Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|improve this question
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 '14 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 '14 at 15:12
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 '14 at 17:52
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 '14 at 20:59
Tidally locked planets in eccentric orbits WITHOUT SYNCHRONOUS ROTATION, is what I'm interested in here. I.e planets which rotate relative to their star, although they are tidally locked. Conditions on the surface of such planets should be much more like on non-tidally locked planets, with globally distributred heat. I don't know of any moons in the solar system which have Mercury-like orbit and rotation. – LocalFluff Feb 20 '14 at 9:16
up vote 3 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.

share|improve this answer
Isn't that a non-trivial version of being tidally locked? – Marc Feb 20 '14 at 19:22
@Marc It's kind of tidally locked. – Gerald Feb 20 '14 at 20:47

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.