Definition of a Planet

From: Richard S. R.
Sent on: Tuesday, August 7, 2012 8:58 PM
Apropos of the discussion at tonight's confab, according to Wikipedia, the IAU has only 3 criteria for planethood:

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The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:
  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has "cleared the neighbourhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB). Initial drafts planned to include dwarf planets as a subcategory of planets, but because this could potentially have led to the addition of several dozens of planets into the Solar System, this draft was eventually dropped. The definition was a controversial one and has drawn both support and criticism from differentastronomers, but has remained in use.
According to the definition, there are currently eight planets and five dwarf planets known in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, where smaller bodies cannot be found yet. Extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.

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The definition specifically says it's "for internal use only" (IE, applying only within the Solar System), thus the use of the word "Sun" rather than the more generic "star". Wikipedia explains why it's so limited:

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The definition may be difficult to apply outside the Solar System. Techniques for identifying extrasolar objects generally cannot determine if an object has "cleared its orbit", except indirectly via Stern and Levison's Λ parameter, and provide limited information about when the objects were formed. The wording of the new definition is heliocentric in its use of the word Sun instead of star or stars, and is thus not applicable to the numerous objects that have been identified in orbit around other stars. However, a separate "working" definition for extrasolar planets was established by the IAU in 2001 and includes the criterion "the minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System."

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I love learning new things.

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Richard S. Russell, a Bright (
2642 Kendall Av. #2, Madison  WI [masked]
608+[masked] • [address removed]

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I am driven by 2 main philosophies:
  (1) Know more about the world than I knew yesterday.
  (2) Lessen the suffering of others.
You’d be surprised how far that gets you.

— Neil deGrasse Tyson, astrophysicist

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