If the definition of a planet is something like the body must clear the path of orbit of debris, how is Jupiter a planet, considering it has like 63 moons and rings? What about Saturn with its moons and its rings?



I’m aware that Neptune and Uranus also have rings, but shouldn’t those disqualify them being labelled planets? Or any celestial body with a satellite?

In: Physics

But satellites do not orbit the Sun in an orbit that crosses the orbit of the planet. That is what is meant by clearing ones orbit. There is an exception but that is for objects with an orbit that is in sync with the planets orbit. All planets do have some of these objects as well but Jupiter have a lot of them.

The idea of “clearing one’s orbit” is obviously open to interpretation, as in all honesty no planet’s orbit is completely clear (even Earth’s). Further, satellites of the planets are considered to be a “part” of that planet (from a gravitational perspective) in this context, as the relationship between those planets and their satellites is much more significant than between those satellites and *other* objects (e.g. the Sun).

The point of the “cleared orbit” was more to showcase that the 8 planets **overwhelmingly** dominate the distribution of mass in their specific orbits. Pluto does not; it’s just one iceball among many. Further, the reason Jupiter’s orbit is not cleared (e.g. the Trojan asteroids) is entirely *because* of how massive it is.

More to the point, the definition of planet was driving primarily by the need to demarcate a difference between the classical planets and the plethora of smaller objects that exist in the asteroid belt and beyond the orbit of Neptune, some of which are larger than Pluto.

The planet’s gravity should dominate its orbit.

All of Jupiter’s moons clearly orbit Jupiter. All of the trojans are gravitationally bound to Jupiter. Jupiter outweighs everything in its orbit by about 5 orders of magnitude

If you look at Pluto, it and Charon orbit a center of gravity outside either of them. Charon is 12% as massive as Pluto, the next highest ratio is Earth’s moon which is just 1.2% the mass of that system. Pluto makes up less than 10% of the mass in the same orbital zone, we only called it a planet because we happened to see it a century ago and didn’t realize that Eris outweighs it out there.

Ceres is probably the best case for “cleared the neighborhood” being a qualitative requirement. Ceres makes up a third of the mass of the asteroid belt, but you probably don’t want to call Ceres a planet because its surrounded by lots of other rocks that really aren’t gravitationally bound to it.

The refinement of the planet definition came about because leaving Pluto as a planet meant we had over a dozen “planets” and potentially 3 in the asteroid belt alone