I'm trying to understand how objects are classified as planets, moons, or dwarf planets. Can someone please explain the differences between them? I'm really curious about why Pluto is a dwarf planet, for example.
The IAU has come up with this definition. It probably won't be revised for quite some time. The reason that Pluto fails as a major planet is because it has not "cleared its neighborhood." Pluto's orbit crosses that of Neptune, a vastly larger body. Pluto also shares its orbit with the Kuiper belt, a swath of small icy bodies beyond Neptune.
A moon is usually understood to be a natural body that orbits a planet, be it major (e.g. Earth), minor (e.g. Pluto), or extra-solar (although no extra-solar moons have been detected at the time of this posting). However, the IAU has not yet formally defined a moon.
Sticky situations and possible future revisions/extensions for the definition of a planet-
Planets are define as gravitationally round bodies that orbit the sun and have cleared their orbital region. In other words, the only bodies that will cross a planet's orbit are temporary transits (i.e. comets) or bodies that are gravitationally controlled by the planet (moons & smaller bodies in orbital resonance with the planet). This resonance requirement includes trojan asteroids, so the previous answer is incorrect in this regard.
Pluto is locked into a 3:2 orbital resonance with the far more massive Neptune, as are other nearby objects in the Kuiper Belt.
This is why Pluto is not classified as a planet, but rather as a dwarf planet.
Please note that the IAU classification of planets applies only to our Solar System as not enough information was known in 2005 about the orbital dynamics of other planetary systems.
A dwarf planet is a minor planet that is round, so it does not dominate its orbit like a real planets do. If an object is more massive than other objects near its orbit and it is gravitationally dominating, then it is called a planet. Otherwise it is a minor planet, if it is not a comet (comets do follow orbits that make their surface ices sublimate when they are close their stars). Round non-dominant objects orbiting a star are called dwarf planets. Actually a dwarf planet is any minor planet or comet that is round. It can also be said that a dwarf planet is any round object that is less massive than other objects near its orbit combined. Round moons are not dwarf planets, because they do not orbit star directly. IAU only wrote about objects orbiting the Sun, but I'm trying to write more generally.
protected by Qmechanic♦ Jan 15 '14 at 9:54
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