The only tidally locked planet in the Solar system is 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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    $\begingroup$ 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. $\endgroup$
    – astromax
    Feb 19, 2014 at 14:30
  • $\begingroup$ @astromax Regardless this is a valid question, but your comment may be the best answer we have for now. $\endgroup$
    – called2voyage
    Feb 19, 2014 at 15:12
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    $\begingroup$ 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. $\endgroup$
    – Gerald
    Feb 19, 2014 at 17:52
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    $\begingroup$ The closer a planet is to the sun, and the fewer other gravitational influences, the sooner a planet will become tidally locked. $\endgroup$
    – Marc
    Feb 19, 2014 at 20:59
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    $\begingroup$ 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. $\endgroup$
    – LocalFluff
    Feb 20, 2014 at 9:16

2 Answers 2


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.

  • $\begingroup$ Isn't that a non-trivial version of being tidally locked? $\endgroup$
    – Marc
    Feb 20, 2014 at 19:22
  • $\begingroup$ @Marc It's kind of tidally locked. $\endgroup$
    – Gerald
    Feb 20, 2014 at 20:47
  • $\begingroup$ Based on newer researches GJ 581d does not exist $\endgroup$ Jun 17, 2023 at 15:02
  • $\begingroup$ @SnackExchange After all, GJ 581d seems to exist $\endgroup$ May 13 at 0:19

(1) While I agree with all said by Gerald in his answer, I have to add an important detail that largely changes the answer. When a close-in terrestrial planet is captured into a higher spin-orbit resonance, it has a tidal bulge running over the planet's circumference. For a really close planet (closer than Mercury to the Sun), this results in gradual tidal overheating. Consequently, the planet changes its rheology, and leaves the tidal trap, and continues its despinning toward spin-orbit synchronism. This process is discussed in detail by Makarov (2015).

For example, in the system TRAPPIST-1, the planets b, d, and e were captured in the 3:2 or higher spin–orbit resonances during the initial spin-down, but slipped further down into the 1:1 resonance. Depending on its rheology, the innermost planet b may be captured in a stable pseudosynchronous rotation (Makarov et al 2018).

To conclude, while temporary capture into higher spin-orbit states of close-in terrestrial planets with appreciable eccentricities may be quite common, many of them eventually slip out of those states and get synchronised or pseudosynchronised. So this phenomenon may not be as common as one may expect it to be.

(2) It has been demonstrated that the tidal decay of the hot Jupiters WASP-12b and Kepler-1658b cannot be produced by tidal dissipation in their host stars. Nor can it be explained by tidal dissipation in these planets, if these planets are synchronised. On the other hand, an assumption that these planets are in a higher spin state explains the measured rate of tidal decay, and endows the planets with values of $k_2/Q$ close to that of our own Jupiter.


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