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My desktop background rotates through a bunch of space and nature scenes, and this one came up.

image of planet

What would be the effect on people walking around down here, if another earth was looming overhead instead of our moon.

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Hello Lakey, Good show with the Avatar image..! –  Waffle's Crazy Peanut Oct 2 '12 at 4:36
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Good point @CrazyBuddy, looking up the fan wiki, I discovered Pandora (from Avatar) is supposed to be the 5th moon of a gas giant. The question, however, is about another planet "looming overhead", implying the two planets orbit each other. The answer is different for multiple gas giant moons vs a super huge moon. –  AlanSE Oct 2 '12 at 4:52
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It's very hard to estimate the size and distance of the planet without knowing the field of vision of the (notional) camera - it could take up a very small part of the sky but be taken with a telephoto lens. –  Nathaniel Oct 2 '12 at 13:55
    
Nathaniel, I simplified the question to just assume it was another planet the size of the earth. Thank you –  Lakey Oct 3 '12 at 1:42
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1 Answer

up vote 6 down vote accepted

The main problem with being near a large planet are the tidal forces produced by its gravity. The Moon is pretty small compared to the Earth, and it's a quarter of a million miles away, but it still produces large movements in sea level i.e. the tides. If you imagine the moon getting bigger and closer the tidal effects would increase. The land feels the same tidal forces as the sea, though being solid it moves much less. However tidal deformations of the land generate heat due to viscous losses.

The classic example of this is the moon Io. This orbits Jupiter at about the same distance as the moon orbits the Earth, but of course Jupiter is much bigger than the Earth. The tidal effects on Io are so big that they heat the body of Io, and it's the most volcanically active body in the Solar System. If Earth orbited Jupiter at the same distance as Io we wouldn't be around to admire the view :-)

There is a distance, called the Roche limit, within which the tidal forces are so great that they pull the moon to bits. The Roche limit for Jupiter is about 50,000 miles so Io is safe (if rather hot).

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Hi John, I went through your answer. But as the user is asking for some giant planet, doesn't the Earth-moon system revolve around it and get tidally locked..? –  Waffle's Crazy Peanut Oct 2 '12 at 13:24
    
Doesn't the Roche limit depend on orbital velocity..? I mean, Once the two bodies attain tidal-locking, could the smaller one still attain the Roche limit..? –  Waffle's Crazy Peanut Oct 2 '12 at 13:52
    
The Roche limit is purely tidal forces. For example the Moon is held together by its gravity. If you bring it close enough for the Earth's tidal forces to exert a bigger pull on the Moon's crust than the Moon's own gravity does then the Moon will start falling to bits. –  John Rennie Oct 2 '12 at 17:49
    
So if, instead of a moon, we had another earth near by, it would become very hot due to tidal forces? –  Lakey Oct 3 '12 at 1:33
    
@JohnRennie: No John, Actually I meant, "Could this limit be attained naturally by an orbiting celestial body at any period or else does it depends on some other external factors..?" –  Waffle's Crazy Peanut Oct 3 '12 at 1:38
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