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Tidally locked planets

17 Mar 2017 17:47

I saw the word "resonance" flash by on the screen a few times while I was scrolling along.. and I wondered... why are so many planets tidally locked?

If planets are in elliptical orbits, how can they remain tidally locked over time? Especially the larger heavier Gas Giants that come in very close then swing way out on the most extreme ellipses? Would not their rotation rates change through out the year? If a planet is in a highly elliptical orbit would not the star grab it and speed it up as it comes in? Slow days when closer and faster days when further out..

I would think also some orbits would freeze and thaw out planets significantly, creating some really weird tidal frictions. I know I have a really hard time remaining still on a waterbed. Also would not other nearby planets prevent tidal locking? I'm sure even tiny rocky worlds close-in to their star would never be perfectly circular in orbits even if in resonance. I wonder if larger moons around planets in Space Engine would prevent any planet at all from being tidally locked to the host star.

Lastly, it seems to me planet systems that are billions of years old would change their motions slightly, without other orbiting bodies influencing them, never staying tidally locked forever.
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Tidally locked planets

17 Mar 2017 18:44

Intuitively, I'd say that the reason for this abundance of resonance or tidal locked planets/stars/objects is that the amount of energy of a system tends to be maintained, so this type of resonances represent an equilibrium from an energetic point of view. The total amount of energy will surely diminish over time, as per entropy rules, but this natural equilibrium could be the best energy profile that a system can reach, leaving entropy a very long time to do its work.
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Tidally locked planets

18 Mar 2017 00:25

Gnargenox wrote:
Source of the post why are so many planets tidally locked?

This is what happens in nature. :) Two nearby masses will distort each other's shapes by their tidal gravity.  If they are not tidally locked, then the gravitational pull on those tidally raised bulges produces a torque which acts to synchronize the spin rate with the orbital rate.  

Think of it this way:  if the planet spins faster than its moon orbits, then its rotation drags the nearest tidal bulge ahead, and therefore the moon's gravity tugs it backwards, which slows the planet's rotation.  (This is what is happening with the Earth and Moon).  The bulge on the Earth is also pulling the Moon forward accelerating it, which expands the orbit outward.  Essentially this is just angular momentum conservation.

The strength of the tidal gravity follows an inverse cube law, while the timescale for tidal locking to occur is proportional to the distance to the 6th power.  So it is a very sensitive function of how far apart the objects are.  This is why moons tend to be tidally locked, while planets generally aren't -- unless they happen to be in a compact system like around a red dwarf star.  Tidal locking around low mass stars is thought to be very common, and Space Engine reflects this.  (Space Engine actually uses a formula to compute the tidal locking).

The eccentricity of the orbit doesn't change the rules for tidal locking very much, though for high eccentricities it may tend to produce a higher order of spin-orbit resonance.  Whether the planet is more solid or liquid doesn't change the fundamental physics very much either, though it does change its efficiency (mainly through the dissipation parameter "Q" in the function given on wikipedia).  The dissipation efficiency with the Earth-Moon system probably changes over geologic time, due to plate tectonics changing the arrangement of landmasses and therefore how the ocean tides "slosh" around the planet.

Gnargenox wrote:
Source of the post Lastly, it seems to me planet systems that are billions of years old would change their motions slightly, without other orbiting bodies influencing them, never staying tidally locked forever.

Maybe, but then you must ask what mechanism would produce the torque to break tidal lock?  Tidal locking itself is a stable equilibrium, since any change in the spin-orbit resonance would produce a counterbalancing torque trying to restore it, much like a spring pulled slightly from equilibrium.  It's a very weak restorative force to be sure, but if you're talking over timescales over billions of years...

Things that can have some effect are massive earthquakes, which redistribute the mass of a planet and therefore the spin rate by conservation of momentum.  But this is a very small effect.  I might imagine on longer timescales that the movement of a mantle plume, or changes in ice coverage across the surface, could act to upset the spin of a planet.  But again these are very small effects and tidal locking will beat them on billion year timescales.

There was a question that one of the planetary scientists at my uni was interested in investigating with some grad students, which was whether terrestrial planets in very close orbits (such that the rocky surface is vaporized on the day side and then freezes out again on the nightside) might involve such a large degree of mass exchange between the day and night side as to prevent tidal locking.  But I'm not sure what the status of that work is.  
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Tidally locked planets

18 Mar 2017 05:02

Watsisname wrote:
Source of the post Maybe, but then you must ask what mechanism would produce the torque to break tidal lock?

Winds on Venus are sometimes considered to be responsible of its non-tidal locking rotation. They produce enough drag to spin up its rotation in retrograde direction. If Venus were has no such dense atmosphere, it would be tidally-locked to the Sun.

Mercury also would be tidally-locked, but its highly eccentric orbit produces 3:2 spin-orbital resonance. Solar tides are 3 times stronger in perihelion than in aphelion, so they controlled its spin rate in that way so it is equal to orbital rate in perihelion. Such non 1:1 resonances will be in 0.981.

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