OK, a while back there was a lot of discussion about reclassifying Pluto’s “planet” status, and that has happened.
So what about planetary moons? Especially in the case of the gas and ice giants, where we keep finding smaller and smaller moons. Has there been any discussion about at what point something no longer becomes a moon, and is just considered space debris?
I don’t think there is a limit. if it’s in orbit around a planet; it’s a moon. (Barring man made objects of course.)
At least that’s my interpretation of it from the dictionary.
Much like “planet,” “moon” is a term that’s more of a general appelation without a specific scientific definition. There are plenty of proposed criteria for what should differentiate a “moon” from a “moonlet” from “orbital debris” (or for that matter, how to differentiate a planet with an enormous moon from a double planet).
For what it’s worth, the IAU currently recognizes 165 planetary satellites, and is worried about running out of names as we are able to identify smaller and smaller bodies orbiting planets.
Aren’t they actually called “natural satellites” and the earth’s one is called “Moon”?
Meaning, if it orbits a planet, it’s a satellite - size is not an issue.
I’d guess that the distinction is more semantic than planetological – a natural satellite, or ‘moon’ if you will, is a discretely observable object in a defined orbit. Saturn’s rings (and Jupiter and Uranus’s) are composed of miniscule moonlets in common orbits, not individually resolvable from Earth. It also has a scad of actual satellites in unique orbits (well, two of them share an orbit) which are individually resolvable.
The question of large satellite vs. double planet is an interesting one. If I had to draw the distinction, I’d say that a double planet is a set of gravitationally bound satellites of a star which revolve around a common center of mass outside the surface of the larger of the two. The Earth-Moon and Pluto-Charon pairs are close, but the common center of gravity for each is within Earth and Pluto respectively. (FWIW, whether “it orbits the Sun or a planet” is not a valid distinction – the Moon’s orbit is defined by gravitational forces that are 52% contributed by the Sun and 48% by Earth, and while I don’t have the figures, I’m quite sure much the same is true for Charon re Pluto.)
What is space trash? b Here are all the moons we have created.