunian objects (and even some small asteroids) have satellites, so this is not a distinct planetary quality.
Clearing the Neighborhood of Its Orbit
- Another characteristic of a planet, and the one that ultimately disqualified Pluto from planetary status, is that the object must "clear the neighborhood of its orbit." This means that the object is the dominant gravitational body in its orbit. Smaller objects nearby must be consumed or flung away by the gravity of the larger object. Pluto's mass is only 0.07 times the mass of other objects in its orbit. It is one of a group of other objects in the neighborhood, and can not "clear its area." In contrast, Earth's mass is 1.7 million times the objects in its orbit.

Astronomy