How big could a moon be before its orbit became too unstable or tidally locked to a planet about 1.6 times larger than Earth.

Planet BlinketyBlink is on a similar orbit (to that of earth's) around its sun, its landmass is 12% to 80% water and 8% ice.

Alternatively, if the Moon is about 27% the size of Earth, how large would it have to be before it became unstable or tidally locked?

I need a large moon that is not tidally locked (yet).

How large could it be?

My world is younger than Earth about it's age when Pangea was around, and its moon as well I guess, it doesn't really matter. I just need my planet's atmosphere and weather conditions to be similar to that of Earth's right now, but the moon to not be tidally locked and be larger than ours.


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    $\begingroup$ Tidal locking is inevitable. Size just dictates how long it takes. $\endgroup$ – Joe Bloggs Sep 30 '17 at 14:23
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    $\begingroup$ I think the confusion here is that tidal locking takes time as part of it's definition. Our moon is already tidally locked, because it's been around long enough that whatever it's initial spin was has been changed to match its orbital period. If our moon had been 27% larger when it formed it would have become tidally locked sooner if it had the same initial angular momentum. $\endgroup$ – Joe Bloggs Sep 30 '17 at 15:09
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    $\begingroup$ Add the age etc to the question and I'm sure we can work something out. :-) $\endgroup$ – Joe Bloggs Sep 30 '17 at 15:25
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    $\begingroup$ Size won’t make a moon's orbit unstable. As it gets bigger, it becomes a double planet rather than planet/moon, and bigger still and it becomes the planet, but all those configurations can have orbital stability. $\endgroup$ – Mike Scott Sep 30 '17 at 15:29
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    $\begingroup$ Not an issue, often working out the right way to ask a question is just as difficult as answering the question. $\endgroup$ – Joe Bloggs Sep 30 '17 at 16:21

A planet and its moon both rotate about their combined centre of gravity or barycentre. In the case of the Earth Moon system the barycentre is quite close to the Earth as the Earth is so much more massive than the Moon. But if the mass of the Moon were to increase the barycentre would migrate away from the Earth towards the Moon.

Earth - Moon

If the Moon were the same mass as the Earth the barycentre would fall exactly between the Earth and the Moon. In this situation the system would better be described as a binary planet. If our Moon were more massive than the Earth, our Moon would be classified as a planet and the Earth would be classified as a moon. So to answer your question just under 1.6 times the mass of the earth (assuming mass rather than size) any bigger than that and it would no longer be considered to be the moon, it would be the planet instead.

If a planet were to gravitationally capture a moon which already had a significant spin, tidal forces could take millions or billions of years to slow it down, depending on the detailed circumstances. Moons in more distant orbits would take longer to become tidally locked.

  • $\begingroup$ +1 for noting spin speed is more important than size for tidal locking. $\endgroup$ – Joe Bloggs Sep 30 '17 at 16:22
  • $\begingroup$ Is there a formula to calculate the things I need to know? I will enjoy spending a few days working this out :P Or throw my note pad through the window and just write that the moon was very large... $\endgroup$ – shieldedtulip Sep 30 '17 at 16:26
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    $\begingroup$ I think you might find this post of interest: astronomy.stackexchange.com/questions/1859/… Especially the last paragraph. Your primary concern is going to be how close the moon needs to be. If its orbit is further out, the tidal locking will occur more slowly. Do you want to do the exact calculations? $\endgroup$ – Slarty Sep 30 '17 at 16:36
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    $\begingroup$ The calculation you need can be found here, but there are a number of "fiddle factors" en.wikipedia.org/wiki/Tidal_locking#Timescale another factor to think about is that the Earths moon has been spiraling away from earth since it formed, so it was originaly a lot closer (and would have slowed down much faster) $\endgroup$ – Slarty Sep 30 '17 at 16:45
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    $\begingroup$ Yes a closer moon would appear much larger. The primary effect of a much closer moon would be bigger tides. Would this pose a problem? A very large and very close moon would create huge tidal forces. You can use this calculator to find the size of the effects of any particular arrangement: keisan.casio.com/exec/system/1360312100 Another effect would (depending on the precise orbit) probably be far more eclipses each covering a wider area and lasting for longer. In extremis this could cause problems as it would reduce light levels significantly. $\endgroup$ – Slarty Sep 30 '17 at 17:46

Tidal braking will take always place; tidal locking will be a matter of time. As another answer said, you could have a moon that is not tidally locked if it was recently captured by the planet (meaning a few tens of millions of years ago, probably). Since a rogue object can come from anywhere, recently captured objects tend to have remarkable orbits, often very excentric and rather tilted with respect to the plane of the planet's equator. In time these things tend to become more "normal"; tidal braking in particular tends to circularize orbits and to shift them towards the equatorial plane of the planet.

One thing you need to consider is that tidal braking is produced by dissipation of rotational energy, which is most intense at the beginning of the process. Imagine a car rolling downhill with the driver always applying the brakes. In the case of your captured moon, this energy that is being dissipated as the moon brakes will probably melt it (as is theorized to have happened to Triton after being captured by Neptune), or at least turn it into a very active place (as is happening to Io).

A large, close-in satellite will not necessarily make your planet a worse place to live, but it will certainly have important effects. Tides will be stronger, and of course your planet will also suffer from tidal heating, possibly becoming more active geologically, since both bodies (planet and satellite) will be working toward tidally locking the other.


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