A question from the Piper Cub: why do the two planets closest to the Sun not have moons, but all the other planets (and one dwarf planet, for those who have accepted the IAU’s definitions) have moons? I’m guessing it’s something to do with the massive gravitational pull of the Sun, but can anyone clarify?
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.
I would also be interested to know the answer to this. Since no one has given a definitive answer yet, I’ll jump in with a guess and say maybe the Sun makes it much less likely that a big enough rock will end up getting caught in the orbits of Mercury or Venus, or that one will smack into the planet in question to form a moon (as probably happened with Earth’s moon).
In fact, perhaps with Mercury at least, it would be completely unable to keep a moon in orbit without the Sun pulling it away.
I believe Venus did have a moon at one point but it was lost when the planet’s spin changed direction.
Note that our moon was formed by a collision with Earth, but one which neither destroyed our fledgling planet (and/or the moon itself) nor resulted in said moon leaving Earth orbit by virtue of its momentum after the collision. Most of the moons around the outer planets were captured-all perhaps? Phobos and Deimos around Mars were also captured.
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Both Mercury and Venus rotate very slowly, which means that they have “geosynchronous” orbits that are very large.
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Moons that orbit inside the geosynch orbits of their planets experience tidal deceleration (Tidal acceleration - Wikipedia) and over geological ages would tend to crash into their planet (our moon is well outside Earth geosynch orbit, and experiences tidal acceleration, a slower process that does not end with a crash.
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A moon that was outside Mercury or Venus’s geosynch orbit would very likely be outside Mercury or Venus’s “Hill Sphere” (Hill sphere - Wikipedia) - the volume of space in which the planet can successfully keep a moon in orbit (vice losing it to independent orbit around the Sun).
It’s difficult to say with such a small sample size, but Mercury and Venus appear to be the typical case for rocky planets, with Earth and Mars being the unusual ones. Earth’s moon is, so far as anyone can figure, a total fluke: One should not expect other rocky planets with large moons to be common. And Mars has moons by virtue of being so close to the asteroid belt that it’s had a chance to capture a couple.
But really, any definitive answer would require extensive observation of a large number of fairly-selected extrasolar planets. The ones we know of so far have some extreme biases in what we find, and we usually can’t tell enough to determine if they have even large moons (small ones like Phobos and Deimos would be hopeless to detect, with current technology).
I wonder if a double planet (say Venus as Earth’s companion) is possible…
Don’t some people argue that the Earth Moon combination really is a double planet? because the size of the Moon compared to Earth is the biggest satellite : planet ratio in the Solar System?
Which means that there may not be a good reason except random chance. Some planets have moons, some don’t.
Ask The Piper Cub why the sky is blue and why he likes chocolate. 
No, because the common center of orbit (barycenter) of both is inside the Earth. Not so with Pluto/Charon and some other bodies.
Mercury and Venus were revoked of their moons for egregious public indecency. Earth is most certainly pushing this mooning business, but we’re currently being overlooked since the outter planets seem to be flagrantly disregarding the current planetary laws. Especially, well… you know… I hear he’s about to be read his Miranda Rights…
I hear that other one’s double mooning incident was so bad he’s on the hook for causing fear and panic.
Phobos and Deimos also very atypical of what we think of as a “moon.” Really, Earth is the only inner planet with a proper moon.
I am not certain if Luna is really all that ‘proper’.
I saw her the other night and she was acting rather ‘blue’.
She has a real dark side too.
Not only are Mars’ satellites flukes due to being close to the asteroid belt, but they are only temporary. Phobos’ orbit, for example is expected to last a mere 11 million years more. So it’s practically a bit of luck that we have been able to observe it at all, having evolved enough to build telescopes at the right time.
Again, it comes down to the Earth/Moon system being the oddball. Fun fact: the Moon’s orbit around the Sun is so nearly circular that it is convex the whole way.
Do all of the moons in our solar system NOT rotate on their axes like Earth’s does?
For Venus, the Hill Sphere has a radius of about 620,000 miles - and the geosynch radius is about 950,000 miles (if I’ve done the math right), so any moon of Venus will either be lost to the Sun, or experience tidal deceleration that leads it to eventually crash into the planet.
For Mercury, the Hill sphere is about 16,000 miles - and the geosynch radius is 150,000 miles, so so any moon of Mercury will either be lost to the Sun, or experience tidal deceleration that leads it to eventually crash into the planet.
So, it’s not very surprising that we don’t see any moons for these planets - unlike Earth, Jupiter, Saturn, etc., which can hold moons for durations compable to the life of the solar system, Venus and Mercury can’t.
You mean it’s uphill both ways?
Thanks very much for all the replies - very interesting!
Moons are a chance event. Space is vast and the odds of anything coming close to a planet is very small so it isn’t surprising Mercury and Venus lack moons. Venus has a runaway greenhouse going on which a moon could reduce by stripping away some atmosphere. Not that we’ll ever see that.
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.