18 March 2008

Fire and Ice: Tidally Locked Exoplanets

Posted by Ryan Anderson

What would the climate be like if the earth was closer to the sun than Mercury, and was tidally locked, so that the same side of the planet always faced the sun?

This was the question that Anita Ganesan and colleagues set out to answer with their poster last Thursday night at LPSC. I didn’t get the chance to talk to them directly, but I read their poster and abstract with interest, and thought I would share it here.
Just as the tides between the Earth and the Moon have slowed the Moon’s rotation until it is “locked” with the same side always facing toward us, the same thing could happen to planets that orbit close enough to their star. Ganesan showed the result of computer models of tidally locked “super-earths” (masses up to 20 time Earth’s mass) around hypothetical stars.


Ganesan’s computer model shows that the sun-lit side can get extremely hot, and the dark side can get extremely cold, but temperatures might be moderate right on the day-night line.

The computer models showed that the side of the planet facing the star could become extremely hot, possibly resulting in a sea of molten rock. Since molten rock is less dense than solid rock, the solid mantle around the sunny magma sea would slowly flow in to replace the melting rock while at the same time, the magma near the surface would flow away from the center of the molten sea and out onto the cooler surface nearby. If I am understanding it correctly, this would mean a constant recycling of the rocks in and around the magma sea, which could have some interesting implications for the composition of the magma.

The “coolest” part of the poster and abstract was that their simulations show that even on very hot tidally locked planets, there could be some locations on the surface where the temperature is suitable for life! A quote from the abstract sums this up:

There could exist an annulus on the surface of a hot
tidally-locked exoplanet corresponding to a habitable
region, where temperatures are intermediate between
those resulting from the intense stellar heating and the
cold, dark side. This has broad implications for the
search for extraterrestrial life, in that these planets
should also be considered. In the example from Figure
1, if the nondimensional temperature scaled to 1 corresponds
to 1000°C, then a region in the planet at temperatures
between 20 and 40°C exists permanently at
the surface close to the equator, and extends into the
planet at higher latitudes and greater depths.

It’s a little confusing, because in their model, the sun is shining directly down on the planet’s pole so that everything is symmetric when the computer does its calculations. So when they talk about the “equator” being habitable, what they mean is places on the surface that are about 90 degrees in latitude away from where the sun is shining straight down.

In other words, if you took earth, moved it in closer to the sun and tidally locked it, the habitable zone would be a ring that follows the boundary between day and night. The hottest part of the planet would be the side facing the sun, and it might be thousands of degrees. The coldest part would be in the middle of the night side, and would be well below zero.

I was particularly interested in this research because, a few years ago as an undergraduate, my friend and I were talking about this very topic. In particular, we were brainstorming about whether or not you could have a habitable, tidally locked planet. We decided that it would be great material for a science fiction story because it raises so many interesting questions!

What would the weather be like on a planet like this? Could there be oceans? Would all of the atmosphere freeze out on the night side? Could a planet like this keep its atmosphere for a long time? Could life evolve? What would it be like to live on a world where the only habitable places are constantly in twilight? Would the inhabitants of that planet fear bright light? Would they be afraid of the dark? How would cultures and religions be affected by living on a world like this?

I may be a Martian, but the study of exoplanets always excites me because it can lead to all sorts of questions and I can’t help but let my imagination run wild. Science fiction has done Mars to death, but I have yet to see a story about the Ice People from the dark side of Gliese 581 c. Maybe that’s my cue to get writing…

(Check out the LPSC abstract for this research here)