Can a solar system exist where the second planet rotates fast, and the third planet is tidally locked to their star?

Question

Today I read about the Romulan home star system and it looks like it might be impossible for such a star system to exist.

Memory Alpha describes Remus:

Remus was tidally locked, with one hemisphere always facing its primary while the other always faced away from it. The civilization of the Remans was located on the night side.

Remus is farther from their star than Romulus. Romulus rotates many times during a single orbit of their star while Remus is tidally locked and rotates once per orbit of their star.

If a planet orbits even deeper in the gravity well of their star than Remus it should be even more likely to be tidally locked than plants further out.

Could tidal interactions with neighboring planets and/or its moons prevent a planet from becoming tidally locked while being unable to prevent planets further out from being tidally locked?

Answer 1

The tidal locking timescale depends on several factors: $$\tau_{lock} \approx \frac{0.4 \omega_0 a^5 m Q}{3 G M^2 k_2 r^3}$$ such as the initial spin rate $\omega_0$, the semimajor axis $a$, the mass $m$, the solar mass $M$, the radius $r$ and various dissipation parameters $Q$ and $k_2$.

Two planets that merely differ in $a$ will have the inner one lock much earlier than the outer one. But give them slightly different $\omega_0$, $m$ and especially $r$ and the inner might dissipate more slowly than the outer.

There is also the possibility of spin-orbit resonances. Mercury is in 3:2 resonance, and tidal forces might lock planets into 2:1 or 5:2 resonances. That way the inner planet may be in a resonant lock while the outer is tidally locked 1:1. In fact, Venus is turning very slowly (and retrogradely) so it is not too far off, although the causes of the retrograde spin are unclear.

So, yes, you can have tidally locked planet orbiting outside non-locked planets.

Answer 2

While it's true that planets closer to their star, being under the influence of a much stronger gravitational field than outer planets, are more likely to become tidally locked, that's not a hard and fast law.

There's no a priori reason that a more distant planet couldn't have been formed, e.g., as the collision of two solid but highly dissimilar (in density) objects. The gross asymmetry in circumferential density would enhance tidal lock. But I fear this is veering into WorldBuilding.SE more than astronomy.

Answer 3

The answer is: yes.

An obvious example is a situation where the second planet has a moon so massive that the planet's rotation is synchronised with the moon's orbital motion around the planet.

Answer 4

Our own solar system demonstrates how widely variable the effects can be:

• Mercury, despite being quite close to the sun, isn't synchronously locked. Instead, it has a 3:2 spin-orbit resonance, rotating 3 times every 2 orbits, giving it a solar day of 176 days.
• Venus isn't locked in any way, and actually rotates retrograde with a solar day of 116.75 days.

This isn't a direct counterexample, since the solar day of Mercury is still longer than that of Venus, but Mercury's rotation is stabilized by a resonance with its elliptical orbit, while Venus could very well have ended up being synchronously locked if the circumstances that led to its retrograde rotation turned out a bit different.

A resonance with another inner planet could also help keep the inner world spinning faster, though it might be unlikely to find such an arrangement within the orbit of another planet that itself is close enough to be tidally locked. However, a large moon (or a mutually-locked binary planet arrangement) could also help.