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According to a geophysical planet definition, as well as the IAU planets and dwarf planets, the largest moons are also planets: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton. https://en.wikipedia.org/wiki/Geophysical_definition_of_planet Although other moons of Saturn and Uranus and also Charon look round to me also.


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The other answers told how many planets are recognized, how many dwarf planets that aren't IAU-recognized planets are recognized and one that indeed includes Eris. In a month it will be fifteen years since the infamous decision for an extremely vague and therefore stretched definition that was trying hard to keep the number of planets as low as possible for ...


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There's nothing wrong with asking a question to which the answer is "nobody knows" -- so long as you are willing to accept that the answer is essentially "nobody knows". The only thing we do know is that the probability you are asking about is non-zero because life does exist on the Earth. Some argue that the probability you are asking ...


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The answer is, it could be non-zero (some would argue it must be non-zero), but since we don't know what the probability of life emerging on Earth was, it is impossible to quantify. This is why this question is normally turned around - if we find life elsewhere in the Solar System (and it is independently developed), then what is the probability that life ...


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This is extremely unlikely. And if it happens, it's a state of limited duration. To explain "space of measure zero" David Hammen mentions in his comment, broken down for this example: there is exactly ONE state (exactly matching rotation) while there are infinitely many other possible rotation periods: just count all numbers, fractional and ...


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Kepler's laws and the associated orbit only hold for a two-body problem, so the 'barycenter' in the question can only be understood as the center of mass of the sun and a particular planet (ignoring the other ones), not as the solar system barycenter. And in this sense the convention is to take the sun at the focus, which means the semi-major axis is the (...


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I can say that obviously the planets of other stars do not have orbits that are either 'in" or "parallel to" the galactic plane. And I explain why below. If you look at maps at the sky you will see that realtively nearby stars are see in in all directions from Earth. Or you could go out on a clear night at a place with an unobstructed view of ...


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Most of the irregular moons do not quite have synchronous rotation: Regular satellites are usually tidally locked (that is, their orbit is synchronous with their rotation so that they only show one face toward their parent planet). In contrast, tidal forces on the irregular satellites are negligible given their distance from the planet, and rotation periods ...


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You are asking about tidal locking. Some moons are tidally locked, others are not. There are several factors that lead to or militate against tidal locking: The distance at the planet about which the moon orbits. The time needed to tidally lock a moon is proportional to the distance at which the moon orbits the planet, raised to the sixth power. Close-in ...


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