planet satellites?

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noozoo
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planet satellites?

Postby noozoo » Thu Feb 23, 2017 6:13 pm

Among the 3000+ exoplanets discovered, have they found any which have two (or more) equal sized planets orbiting each other rather than a smaller satellite orbiting a larger planet?

Just curious, as it occurs to me this could occur. But does it? Earth and the Moon are probably the best examples of this close by, but they are still quite different in mass.

BDanielMayfield
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Re: planet satellites?

Postby BDanielMayfield » Thu Feb 23, 2017 7:19 pm

No. In fact, I don't think even any exo-moons have been found yet, though surely they must exist. Even at least a few pairs of near massed exoplanets should be found in time, since binary star systems with near masses are not uncommon.

In our system the best example of a double planet is Pluto and it's large moon Charon.

Bruce
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neufer
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Re: planet satellites?

Postby neufer » Thu Feb 23, 2017 11:06 pm

BDanielMayfield wrote:
I don't think even any exo-moons have been found yet, though surely they must exist. Even at least a few pairs of near massed exoplanets should be found in time, since binary star systems with near masses are not uncommon.

In our system the best example of a double planet is Pluto and it's large moon Charon.

Almost all known exoplanets are tidally locked (or nearly so: a la Mercury & Venus) around their suns.

It is difficult for a tidally locked (or nearly tidally locked) planet to maintain a moon
(other than at their L4 or L5 Lagrangian points).

https://en.wikipedia.org/wiki/Exomoon wrote:
<<Planets close to their stars on circular orbits will tend to despin and become tidally locked. As the planet's rotation slows down the radius of a synchronous orbit of the planet moves outwards from the planet. For planets tidally locked to their stars, the distance from the planet at which the moon will be in a synchronous orbit around the planet is outside the Hill sphere of the planet. The Hill sphere of the planet is the region where its gravity dominates that of the star so it can hold on to its moons. Moons inside the synchronous orbit radius of a planet will spiral into the planet. Therefore, if the synchronous orbit is outside the Hill sphere, then all moons will spiral into the planet. If the synchronous orbit is not three-body stable then moons outside this radius will escape orbit before they reach the synchronous orbit.>>
Art Neuendorffer

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Re: planet satellites?

Postby BDanielMayfield » Fri Feb 24, 2017 12:05 am

neufer wrote:
BDanielMayfield wrote:
I don't think even any exo-moons have been found yet, though surely they must exist. Even at least a few pairs of near massed exoplanets should be found in time, since binary star systems with near masses are not uncommon.

In our system the best example of a double planet is Pluto and it's large moon Charon.

Almost all known exoplanets are tidally locked (or nearly so: a la Mercury & Venus) around their suns.

It is difficult for a tidally locked (or nearly tidally locked) planet to maintain a moon


Your point depends on the huge selection bias reflected in current exoplanet findings. Close in planets are easier to find. Sure, close in planets aren't likely to have moons. But most of the planets in our system aren't close in, and these non close in planets do have moons. Are you suggesting that our system with its numerous moons is a rare outlier?
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neufer
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Re: planet satellites?

Postby neufer » Fri Feb 24, 2017 12:32 am

BDanielMayfield wrote:
neufer wrote:
BDanielMayfield wrote:
I don't think even any exo-moons have been found yet, though surely they must exist. Even at least a few pairs of near massed exoplanets should be found in time, since binary star systems with near masses are not uncommon.

In our system the best example of a double planet is Pluto and it's large moon Charon.

Almost all known exoplanets are tidally locked (or nearly so: a la Mercury & Venus) around their suns.

It is difficult for a tidally locked (or nearly tidally locked) planet to maintain a moon


Your point depends on the huge selection bias reflected in current exoplanet findings. Close in planets are easier to find. Sure, close in planets aren't likely to have moons. But most of the planets in our system aren't close in, and these non close in planets do have moons. Are you suggesting that our system with its numerous moons is a rare outlier?

I'm agreeing with your original statement by adding more detail.
Art Neuendorffer

BDanielMayfield
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Re: planet satellites?

Postby BDanielMayfield » Fri Feb 24, 2017 1:01 am

Ah, sorry that I read too much into your post. We are in accord. Copernicus can Rest In Peace.

noozoo, hopefully the above exchange wasn't too confusing. Fire away if you still have questions.

Bruce
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