# Why do the two inner planets not have moons, but all the others do? **URL:** \ **Category:** Factual Questions\ **Created:** [September 1, 2012, 4:41pm UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379 "2012-09-01T16:41:41Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [September 2, 2012, 5:25pm UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/21 "2012-09-02T17:25:20Z")
> [@FoieGrasIsEvil](#): > > Do all of the moons in our solar system NOT rotate on their axes like Earth’s does? The moon does rotate on its axis, just enough to stay in tidal lock with the Earth. So I suppose you’re really asking if there are moons that are not in 1:1 tidal lock with their planets. Mercury’s spin has a 3:2 resonance with its orbit, so I bet that might happen for some planetary moons as well. I can’t do the math, but I believe that 1:1 is more typical for more cirular orbits, whereas other resonances are associated with more elliptical ones. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 3, 2012, 2:11am UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/22 "2012-09-03T02:11:25Z")
The other resonances aren’t resonances at all unless you’ve got an elliptical orbit. And Mercury’s orbit is the most elliptical of any of the rockballs or gasballs, so it’s not surprising that it’s got a resonant lock. That said, while it wouldn’t be too surprising to find a moon in a resonant lock with its planet, such a situation is not known in our Solar System (though of course, there might be some wholly unremarkable chunk of ice around one of the gas giants that we’ve just never noticed). ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [September 3, 2012, 10:09am UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/23 "2012-09-03T10:09:12Z")
> [@Ken001](#): > > The outer planets are giants compared to Earth and have deep gravity wells. That means that if a large wandering mass passes by then it could be captured. Thus Saturn has 61, Jupiter 63, Uranus 27, and Neptune 13 moons. Minor nitpick. The major moons of the gas giants are thought to have formed in place, with the exception of [Triton](http://en.wikipedia.org/wiki/Triton_(moon)#Capture). Of the smaller satellites, some formed in place, some were captured and others are thought to be the relics of collisions. ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [September 4, 2012, 12:02pm UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/24 "2012-09-04T12:02:28Z")
> [@Incubus](#): > > There’s no reason Venus can’t have a moon; its close in size and proximity to earth. I know there are a few theories on how moons come to be (captured moon, debris from collision, planetary leftovers forming into a moon, etc.) > > Is Charon considered a ‘moon’? Doesnt Pluto and Charon have a common axis of orbit? They arent that different in size. Even besides Charon, Pluto has (at last count) three additional satellites. All are tiny, and orbit farther out than Charon does. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [September 4, 2012, 1:17pm UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/25 "2012-09-04T13:17:06Z")
> [@Diceman](#): > > Even besides Charon, Pluto has (at last count) three additional satellites. All are tiny, and orbit farther out than Charon does. [Found another in July](http://en.wikipedia.org/wiki/Moons_of_Pluto). Looks like we’re up to four (tiny ones) now. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 5, 2012, 7:13am UTC](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379/26 "2012-09-05T07:13:05Z")
The OP implies that only one dwarf planet has a moon. Not true. Haumea has two moons and Eris has one. A variety of other asteroids and TNOs have them as well. [Previous page](https://boards.straightdope.com/t/why-do-the-two-inner-planets-not-have-moons-but-all-the-others-do/633379.md?page=1)