<p>The orbit of a moon of a close-in planet becomes stable for an indefinite period of time within two scenarios.</p>
<p>In Scenario 1, the moon is captured or formed above synchronism. Orbiting away, like our Moon, it is simultaneously slowing down the planet's rotation rate. If the mutual tidal synchronisation takes place before the moon crosses the reduces Hill radius, the stable configuration is reached and the moons survives infinitely. For this to happen, the moon must be massive (or the planet must be small). The minimal ratio of their masses must obey
<span class="math-container">$$
\frac{M_m}{M_p} &gt; \left(\frac{R_p}{r_H^{\,\prime}}\right)^2\,\;,
$$</span>
<span class="math-container">$R_p$</span> and <span class="math-container">$r_H^{\,\prime}$</span> being the planet's radius and the reduced Hill radius (<a href="https://www.aanda.org/component/article?access=doi&amp;doi=10.1051/0004-6361/202245533" rel="nofollow noreferrer">Makarov &amp; Efroimsky 2023</a>, eqn 52), also available in <a href="https://arxiv.org/abs/2302.04646" rel="nofollow noreferrer">ArXiV</a>.</p>
<p>In Scenario 2, the moon is captured or formed below synchronism. Spiraling downwards, it is reducing the rotation rate of the planet. If the mutual synchronism is attained before the moon crosses the Roche radius, the configuration becomes stable. In this case, again, the ratio <span class="math-container">$M_m/M_p$</span> should not be too small, see Section 9.2 in <em>Ibid</em>.</p>
<p>As an entertaining aside, I would mention that a planet synchronised by its moon will thereby be nonsynchronous with respect to its host star. Now recall that a rotation rate faster than the mean motion works to tidally increase the semimajor axis in the tidal two-body problem. This will prevent a close-in planet from falling on its star. So, if you ever come across a planet, which is very close to its star but is not tidally descending, it may be right to enquire if that planet has a synchronous moon braking its tidal descent. Another possible feature of such a configuration would be the planet's finite eccentricity. (Fast rotators tend to boost <span class="math-container">$e$</span>.)</p>

CC BY-SA 4.0
2023-03-13T21:46:51.173Z
2023-03-15T22:36:44.693Z
2023-03-15T22:36:44.693Z
Michael_1812
<p>This question asks about the phases of a hypothetical moon of Proxima Centauri b.</p>
<p><a href="https://astronomy.stackexchange.com/questions/53112/what-phases-of-a-moon-orbiting-proxima-centauri-b-could-be-seen/53118#53118">What phases of a moon orbiting Proxima Centauri b could be seen?</a></p>
<p>And In my answer I suggest that Proxima Centauri b can't still have any moon it might have had soon after it formed, unless tidal interactions had not pushed that moon out beyond the true region of stability of Proxima Centauri b.</p>
<p>I suggest that a moon of Proxima Centauri b which formed with the same mass as Earth's Moon and orbiting at the same distance from Proxima Centauri b as the Moon from Earth would have been pushed beyond the true region of stability of Proxima Centauri b by now.</p>
<p>So can anyone calculate the ranges for the mass and orbit of a moon still retained within the region of true stability of Proxima Centauri b?</p>

<orbit><orbital-migration>
The possible Masses and Orbits of Hypothetical Moons of Proxima Centauri b
M. A. Golding
# The possible Masses and Orbits of Hypothetical Moons of Proxima Centauri b

(Asked by: username_0 on 2023-03-09)

This question asks about the phases of a hypothetical moon of Proxima Centauri b.

[What phases of a moon orbiting Proxima Centauri b could be seen?](https://astronomy.stackexchange.com/questions/53112/what-phases-of-a-moon-orbiting-proxima-centauri-b-could-be-seen/53118#53118)

And In my answer I suggest that Proxima Centauri b can't still have any moon it might have had soon after it formed, unless tidal interactions had not pushed that moon out beyond the true region of stability of Proxima Centauri b.

I suggest that a moon of Proxima Centauri b which formed with the same mass as Earth's Moon and orbiting at the same distance from Proxima Centauri b as the Moon from Earth would have been pushed beyond the true region of stability of Proxima Centauri b by now.

So can anyone calculate the ranges for the mass and orbit of a moon still retained within the region of true stability of Proxima Centauri b?

**Tags:** orbit, orbital-migration


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## 1 answers

### Answer by username_1 on 2023-03-13. Score: 3

The orbit of a moon of a close-in planet becomes stable for an indefinite period of time within two scenarios.

In Scenario 1, the moon is captured or formed above synchronism. Orbiting away, like our Moon, it is simultaneously slowing down the planet's rotation rate. If the mutual tidal synchronisation takes place before the moon crosses the reduces Hill radius, the stable configuration is reached and the moons survives infinitely. For this to happen, the moon must be massive (or the planet must be small). The minimal ratio of their masses must obey
$$
\frac{M\_m}{M\_p} > \left(\frac{R\_p}{r\_H^{\,\prime}}\right)^2\,\;,
$$
$R\_p$ and $r\_H^{\,\prime}$ being the planet's radius and the reduced Hill radius ([Makarov & Efroimsky 2023](https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202245533), eqn 52), also available in [ArXiV](https://arxiv.org/abs/2302.04646).

In Scenario 2, the moon is captured or formed below synchronism. Spiraling downwards, it is reducing the rotation rate of the planet. If the mutual synchronism is attained before the moon crosses the Roche radius, the configuration becomes stable. In this case, again, the ratio $M\_m/M\_p$ should not be too small, see Section 9.2 in *Ibid*.

As an entertaining aside, I would mention that a planet synchronised by its moon will thereby be nonsynchronous with respect to its host star. Now recall that a rotation rate faster than the mean motion works to tidally increase the semimajor axis in the tidal two-body problem. This will prevent a close-in planet from falling on its star. So, if you ever come across a planet, which is very close to its star but is not tidally descending, it may be right to enquire if that planet has a synchronous moon braking its tidal descent. Another possible feature of such a configuration would be the planet's finite eccentricity. (Fast rotators tend to boost $e$.)


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