Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Sep 29, 2010 #1
    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?
     
  2. jcsd
  3. Sep 30, 2010 #2
    I am not claiming to understand all the math, but it is based on mass, orbital period and distance from the star.
     
  4. Sep 30, 2010 #3

    Borg

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    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" [Broken]
     
    Last edited by a moderator: May 5, 2017
  5. Sep 30, 2010 #4
    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.
     
    Last edited: Sep 30, 2010
  6. Sep 30, 2010 #5

    Ich

    User Avatar
    Science Advisor

    There are only a few planets - like http://en.wikipedia.org/wiki/HD_189733" [Broken] - 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.
     
    Last edited by a moderator: May 5, 2017
  7. Oct 1, 2010 #6
    Thanks Ich, that's what I was looking for.
     
  8. Oct 2, 2010 #7

    Borg

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Sorry, I just quoted the beginning of the article. The page is the same as the calculations/estimations link that Ich supplied.
     
  9. Nov 1, 2010 #8
    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.

    ============
    BTW: Am I correct in assuming that a tidally locked planet can't have a moon?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook