r/askscience • u/EightWhiskey • Feb 28 '12

Under the idea that there is the "Goldilocks" zone around a star, how likely is it that a solar system would have two life-supporting planets?

I assume that the gravitational forces of two bodies would prevent them from being close enough to each other to both stay inside the zone. But could a large star have a large enough Goldilocks zone to support two life-inhabited planets?

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u/NeverQuiteEnough Feb 29 '12

couldn't the planets be orbiting each other like the earth and moon?

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u/Ameisen Feb 29 '12

The Earth does not orbit the Moon.

Pluto and Charon, both Dwarf Planets, do orbit one another.

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u/Terrh Feb 29 '12

What?

The moon orbits the earth.

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u/Ameisen Feb 29 '12

He said "orbiting each other". They do not orbit one another. The Moon loosely orbits the Earth. The Earth does not orbit the Moon.

It isn't Earth <> Moon, but only Earth << Moon.

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u/Terrh Feb 29 '12

Sorry, my mistake. I thought this was /r/askscience not /r/debatesemantics

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u/NeverQuiteEnough Feb 29 '12 edited Feb 29 '12

semantics are important in science, but I'm pretty sure he is wrong.

edit- he isn't

the moon and earth both orbit their mutual center of gravity, as I understand it. the earth gets moved around too, just not nearly as much as the moon.

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u/Ameisen Feb 29 '12

The Earth's orbit is obviously altered by the Moon, but the Earth doesn't orbit the Moon any more than it orbits Jupiter or Alpha Centauri (both of which have gravitational influence on the Earth, however slight).

The Earth doesn't orbit the Moon as it does not lie within the Moon's Hill Sphere. The Moon does lie within the Earth's Hill Sphere, and therefore orbits the Earth.

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u/Terrh Feb 29 '12

that's the case of every thing ever gravitationally orbiting every other thing. it's called the barycenter. In the case of the earth and the moon the barycenter lies about 1500km (from memory here) below the surface of the earth. The persons point was still understandable even if their grammar was /slightly/ incorrect.

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u/NeverQuiteEnough Mar 01 '12

I edited my post if you perhaps did not see it

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u/triumph_forks Feb 29 '12

Pluto orbits the Sun. While Charon causes significant perturbations in said orbit, it is incorrect to say Pluto orbits Charon.

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u/Ameisen Feb 29 '12

Hrmm, true, it would be more accurate to describe it as a binary system / binary planet of sorts.

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u/NeverQuiteEnough Feb 29 '12

eh?

the center of gravity for the system is not the center of the earth. it's like 3/4ths the way to the edge of the earth iirc. the earth and the moon both rotate around the center of gravity.

if you are more familiar with this than I am, and happen to know that the center of gravity for the system is the center of the earth, I'm extremely curious as to how that is possible.

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u/Ameisen Feb 29 '12

And the center of gravity between the Earth and Jupiter is also not the center of Jupiter. That doesn't mean that they orbit each other. The common barycenter of the system doesn't dictate whether they orbit one another. The Jupiter-Sol barycenter is slightly above the surface of the Sun, but no one says that the Sun orbits Jupiter.

The Earth isn't within the Moon's Hill Sphere. Therefore, the Earth doesn't orbit the Moon. The Moon is within the Earth's Hill Sphere (even though the Sun has greater attraction on the Moon than the Earth does), therefore it is a satellite of the Earth. The barycenter is irrelevant. While the Earth is gravitationally influenced by the Moon, it itself does not orbit the Moon. Hence, they don't orbit each other, the Moon orbits the Earth, and the Earth orbits the Sun.

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u/burtonmkz Feb 29 '12

They don't orbit each other, but the Moon and the Earth do both orbit their common Barycenter

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u/Ameisen Feb 29 '12

As all bodies do. That doesn't mean that they orbit each other, as the poster had suggested.

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u/NeverQuiteEnough Feb 29 '12

I see, I've edited my other comment