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It has to clear the Kuiper belt? That's a tall order!

garmaine 8y ago
Who knows. The definition is vague to the point of uselessness. Pluto, apparently, is not a planet because it hasn't cleared Neptune out of its orbit. But then by symmetry neither has Neptune cleared Pluto out of its orbit, so if we're to have any consistency then we can't call Neptune a planet either. Oops. The reality is that the IAU definition of "planet" was a very poor decision made by people who aren't planetary scientists. The geophysical definition is much better: an object large enough to assume spherical shape from its own gravitational force acting on its constituent material, and too small to have initiated fusion and be a star. So Pluto is a planet, as is Ceres, the Moon, and a thousand other known objects.
- skykooler 8y ago
  The definition isn't exactly "cleared its orbit", but rather "gravitationally dominates its orbit". Neptune fits these criteria, because while Pluto does cross its orbit it is locked in a resonant orbit with Neptune. Similarly, the Moon, as well as Cruithne and several other near-earth objects, are all locked in a resonant orbit with the Earth. However the asteroid belt is not resonant with Ceres.
  - garmaine 8y ago
    Alternatively: Neptune is locked in a resonant orbit with Pluto. Likewise, and even more so for the Earth and Moon, and Pluto and Charon. It's not a quantitatively exact specification.
  - Latteland 8y ago
    what if the earth was not a moon of another planet but was in an orbital resonance with that larger planet. we'd still not be considered a planet. this planet definition seems pretty arbitrary to me. the astronomy world feels like it's self-defining something to justify pluto not being a planet.
- yaks_hairbrush 8y ago
  There are eight currently known bodies (apart from the sun) in our solar system which can tug hard enough to dictate terms of sharing/crossing their orbit. Pluto is locked into a 3:2 orbital resonance with Neptune. Neptune dictates the terms under which Pluto can have an orbit with anything close to its current parameters. That's what it means to be a planet. While there is theoretical space for borderline cases, none exist in our solar system. I understand you'd like to focus on objects in hydrostatic equilibrium. Pluto, Ceres, the Moon, and Titan are really neat, but something happens when you tell their life stories: you end up referencing one or more of the eight heavyweights. By contrast, these smaller objects just don't get referenced very much when we talk about other bodies' life stories. The large scale structure and history of the solar system has just 8 known characters that really matter, and that's what the definition of the planet captures.
  - dotancohen 8y ago
    The large scale structure and history of the solar system has just 1 character that really matters, a further two bits of debris that are worth mentioning, and a further six specks that might get a mention in an appendix.

Who knows. The definition is vague to the point of uselessness. Pluto, apparently, is not a planet because it hasn't cleared Neptune out of its orbit. But then by symmetry neither has Neptune cleared Pluto out of its orbit, so if we're to have any consistency then we can't call Neptune a planet either. Oops. The reality is that the IAU definition of "planet" was a very poor decision made by people who aren't planetary scientists. The geophysical definition is much better: an object large enough to assume spherical shape from its own gravitational force acting on its constituent material, and too small to have initiated fusion and be a star. So Pluto is a planet, as is Ceres, the Moon, and a thousand other known objects.

skykooler 8y ago
The definition isn't exactly "cleared its orbit", but rather "gravitationally dominates its orbit". Neptune fits these criteria, because while Pluto does cross its orbit it is locked in a resonant orbit with Neptune. Similarly, the Moon, as well as Cruithne and several other near-earth objects, are all locked in a resonant orbit with the Earth. However the asteroid belt is not resonant with Ceres.
- garmaine 8y ago
  Alternatively: Neptune is locked in a resonant orbit with Pluto. Likewise, and even more so for the Earth and Moon, and Pluto and Charon. It's not a quantitatively exact specification.
- Latteland 8y ago
  what if the earth was not a moon of another planet but was in an orbital resonance with that larger planet. we'd still not be considered a planet. this planet definition seems pretty arbitrary to me. the astronomy world feels like it's self-defining something to justify pluto not being a planet.
yaks_hairbrush 8y ago
There are eight currently known bodies (apart from the sun) in our solar system which can tug hard enough to dictate terms of sharing/crossing their orbit. Pluto is locked into a 3:2 orbital resonance with Neptune. Neptune dictates the terms under which Pluto can have an orbit with anything close to its current parameters. That's what it means to be a planet. While there is theoretical space for borderline cases, none exist in our solar system. I understand you'd like to focus on objects in hydrostatic equilibrium. Pluto, Ceres, the Moon, and Titan are really neat, but something happens when you tell their life stories: you end up referencing one or more of the eight heavyweights. By contrast, these smaller objects just don't get referenced very much when we talk about other bodies' life stories. The large scale structure and history of the solar system has just 8 known characters that really matter, and that's what the definition of the planet captures.
- dotancohen 8y ago
  The large scale structure and history of the solar system has just 1 character that really matters, a further two bits of debris that are worth mentioning, and a further six specks that might get a mention in an appendix.