# Only Mercury and Venus have no moons. How come? **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2007, 2:25pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935 "2007-01-14T14:25:59Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Elendil\_s\_Heir](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)\ **Post date:** [January 14, 2007, 2:25pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/1 "2007-01-14T14:25:59Z")
In our Solar System, only Mercury and Venus have no moons. Is it because they’re the two worlds closest to the Sun, or is there something else going on? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [January 14, 2007, 2:42pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/2 "2007-01-14T14:42:14Z")
[QUOTE=Elendil’s Heir] In our Solar System, only Mercury and Venus have no moons. Is it because they’re the two worlds closest to the Sun, or is there something else going on? [/QUOTE] IANAAstronomer, but I would say three reasons. One, the larger planets assert more gravitational influence in nearby space and are more likely to capture junk passing by; two, due to its size, the sun is more likely to capture flybys than nearby planets; and three, random chance. Notice that the second choice is suggested by the number of moons around the inner planets vs. outer. As you go from inner to outer, at least for the first five planets, the number of moons increases (Mercury, 0; Venus, 0; Earth, 1; Mars, 2; Jupiter, a whole bunch that increases frequently as we discover more). Maybe Phil Plait will be along to tell us if it is impossible for Mercury to retain a moon, or just unlikely. ---
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [January 14, 2007, 3:45pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/3 "2007-01-14T15:45:07Z")
Pluto has Charon, though; IIRC, they were declared a double dwarf planet system, but I may not be RC. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [January 14, 2007, 5:55pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/4 "2007-01-14T17:55:05Z")
Mars’ two moons are almost certainly captured asteroids, and perhaps won’t be there indefinitely. It would be entirely plausible for Mars to have no moons except as a historical accident. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [January 14, 2007, 5:57pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/5 "2007-01-14T17:57:59Z")
[QUOTE=Lumpy] It would be entirely plausible for Mars to have no moons except as a historical accident. [/QUOTE] Ditto for the Earth, if the collision theory of Lunar formation is correct. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [January 14, 2007, 7:28pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/6 "2007-01-14T19:28:38Z")
Have they defined “what _is_ a moon” yet? ---
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [January 14, 2007, 7:36pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/7 "2007-01-14T19:36:30Z")
[QUOTE=DrDeth] Have they defined “what _is_ a moon” yet? [/QUOTE] Google on “Goatse.” Now **that’s** a **moon!** ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [January 14, 2007, 7:41pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/8 "2007-01-14T19:41:19Z")
I think it’s actually Earth that’s the outlier here. It’s the only inner rocky planet that has a real moon. Mars’ moons are really just captured asteroids, as **Lumpy** said. If Earth’s moon were a little bigger, we’d actually be a binary planet system, not a planet with a moon. There are no other planets in the solar system that have a moon even approaching the relative size of Earth’s moon to Earth (i.e. Jupiter having a Neptune-sized moon). ---
**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\ **Post date:** [January 14, 2007, 11:23pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/9 "2007-01-14T23:23:54Z")
[QUOTE=Captain Carrot] Pluto has Charon, though; IIRC, they were declared a double dwarf planet system, but I may not be RC. [/QUOTE] Amazing! Almost 8 hours later and I’m still the first to jump in and say that Pluto is no longer a planet?! ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [January 14, 2007, 11:29pm UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/10 "2007-01-14T23:29:56Z")
Like anything else in life, if you don’t keep up the payments, it gets repossessed. Sends a warning to the other planets, too. ---
**Author:** ![Nancarrow](https://avatars.discourse-cdn.com/v4/letter/n/c37758/32.png) [@Nancarrow](https://boards.straightdope.com/u/Nancarrow)\ **Post date:** [January 15, 2007, 12:05am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/11 "2007-01-15T00:05:12Z")
[QUOTE=Canadjun] Amazing! Almost 8 hours later and I’m still the first to jump in and say that Pluto is no longer a planet?! [/QUOTE] I think the rest of us are in denial. 😛 ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [January 15, 2007, 12:14am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/12 "2007-01-15T00:14:44Z")
[QUOTE=Canadjun] Amazing! Almost 8 hours later and I’m still the first to jump in and say that Pluto is no longer a planet?! [/QUOTE] Well, no, that was mentioned back in the 3rd post. Maybe your should have bothered to read the thread before ‘jumping in’ with your (inaccurate) post. ---
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [January 15, 2007, 12:18am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/13 "2007-01-15T00:18:39Z")
I think **Canadjun** read ‘dwarf planet’ as ‘planet’ erroneously. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [January 15, 2007, 12:39am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/14 "2007-01-15T00:39:05Z")
