<p>(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 <a href="https://iopscience.iop.org/article/10.1088/0004-637X/810/1/12" rel="nofollow noreferrer">Makarov (2015)</a>.</p>
<p>For example, in the system TRAPPIST-1, the planets <em>b, d,</em> and <em>e</em> 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 <em>b</em> may be captured in a stable <em>pseudosynchronous</em> rotation <a href="https://iopscience.iop.org/article/10.3847/1538-4357/aab845" rel="nofollow noreferrer">(Makarov et al 2018)</a>.</p>
<p>To conclude, while <em>temporary</em> 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.</p>
<p>(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 <span class="math-container">$k_2/Q$</span> close to that of our own Jupiter.</p>

CC BY-SA 4.0
2024-05-13T00:39:44.360Z
2024-05-13T00:39:44.360Z
Michael_1812
<p><a href="http://arxiv.org/abs/1208.0814" rel="noreferrer">GJ 581d</a> and <a href="http://arxiv.org/abs/1311.4831" rel="noreferrer">GJ 667c</a> are candidates for showing spin-orbit resonance.
<a href="http://gravitation.web.ua.pt/sites/gravitation.web.ua.pt/files/Colluci08102012.pdf" rel="noreferrer">This paper</a> mentions GJ 3634b and 55 Cnc b as two further candidates.</p>

<p>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.</p>

CC BY-SA 4.0
2023-06-17T15:02:20.893Z
Based on newer researches GJ 581d does not exist
Snack Exchange
CC BY-SA 4.0
2024-05-13T00:19:35.200Z
@SnackExchange After all, GJ 581d [seems to exist](https://iopscience.iop.org/article/10.3847/2515-5172/ad1de4)
Michael_1812
CC BY-SA 4.0
2024-06-17T07:09:15.840Z
@Michael_1812 That paper was published 7 months after my comment. The planet was considered non-existent when I wrote here.
Snack Exchange
CC BY-SA 4.0
2024-06-17T12:46:43.807Z
@SnackExchange Understood. Thank you.
Michael_1812
CC BY-SA 3.0
2014-02-20T19:22:46.993Z
Isn't that a non-trivial version of being tidally locked?
Marc
CC BY-SA 3.0
2014-02-20T20:47:44.647Z
@Marc It's kind of tidally locked.
Gerald
CC BY-SA 3.0
2014-02-20T17:35:01.297Z
2014-02-20T17:35:01.297Z
Gerald
<p>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?</p>

<p>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?</p>

CC BY-SA 3.0
2014-02-19T14:30:50.570Z
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
CC BY-SA 3.0
2014-02-19T15:12:56.107Z
@astromax Regardless this is a valid question, but your comment may be the best answer we have for now.
called2voyage
CC BY-SA 3.0
2014-02-19T15:39:16.640Z
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
CC BY-SA 3.0
2014-02-19T17:52:22.733Z
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
CC BY-SA 3.0
2014-02-19T20:56:53.263Z
While we don't know for sure if there are tidally locked exoplanets (probability is high, though), there is at least one proof of star tidally locked to its planet, tau Boo: http://www.space.com/1108-role-reversal-planet-controls-star.html
OnoSendai
CC BY-SA 3.0
2014-02-19T20:59:40.730Z
The closer a planet is to the sun, and the fewer other gravitational influences, the sooner a planet will become tidally locked.
Marc
CC BY-SA 3.0
2014-02-20T09:16:42.590Z
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
<exoplanet><tidal-forces><mercury>
Are many exoplanets nonsynchronously tidally locked like Mercury?
LocalFluff
# Are many exoplanets nonsynchronously tidally locked like Mercury?

(Asked by: username_0 on 2014-02-19)

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?

**Tags:** exoplanet, tidal-forces, mercury

> **Comments:**
> **username_1**: 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.  
> **username_2**: @username_1 Regardless this is a valid question, but your comment may be the best answer we have for now.  
> **username_0**: 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?  
> **username_3**: 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.  
> **username_4**: While we don't know for sure if there are tidally locked exoplanets (probability is high, though), there is at least one proof of star tidally locked to its planet, tau Boo: http://www.space.com/1108-role-reversal-planet-controls-star.html  
> **username_5**: The closer a planet is to the sun, and the fewer other gravitational influences, the sooner a planet will become tidally locked.  
> **username_0**: 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.  

---

## 2 answers

### Answer by username_3 on 2014-02-20. Score: 6 | Accepted answer

[GJ 581d](http://arxiv.org/abs/1208.0814) and [GJ 667c](http://arxiv.org/abs/1311.4831) are candidates for showing spin-orbit resonance.
[This paper](http://gravitation.web.ua.pt/sites/gravitation.web.ua.pt/files/Colluci08102012.pdf) 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.

> **Comments:**
> **username_5**: Isn't that a non-trivial version of being tidally locked?  
> **username_3**: @username_5 It's kind of tidally locked.  
> **username_7**: Based on newer researches GJ 581d does not exist  
> **username_6**: @SnackExchange After all, GJ 581d [seems to exist](https://iopscience.iop.org/article/10.3847/2515-5172/ad1de4)  
> **username_7**: @username_6 That paper was published 7 months after my comment. The planet was considered non-existent when I wrote here.  
> **username_6**: @SnackExchange Understood. Thank you.  

---

### Answer by username_6 on 2024-05-13. Score: 0

(1) While I agree with all said by username_3 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)](https://iopscience.iop.org/article/10.1088/0004-637X/810/1/12).

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)](https://iopscience.iop.org/article/10.3847/1538-4357/aab845).

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.


---

