# Is there any way for a planet orbiting a red dwarf in the habitable zone to not be tidally locked?

Is there any way to avoid the tidal locking of a planet orbiting a red dwarf in the habitable zone?

For example, could a planet with a 90° obliquity and large moon avoid such a situation?

• BTW I don't think tidally locked terrestrial planets orbiting in a red dwarf star's habitable zone would be completely uninhabitable. The planet will still be rotating with a period equal to its orbital period, which for red dwarfs is on the order of several days or weeks. This seems fast enough for atmospheric circulation to redistribute heat and prevent the atmosphere freezing out on the night side. Jul 6 '20 at 15:06

## 4 Answers

Leconte et al. (2015) suggested that the presence of an atmosphere could prevent or at least slow tidal locking. The star should exert two separate torques: one on the atmosphere and one on the solid body of the planet: $$T_a=-\frac{3}{2}K_ab_a(2\omega-2n),\quad T_g=-\frac{3}{2}K_gb_g(2\omega-2n)$$ where $$K_a\equiv\frac{3M_*R_p^3}{5\bar{\rho}a^3},\quad K_g\equiv\frac{GM_*R_p^5}{a^6}$$ for stellar mass $M_*$, planetary radius $R_p$, mean density $\rho$, semi-major axis $a$, mean motion $n$, rotation rate $\omega$, and response to torques $b_a$ and $b_g$. The two torques could be equal, and assuming that the atmosphere transfers some angular momentum to the surface of the planet, this could prevent tidal locking. There are several equilibria at which this could occur:

• Would I be wrong to say this is very possibly occurring on Venus? Feb 3 '17 at 17:25

Yes: It has a companion planet or an excessively large moon, with the two bodies orbiting their common center of mass (much like the Earth and the Moon). They could be tidally-locked to each other, but they cannot be tidally-locked to their star.

• Can you provide some evidence for why this would work? Why couldn't it be that each body becomes tidally locked with the star first, thus preventing the planets from being tidally locked with one another? Nov 13 '16 at 19:57
• I don't believe this answer is deserving of being the best answer. It does not attempt to validate or cite references for its statements. After all, anyone can say anything on the internet. I would prefer more proof for the statements above. Nov 14 '16 at 14:17
• @JDługosz Iván can't edit yet.
– Tim
Feb 3 '17 at 21:18
• I thought anyone can edit; it just goes to the review queue. Feb 4 '17 at 5:11

The more likely case is actually a spin-orbit resonance that is not 1:1 but a half odd multiple, like the 3:2 case of our own Mercury. Having eccentricity in the orbit encourages this situation.

I’ve been meaning to write this up on the Worldbuilding.SE but I have not re-found enough references. But see this video.

• very exciting video. U should have posted that information on WB, what if I didn't looked here? Nov 19 '16 at 12:23
• Mercury is a great example, and around a red-dwarf with a shorter orbital period (like the 11 day period of proxima centauri b), with a mercury orbit that would be 264 hours of sun, 264 hours of night. Not ideal, but a happy medium between Earth's 12 hours of sunlight and Mercury's several months. Feb 3 '17 at 17:33

If the star is large enough and the habitable zone is able to extend quite a bit outwards and the planet orbits the outer edge of that habitable zone then yes.