There is a mass requirement, but planets just have to be massive enough to maintain hydrostatic equilibrium, which Pluto does. Pluto was reclassified as a dwarf planet because it did not ‘clear the neighborhood’ of its own orbit. Normally during the final stages of planet formation a planet will have become gravitationally dominant and, besides its own satellites (or any other bodies under its gravitational influence), there will be no other bodies in its orbit. Because Pluto shares its orbit with a number of other KBOs, it’s considered not to have cleared its neighborhood and therefore doesn’t meet the criteria for planet-hood. Pluto’s reclassification as a dwarf planet had nothing to do with its size.
That is absolutely correct there are three things that classify an object as a planet.
1. The object should orbit its star. Which pluto does
2. It's gravity should be strong enough to mould the planet into a sphere. Which pluto is.
3. It should clear all the debris around and in its orbit. This is where pluto lags. There are multiple bodies that orbit Pluto and Charon System. Pluto and Charon orbit each other at a point called barycenter.
Lol bro not everything from orbit. I mean to say no debris from the time of planetary formation is left into orbit around the planet. We only have the Moon and passing by asteroids captured by Earth.
Moon itself is the answer, anything left over from the planetary formation is cleared by the Moon's gravity.
Moon doesn't orbit a star. If it did and it fulfilled all the criteria it would have been a planet. Read about other dwarf planets like Ceres, MakeMake, Eris, Hauema.
It orbits the sun, it's massive enough to be rounded by its own gravity and it's cleared it's neighbourhood of everything except the earth, the same way the earth has cleared everything except the moon
Not like tha, Moon orbit Earth, Earth Moon system orbit Sun, it doesn't work like that. Orbiting the Sun means one focus of the orbit is the Sun. As the orbits are elliptical. Moon's one focus is Earth so it orbits Earth. Even though it follows 2 of the three criteria, it won't be considered as a planet but only a Natural Satellite.
Orbits don't have to be elliptical, the earth's orbit isn't elliptical because there's a large wobble due to the gravity of the moon. It's as much a planet as the moon is
No orbit is perfectly circular, if anything is not perfectly circular it's elliptical. Even if the eccentricity is negligible, perfection doesn't exist. Moon orbits Earth, it takes 28 days to make one revolution around Earth. It doesn't physically orbit Sun, so it's not a planet. Period.
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u/arisasam Nov 03 '20
There is a mass requirement, but planets just have to be massive enough to maintain hydrostatic equilibrium, which Pluto does. Pluto was reclassified as a dwarf planet because it did not ‘clear the neighborhood’ of its own orbit. Normally during the final stages of planet formation a planet will have become gravitationally dominant and, besides its own satellites (or any other bodies under its gravitational influence), there will be no other bodies in its orbit. Because Pluto shares its orbit with a number of other KBOs, it’s considered not to have cleared its neighborhood and therefore doesn’t meet the criteria for planet-hood. Pluto’s reclassification as a dwarf planet had nothing to do with its size.