Could an earth-sized planet have 2 moons?

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Could an earth-sized planet have 2 moons?

Post #1by starforce84 » 16.04.2007, 21:25

Based on what I've been reading, it sounds like the earth couldn't support a second natural satellite outside of the moon's orbit unless the second moon had a retrograde orbit. Is that correct? (I'm also assuming a second moon inside the current moon's orbit wouldn't work either.)

Would there be any other scenario (if you changed the moon's orbital radius, mass, inclination, etc. or if you slightly changed the earth's mass, distance from the sun, etc.) that would allow earth to handle 2 moons?

As a corollary -- is there a resource (book, website, etc.) that would allow me to investigate the physics involved (maybe tinker with some numbers and formulas) so I could get a better grasp on the concepts involved in a question of this nature?

Thank you!

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Post #2by selden » 16.04.2007, 21:31

I can't answer your question directly, but you might take a look at Tony Dunn's gravity simulator at

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Post #3by Hungry4info » 17.04.2007, 23:31

Hmm. The moon is far outside the roche limit. I don't see why a small moon couldn't exist within the orbit of the moon. Mars has two moons, why can't Earth?

Maybe Earth's moon, being as massive as it is, would prevent the long-term stability of a satellite within it's orbit.
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Post #4by Colin_hutcheson » 20.04.2007, 12:48

I'm assuming you mean a moon of similar mass to our Moon. Otherwise you could argue that the Earth already has thousands of moons, albeit artificial!
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Post #5by ajtribick » 20.04.2007, 13:49

Depends on a lot of factors. If we're going for large moons, then it seems that you'd have to have a big collision - the discs around terrestrial planets probably are not substantial. Problem is, a moon-forming collision would almost certainly change the rotation of the planet - which could potentially change the direction the tidal evolution of the moons goes.

If the result is that both moons end up below the geostationary orbit (1), or in retrograde orbits (2), they would tend to head downwards and get ripped apart at the Roche limit/collide with the planet.

The first moon-forming collision would probably get the planet rotating in the direction of orbit of the first moon. Let's assume it survives in a prograde orbit above the Roche limit and starts heading outwards. Then a second moon-forming impact occurs - the new moon forms inside the orbit of the old one. If the second collision is in the same direction as the planet, it would speed it up further, which might cause situation (1) - one or both of the moons may end up heading down - best case is that you end up with one moon. A collision in the reverse direction could cause situation (2) - even though the inner moon may be heading outwards, the outer moon starts heading inwards, which would destabilise/destroy the system - instead of getting two moons, you end up with none.

I'd guess the fraction of terrestrial planets with more than one large moon is very small - requires very specific formation history. In fact, one suggestion for why Venus doesn't have a moon is that it had two moon-forming collisions, which resulted in both moons getting destroyed. Testing this idea would be very difficult though.

Capturing moons is also tricky - a two-body encounter (planet+asteroid) wouldn't work, so you'd probably need some kind of binary asteroid - one member of the binary asteroid gets captured into orbit around the planet, one gets ejected. Do this a few times and you might end up with a system of small moons orbiting the planet.

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Post #6by tony873004 » 22.04.2007, 22:28

The biggest problem with a natural moon interior to the Moon's orbit is that it is theorized that the Moon was formed just outside Earth's Roche Limit, and spiraled out to its current position. So it would have swept this region clear.

It's possible that the Earth had more than one moon billions of years ago. But as they spiraled out from tidal forces, moons exterior to the current Moon would have become unstable and escaped. They must have been smaller though, because after escaping, their likely fates would be a collision with the Earth or Moon.

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Post #7by psCargile » 23.04.2007, 01:39

Using Gravity Simulator I placed a small moon 175 km radius and .005 Earth masses at a SMA of 500,000 km. This caused quite some havoc with the Moon's orbit and eventual both orbits became very elliptical with the new moon coming very close to the Earth.

The same moon 350 km radius doesn't remain in orbit very long.

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Post #8by Planet X » 23.04.2007, 13:08

Of course, there's always the possibility of a second trojan satelite sharing the same orbit as Luna. It would have to be either 60 degrees ahead of or behind our moon for the situation to have a chance of reasonable stability, however. Later!


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Post #9by Spaceman Spiff » 23.04.2007, 16:41

Perhaps you should consider when the moons are in resonance. Pluto's moons seem to be in resonant orbits, which helps stability.

What happens with a moon one third the mass and period of our moon, for example?


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Post #10by Planet X » 23.04.2007, 18:18

The 3:1 orbital resonance is actually quite an ideal situation for two relatively massive moons to be in. The outer moon would orbit 2.08 times further out from the parent planet than the inner one. This situation is especially ideal with the less massive moon orbiting closer to the much more massive parent planet. Consequently, it is more influenced by the planet than the outer moon. Finally, the more massive outer moon would be too distant to both have it's own orbit disturbed and disturb the orbit of the inner moon. Later!


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Post #11by MKruer » 24.04.2007, 08:40

I had a stable moon system with a 2:1 ratio, using a heavy inner moon and a lighter outer moon. (the Inner moon was a little over twice as heavy as the outer one) Granted that the main planet was also about 50% more massive then the earth.

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