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.
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