[CamperBob2]

That's pretty cool. It's wildly counterintuitive, but if it weren't the case, the planets would be orbited by lots of captured asteroids and debris, instead of/in addition to being covered in craters. The only explanation for why that doesn't happen is that it can't happen.

[shkkmo]

The presence of natural satellites indicates this can indeed happen. It just requires a pretty unlikely orbital configuration.

[stickfigure]

Or an impact:

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

[dragonwriter]

> The presence of natural satellites indicates this can indeed happen

No, it doesn't, because natural satellites are generally not captured, and for those that are captured, the process involves interactions with other bodies.

[perl4ever]

The first hit when I search "planets capturing moons" says natural satellites generally are captured.

"Most satellites of the outer solar system didn’t form with their host planets"

https://astronomy.com/news/2016/12/captured-moons-of-the-gia...

Even Triton, which is the size of a planet and in an almost circular orbit, is thought to be captured, the last I heard.

[CamperBob2]

One of the criteria for planethood is an assumption that the body clears its own orbit. Moons don't just come hurtling out of the cosmos; they either result from a collision of some other body with the planet, as with our Moon, or they're already close to the planet's orbit at the time they are captured.

[perl4ever]

>One of the criteria for planethood is an assumption that the body clears its own orbit.

Is that so?

"The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets"

https://en.wikipedia.org/wiki/Centaur_(small_Solar_System_bo...

There are tens of thousands, so perhaps the definition of a planet is even more abstruse than people let on.

And apparently at least dozens have been identified as probably of interstellar origin, while it is thought that a centaur can become a moon, (e.g. Phoebe) so I wonder if we can really rule out that moons "come hurtling out of the cosmos":

"Being able to tell apart interstellar asteroids from native asteroids born in the Solar System has long eluded astronomers, but the team’s results identified 19 asteroids of interstellar origin. These are currently orbiting as part of the group of asteroids known as Centaurs, which roam the space in between the giant planets of the Solar System."

https://ras.ac.uk/news-and-press/research-highlights/interst...

[shkkmo]

> Is that so?

Yes, it is. The definition of "clearing an orbit" isn't precisely defined, but it doesn't have to be since there appears to be a large natural gap in how much orbit clearing an planet does vs. a dward planet.

> A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. That includes Pluto, whose orbit intersects with Neptune's orbit and shares its orbital neighbourhood with many Kuiper belt objects. The IAU's definition does not attach specific numbers or equations to this term, but all IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet or candidate for dwarf planet.[0]

[0] https://en.m.wikipedia.org/wiki/Clearing_the_neighbourhood

[hinkley]

Jupiter’s moons are I’m harmonic orbits because they trade momentum until they balance out.

From what I understand any eccentric orbits would either flatten out or crash into Jupiter.

[CamperBob2]

No, it doesn't. No (rocky) planet has more satellites than craters, but every planet's gravitational field is larger than its surface.

[8note]

Do we know how many of those craters used to be satellites?