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The only tidally locked planet in the Solar system i Mercury. But it is synchronously tidally locked 3:2, because of the relatively high eccentricity of its orbit, so doesn't turn the same side towards the Sun. I wonder if this is common for exoplanets?

Many discovered exoplanets are close to their star and must be tidally locked. Has any of them been determined to be synchronously tidally locked, like Mercury? Is Mercury a rare freak in this respect, or a representant of a common phenomenon?

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I think they're still trying to nail down things like orbital parameters and masses - I'm not sure if that level of detail is possible to study just yet. –  astromax Feb 19 at 14:30
    
@astromax Regardless this is a valid question, but your comment may be the best answer we have for now. –  called2voyage Feb 19 at 15:12
    
Are "hot Jupiters" more, or less, likely to be synchronously tidally locked, than are small planets like Mercury? Are planets overall expected to have more, or less, eccentric orbits when they are close to a star? Are there any clear theoretically motivated expectations today? –  LocalFluff Feb 19 at 15:39
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I'm not the time at the moment to look for sources, but what I can say from memory: Other planetary systems can look totally different from ours, including planets with highly excentric orbits; there should be an overview somewhere. For at least of one giant planet close to a star a rough surface map could be reconstructed, exploiting a series of different occultations. –  Gerald Feb 19 at 17:52
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The closer a planet is to the sun, and the fewer other gravitational influences, the sooner a planet will become tidally locked. –  Marc Feb 19 at 20:59

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up vote 2 down vote accepted

GJ 581d and GJ 667c are candidates for showing spin-orbit resonance. This paper mentions GJ 3634b and 55 Cnc b as two further candidates.

Spin-orbit resonances different from 1:1 are expected to be rather common among rocky planets orbiting close to a star. Direct observational evidence is difficult to obtain; results are based on model calculations.

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Isn't that a non-trivial version of being tidally locked? –  Marc Feb 20 at 19:22
    
@Marc It's kind of tidally locked. –  Gerald Feb 20 at 20:47

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