Tidal Locking

I've mentioned tidal locking several times when discussing Earth-like exoplanets. The definition of tidal locking is that a planet or moon's hemisphere always faces its orbital partner. Our moon is a good example of this tidal or gravitational locking phenomenon. It's the reason we can't see the backside of the moon.

Basically, a tidally locked body takes just as much time to orbit a dominant body as it does to rotate on its axis. Mostly, this is what happens to a smaller body orbiting a larger body, but if the two bodies are sufficiently separated it could cause both to be totally locked. A good example of this is Pluto and its moon Charon.

The reason that tidal locking takes place is because of gravity. What happens is that that gravitational force between the orbiting bodies causes bulges in their surfaces that cause a dragging effect. Eventually, the system becomes stable and the gravitational forces between the two bodies falls into a much less changing situation. That's what has happened in the case of our moon, but the slowing effect is still operating and it's causing our moon to slowly move away. The tidal forces of the moon are felt on Earth as —you guessed it—the tides. The moon causes the oceans to bulge out on both sides of the planet, and this causes the tides on both sides of the Earth.

However, these tidally locked situations are not perfect. There is a resonance effect that causes a slight warble in the moon, for example. It causes a slight variation in the period of the moon's orbit relative to its rotation period. In the moon's case the ratio is 1.1. This is why we can see 59% of the moon's surface.

Mercury is also tidally locked to the Sun, but the ratio is much higher at 3.2.

Now we get to the good part. As you might know, many of the candidate extraterrestrial planets orbit in close to much dimmer red dwarf stars. This is because red dwarfs are much more populace in our galaxy and probably all other galaxies. Red dwarfs (M-type and K-type) stay on the main sequence where they fuse hydrogen to helium much longer than our G-type sun. That means that the planets that obit them have climates that are stable for many billions of years, much longer than our Earth. This would give life a better chance to evolve into intelligence.

But, what would life be like on these tidally locked planets. For one thing, one side would face the star and the other side would be in perpetual darkness. The dark side would freeze and the star facing side would be hot. Could life develop and flourish on such a planet.

Gliese 581 is a good example of such a planet. It orbits a red dwarf star. In this case the side facing the star is cooked and any water would be vaporized. On the other side water would be frozen. One theory is that this situation on a planet with an atmosphere would result in winds forming on the hot side and flowing to the cold side, and effect that would modify the climates of both sides. However, this weather system would lead to strong storms.

One possibility for life would be in the twilight boundaries between the two extremes, especially of the ratio of orbital period is not quite the same as the rotational period. These boundary areas would have a reasonable climate that could support and sustain life. We have no known planet like this that we can study to determine what the climate would be like. That lack of knowledge could soon change with the newest observational telescopes being planned.

This would make an interesting science fiction plot. How creatures on a tidally locked planet could survive the violent climate changes. Perhaps they would migrate constantly from one side to the other in the boundary areas. Life would not be easy, but that could make them strong.

Thanks for reading.