[QUOTE=Sam Stone] I think it’s actually Earth that’s the outlier here. It’s the only inner rocky planet that has a real moon. Mars’ moons are really just captured asteroids, as **Lumpy** said. If Earth’s moon were a little bigger, we’d actually be a binary planet system, not a planet with a moon. There are no other planets in the solar system that have a moon even approaching the relative size of Earth’s moon to Earth (i.e. Jupiter having a Neptune-sized moon). [/QUOTE] **Sam** has it right, here (with one minor nitpick I’ll get to in a minute). As a practical matter, it’s almost impossible for a small planet like Mars or Mercury to even capture a moon of any size, owing to their relatively small and very weak gravitational sphere of influence. Even if they practically ran into a moon they’d probably not capture it in a stable orbit. The Earth has a peculiarly oversize moon, which is (by current theory) due to an impact with a giant celestial body. Save for Venus, most of the other non-giant planets are scarcely larger than moons themselves. The one nitpick with **Sam** s statement is the Pluto/Charon system, which is probably a binary system with the two worlds of almost equal mass orbiting around a common barycenter. Of course, both of the worlds are properly [Kuiper belt objects](http://en.wikipedia.org/wiki/Kuiper_belt) that move in slow, lazy orbits somewhat out of the plane of the ecliptic, and as such really shouldn’t qualify as planets, at least from a commonality of origins approach. Whether the Solar System is really a representation of a typical planetary system is, at this point almost totally unknown, so how likely or unlikely it is that the rocky worlds have so few and small moons is speculative, but it’s almost certainly the case that situations like the Earth-Luna system (a rocky inner world in the ecliptic plane with a dramatically oversized moon) are exceedingly rare. Stranger ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [January 15, 2007, 4:07am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/15 "2007-01-15T04:07:06Z")
[QUOTE=DrDeth] Have they defined “what _is_ a moon” yet? [/QUOTE] Yes, in the same statement that re-defined Pluto as a ‘pseudoplanet’. A moon is a body in a linked orbit around the Sun with another body, which is close enough and small enough (relative to the size of the other body) that their common center of gravity lies outside the other body. So our Moon is, in fact, a moon, as the center of gravity of the Earth/Moon system is inside the Earth. The center of gravity of the Pluto/Charon system, on the other hand, is in empty space between them; so those objects form a binary planet system. (Actually, of course, a pseudoplanet system, as neither Pluto nor Charon is large enough to be considered a planet.) ---
**Author:** ![Dr.Rieux](https://avatars.discourse-cdn.com/v4/letter/d/aeb1de/32.png) [@Dr.Rieux](https://boards.straightdope.com/u/Dr.Rieux)\ **Post date:** [January 15, 2007, 4:32am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/16 "2007-01-15T04:32:00Z")
[QUOTE=Captain Carrot] I think **Canadjun** read ‘dwarf planet’ as ‘planet’ erroneously. [/QUOTE] As far as I’m concerned, a dwarf planet is a planet. “Dwarf” is a modifier. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 15, 2007, 4:39am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/17 "2007-01-15T04:39:46Z")
[QUOTE=SCSimmons] Yes, in the same statement that re-defined Pluto as a ‘pseudoplanet’. A moon is a body in a linked orbit around the Sun with another body, which is close enough and small enough (relative to the size of the other body) that their common center of gravity lies outside the other body. [/quote] I think you mean **inside** there. But even so, this is not correct. One of the preliminary definitions of a planet that the IAU was considering had this, but not the final version. There is no official definition of **moon**. Nor is Pluto considered a **pseudoplanet** ; the official term is **dwarf planet**. BTW, Pluto actually has **three** moons, the other two having been discovered fairly recently (last year or the year before, I forget which). ---
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [January 15, 2007, 4:40am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/18 "2007-01-15T04:40:02Z")
Well, if some of the other planets, and you know who you are, hadn’t gone hogging 13 or 23 or whatever moons, then maybe there would have been some left for Mercury and Venus. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [January 15, 2007, 4:48am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/19 "2007-01-15T04:48:26Z")
[QUOTE=SCSimmons] Yes, in the same statement that re-defined Pluto as a ‘pseudoplanet’. A moon is a body in a linked orbit around the Sun with another body, which is close enough and small enough (relative to the size of the other body) that their common center of gravity lies outside the other body. So our Moon is, in fact, a moon, as the center of gravity of the Earth/Moon system is inside the Earth. ) [/QUOTE] But that includes a rock of soft-ball size as a “moon”. Thus, Saturn has _hundreds of thousands_ of moons, by that. And, Mercury and Venus coudl well have a “moon” of that size. In fact Earth could have a few. ---
**Author:** ![H3Knuckles](https://avatars.discourse-cdn.com/v4/letter/h/ba8739/32.png) [@H3Knuckles](https://boards.straightdope.com/u/H3Knuckles)\ **Post date:** [January 15, 2007, 6:08am UTC](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935/20 "2007-01-15T06:08:48Z")
[QUOTE=DrDeth] But that includes a rock of soft-ball size as a “moon”. Thus, Saturn has _hundreds of thousands_ of moons, by that. And, Mercury and Venus coudl well have a “moon” of that size. In fact Earth could have a few. [/QUOTE] I am not an astronomer, but as I udnerstand it, one of the principle notions of what a planet is, is based on the idea of hydrostatic equilibrium. This is when a body’s gravitic force becomes so great it pulls it’s mass into a sphere (or imperfect nearly-spherical) shape. Ordinary bodies don’t come shaped like the 8 planets, etc. It’s also one of the major distinctions of bodies like our moon, and moons like Jupiter’s big 4, and so on. So if we wanted to make a clear distinction between moons like “the moon,” and ordinary natural satellites, we could stipulate that a moon is a planet-like body that orbits a baricenter that is inside of another planet or dwarf planet. But I don’t believe the IAU has adopted this stance. \*IIRC, the other qualifiers for the planet are that it orbits a star directly, or the baricenter of a multiple-star solar system (can’t orbit another planet or other body), must have cleared it’s orbital path (so Ceres, Eris, and Pluto don’t make it). There isn’t a need for a functional size requirement because of the equilibrium point. But, that’s only my understanding. [Next page](https://boards.straightdope.com/t/only-mercury-and-venus-have-no-moons-how-come/387935.md?page=2)