In a lot of the planned videos for this channel on all the atypical planets or other objects
which might host life, or might be terraformed to host life, we’re going to be talking
about tidal locking a lot. I didn’t see any videos around that gave a good, quick,
and graphic explanation of how tidal locking actually happens in the first place so we’ll
delve into it real quick. Incidentally if you’re new to my videos and have some problems
understanding me you might want to turn on the closed captions for the video.
You’ve probably heard a lot of references to tidal locking of planets or moons over
the years but not gotten an explanation for how it happens, just what the final effect
is. Moons or planets just do it and it has something to do with tides.
But what makes a moon slow down so it always has the same side pointing at the planet below?
If you’re not familiar with tidal locking, it is when a body orbiting another body, like
a moon around a planet or a planet around a star, orbits in a fashion that its own day
and year are equal in length. It spins around its own axis just once for every time it orbits
around the primary body. So it always shows the same face to those looking at it from
that primary body. How does this happen though?
To begin with, any two bodies orbiting together exert tidal forces on each other. That is
to say, since gravity gets weaker with distance, there’s always a slightly stronger tug on
the sides of these objects facing each other. This distorts the bodies, stretching them
toward each other, and gives us tides. This is often called Tidal Flexing.
But that’s not the end of the story because these objects are spinning. Let’s picture
our moon before it was tidally locked and had a fairly regular day length. And let’s
pin the earth and moon in place to keep things simple.
As the moon stretches it is still spinning, and materials take time to stretch and sag,
so our bump isn’t going to be pointing straight at the planet. It will have spun a bit off
center. Now that the bulge is off center the force
of gravity being exerted on it isn’t symmetric anymore. That bump gets pulled on and in the
opposite direction the moon is trying to spin. This is called tidal friction, and is no different
than putting a brake on a wheel. In time the moon slows down… but it never
stops completely spinning because once it’s year and day are the same, once it orbits
once for every time its spin on its axis, that stretched bump does point straight down
at the planet all the time and isn’t off center anymore. So our frictional force goes
away. That’s tidal locking, it is just that simple.
And the larger body gets the same effect just much weaker and slower. Earth has an off-center
bump too it’s just a weaker effect. This is why Earth’s day was originally about
12 hours long when the moon first formed and is now double that, and the moon has become
tidally locked. Eventually, given many billions of years, Earth would slow until not only
did the moon always show us the same side but we showed it the same side as well. The
moon would always be visible to one hemisphere of the planet but not the other though it
would still wax and wane in that first hemisphere since moon phases have to do with which side
the moon is pointing at the sun. There is no dark side of the moon, just a side we cant’s
see. And you can have this happen to a planet with
the star it orbits too. But not for the star because they aren’t rigid objects, they
experience some tidal friction from their planets but they have no faces, so to speak.
They do slow down though, just not as much. Same for a gas giant or a world made entirely
or primarily of water. Incidentally that rotational energy that’s getting removed to slow the
spin of the objects dissipates as heat, frequently a lot of heat, and that’s what is meant
by tidal heating. You can also get this from forces exerted by other moons since many planets
have several, and from the eccentricity of a moon’s orbit, since things don’t usually
orbit in perfect circles. Now let’s look at the math real quick.
Yugh right? Well there’s a simplified form that works
decently, but this isn’t a math lesson. What we’re interested in here are the factors
that control the time tidal locking takes. The composition of the planet, in terms of
its rigidity, matters because water or ice stretch differently than rock or metal. The
mass of the bodies is important. But what really matters is the mass of the primary
body and even more the distance between the objects. Bigger planets, or bigger stars,
lock there satellites faster, but the single biggest variable is the distance. Double the
primaries mass and locking happens 4 times faster, triple it and nine times faster. But
doubling the distance makes it take 64 times longer and tripling it makes it take nearly
a thousand times longer. Mercury is in a partial tidal lock to the sun, a 3:2 resonance instead
of a 1:1, after 4 billion years. Pluto, which is of a similar mass to Mercury, is a hundred
times further away. A hundred to the sixth power is a trillion. So 4 billion years for
Mercury to get part way there and 4 billion trillion for Pluto, a time so long off that
not only would our own sun be long dead, but every star in the universe would be ancient
corpses by then. On the other hand Pluto and its own largest moon, Charon, are already
tidally locked to each other, so that you could run a space elevator right from the
surface of the one to the other. Gas giants with many moons typically have a lot of them
tidally locked especially the nearby ones. This is why we so often refer to planets around
red dwarf stars as being tidally locked. They are much less massive than our own sun, anywhere
from 7% to 60% of the mass, so they lock planets much slower, taking as little as three to
as much as 200 times as long as our sun would for a planet of equal distance.
Key difference is that they aren’t equally distant. For a planet to interest us in this
context though it needs to be warm enough for life, and smaller stars put out way, way
less power than bigger ones. So planets need to be way, way closer for it to be warm. The
smallest of these would need their worlds fifty times closer than Earth is to be warm
enough. Their less massive star might take 200 times longer to lock a planet as far off
as Earth, but one fifty times closer would lock in 50 to the sixth power shorter, meaning
the world would lock many millions of times faster than Earth would around our sun. Same
concept applies for habitable moons around gas giants.
So we often assume worlds in the habitable zones of red dwarf stars will be tidally locked.
Though newer calculations indicate that the atmospheres of worlds interferes with this
more than previously thought. Planetary atmospheres, and oceans, aren’t rigid and move more freely
and circle around so they slow down the locking process. Sort of like in the wheel with a
break analogy as if we dumped motor oil on the wheel. We don’t have a great model yet
for figuring out how much but it will be significant and thicker atmospheres and oceans would slow
that even more, so the notion of all red dwarf planets being tidally locked may be in error,
especially for bigger red dwarves where the planets would be further away.
But that in a nutshell is how tidal locking takes place and why it is so important to
discussion of exoplanets and moons. If you enjoyed this video, don’t forget
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Thanks for watching
and have a great day!