How can you tell that a planet in another star system is tidally locked?

[ladr0n]

My question was piqued by this article: http://news.discovery.com/space/earth-like-planet-life.html

The article claims that the planet discovered is tidally locked, but does not explain how this was determined. Presumably the planet is much too far away to be resolved by any telescope, and its position and mass have been inferred by other means (its periodic effect on the parent star's apparent brightness, etc). How could we determine whether or not a planet like this is tidally locked?

 

[sicarius]

I am not claiming to understand all the math, but it is based on mass, orbital period and distance from the star.

 

[Borg]

From Wikipedia: The change in rotation rate necessary to tidally lock a body B to a larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.
http://en.wikipedia.org/wiki/Tidal_locking"

 

[ladr0n]

Thanks for the answer, Borg, but I do understand what tidal locking is; my question was about how we can detect it in a star system 20 light-years away, since (I am assuming) it would be impossible to resolve the shape of a planet that far away even with our best telescopes.

 

[Ich]

There are only a few planets - like http://en.wikipedia.org/wiki/HD_189733" - where you can derive a circumferential temperature profile to directly confim its tidally locked status.
But there are quite robust http://en.wikipedia.org/wiki/Tidal_locking#Timescale" that say that a large class of planets simply must be tidally locked. This is not observed, it's assumed.

 

[ladr0n]

Thanks Ich, that's what I was looking for.

 

[Borg]

Thanks for the answer, Borg, but I do understand what tidal locking is; my question was about how we can detect it in a star system 20 light-years away, since (I am assuming) it would be impossible to resolve the shape of a planet that far away even with our best telescopes.

Sorry, I just quoted the beginning of the article. The page is the same as the calculations/estimations link that Ich supplied.

 

[Algr]

As I understand it, a planet close enough to a star must be tidally locked because simply being there would cause enough tidal friction to stop rotation in much less then the likely age of the planet. For red dwarf stars, the habitable zone lies inside the tidal lock zone. Planets also have a tidal lock zone for their moons.

We can't really measure Gliese 581g's rotation directly so there is a very slim chance that it (or any exoplanet) may have recently been hit by something big that started it spinning again. But the chance of that is too small to worry about - and it would stop again in a short (astronomically speaking) amount of time.

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BTW: Am I correct in assuming that a tidally locked planet can't have a moon?