FYI.

This story is over 5 years old.

Tech

New Tidal Locking Theory Offers a Habitable Exoplanet Boom

Planets with thin atmospheres spin freely, even very close to their suns.
​Venus Express. ​Image: ESA

A new model offers an out from a seemingly damning problem in the hunt for habitable exoplanets: tidal locking. The idea, which comes courtesy of researchers at the University of Toronto, suggests that it's possible for a planet with the right atmospheric density to orbit its sun very closely without suffering the permanent nights and permanent days that come with being gravitationally frozen in orbit.

Advertisement

Earth is a Goldilocks problem, or so it's usually assumed. This just means that things are balanced in conspicuously perfect ways for life (read: us) to exist . If things were even just a little bit different, our planetary home would be a lifeless rock and it would be that way forever. Everything here is "just right," and isn't that fishy?

One of these just-right circumstances has to do with our Sun: its strength and distance from Earth. To harbor life, complex life in particular, planets usually need to be close enough to their respective stars to maintain a reasonable temperature. A lot of planets meet this requirement, but the catch is that if a planet is too close to its star, as it would need to be if that star were much weaker than ours, it's threatened with the phenomenon of tidal locking. This leaves just a narrow band of just-right for life to generate and thrive: close enough for heat, but far enough away to stay unlocked.

Most stars are known as "dim suns." They're cool, weak. But they also host the most planets, with the catch being that in order for one of those rocks to garner sufficient heat for life, it needs to be pretty close to its sun. These cool and weak stars are still just as massive, however, and so we wind up with the conclusion that most planets, orbiting the most common sort of star, are either frozen over or tidally locked e.g. they don't spin on their axes or they spin super-slow.

Advertisement

Earth, which orbits a relatively bright star, gets something of a free pass.

As Earth whips around the Sun, it spins on its own, offering day and night proportions that are just about perfect (again: ideal). If we were to get closer to the Sun, some screwy things would start to happen. As the gravitational attraction between the two bodies increased, a dangerous imbalance would start to manifest. Simply: the daytime half of Earth would experience gravity's pull more strongly than the nighttime half.

So: think about tidal forces and the oceans, where huge masses of water get pulled in some direction depending on where the Sun and Moon are in relation to Earth.

Past a certain threshold, this gravitational imbalance between daytime and nighttime hemispheres would be enough to keep the planet from rotating at all. The Sun's gravitational pull would clutch daytime Earth strongly enough to keep it from spinning away into night, just like a giant hand. Out here, with our abnormally bright sun, Earth gets enough light/heat to thrive, without the tidal locking consequences.

And tidal locking at its extreme means no rotation at all. Such a planet would have a daytime half and a nighttime half, where one side would be frozen and dead and the other side would be scorched and dead. It can also mean just a super-slow orbit, as is the case with Venus, which rotates once every 243 days, giving it slightly longer days than years. Mercury is a bit better with days lasting about 60 Earth-days.

Advertisement

Either case is enough to practically amount to the "no rotation" extreme. Imagine 243 days of darkness. It would take ​just a week to plunge surface temperatures to below 0°F, and a year later, below -100°F. Some heat leaks over from the daylit side of the planet, but those still aren't very good conditions for life.

Here's how the University of Toronto researchers solve the problem. Imagine a planet's surface and its atmosphere as two nested spheres, coupled by atmospheric density. The heavier an atmosphere is, the more friction exists between it and the surface. So when that giant gravitational hand grabs hold of a planet's atmosphere—which is already bulging outward as it faces the sun—more of the gravitational effect is passed downward to the planet itself. What this means is that tidal locking is at least somewhat a function of atmospheric density, enough even to prevent the phenomenon.

That is, a relatively thin atmosphere—compared to the muck of Venus, for example—will pass on the sun's gravitational torque relatively weakly.

Venus is almost perfectly locked, with a day nearly equal to a year.

"The hottest moment of the day is actually not when the Sun is directly overhead, but a few hours later," Jeremy Leconte, lead author of ​a paper in Science describing the model, and his team explain. "This is due to the thermal inertia of the ground and atmosphere that creates a delay between the solar heating and thermal response, causing the whole atmospheric response to lag behind the Sun."

So: angular momentum is transferred from orbit to atmosphere to planet (and planetary spin) in a way that depends on friction manifested in the atmospheric boundary layer.

None of this matters too much on Earth because we're far enough away from the Sun, but on Venus, the inspiration for this particular investigation, it's a profound effect. Aided by an atmosphere 93 times more dense than our own, Venus is almost perfectly locked, with a day nearly equal to a year. (The reason it's not perfectly equal is thought to be the presence of brutal, continuous upper-atmospheric winds above the planet.)

Swap Earth for Venus and we might find a different relationship, where the planet is better able to spin freely, increasing its habitability chances.

"This has many implications," Leconte and his team write. "On one hand, the difficulties in sustaining a habitable climate far from the star due to the presence of a permanent cold, night side may not be as severe as usually thought. On the other hand, the habitable zone has been recently shown to be more extended near the star for synchronous planets. For these objects, if the atmosphere is thick enough, the non-synchronous rotation that should ensue may thus come to limit the extent of the habitable zone around lower-mass stars."