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Tidally locked exoplanets more habitable than previously thought

Tidally locked exoplanets more...
Of the three possible climate types for tidally locked exoplanets, the wind patterns and climate types pictured center and right are potentially habitable
Of the three possible climate types for tidally locked exoplanets, the wind patterns and climate types pictured center and right are potentially habitable
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The three possible climate types for tidally locked exoplanets
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The three possible climate types for tidally locked exoplanets
Of the three possible climate types for tidally locked exoplanets, the wind patterns and climate types pictured center and right are potentially habitable
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Of the three possible climate types for tidally locked exoplanets, the wind patterns and climate types pictured center and right are potentially habitable

You'd think that a planet with permanent day and night sides would be totally inhospitable. Without a sun to warm it up, the dark side would be freezing cold all the time. And with no respite from the solar onslaught, the light side would be scorching hot. But a new study suggests that exoplanets with this very predicament might in fact be habitable under two out of three possible climate types.

Using 3D models, scientists at KU Leuven in Belgium ran 165 climate simulations on exoplanets known to be "tidally locked" to their star. This means that their rotations are in sync with that of their star, so the same side always faces the it – like the way the same side of the Moon always faces the Earth.

For tidally locked exoplanets to be potentially habitable, they must have a functional planetary "air conditioning system" that balances surface temperatures across the light and dark sides. In one climate type uncovered by the simulations, this air conditioning system is overridden by an eastward wind jet (fast flowing air currents) that messes with circulation in the upper layers of the atmosphere along the equator and prevents heat transfer to the night side.

That only occurs on exoplanets with rotation periods under 12 days, however. The simulations showed that the rest have either two westward jets at high latitudes or a longitudinal "smearing" of the upper atmosphere hotspot across the substellar point (the point at which the star is in zenith, or directly overhead). Both of these other climate types leave the air conditioning system unaffected and thus make the surface potentially habitable.

The finding is valuable because tidally locked exoplanets usually orbit closely to their stars, and exoplanets that are close to their stars are not only easier for researchers to detect and observe but also more likely to contain liquid water than those with a wider orbit.

The study will also help in the tough task of sorting through the growing list of exoplanets discovered (now approaching 2000) to find ones that might be future homes for us humans. Even if they don't look like Earth, tidally locked exoplanets such as 2010 discovery Gliese 581g might just turn out to be viable for our future cosmic sprawl.

A paper describing the study was published in the journal Monthly Notices of the Royal Astronomical Society.

Source: KU Leuven

7 comments
Bob
I wonder if the radiation from such a close star entered into consideration. A planet that close to its star could also get licked by a solar flare on a regular basis. I suspect planets so close to a star would have had their atmosphere and any water stripped away shortly after they formed. Then there is the little problem of taking years to get there only to find it is in a bad neighborhood and plan B is 50 light years away. It will take much, much more than a temperate climate to make it habitable. Actually, there are so many things unique about our earth that finding another place like it within traveling range of light speed spaceships is probably near zero.
cadcoke5
I will add that on the cold side, you would tend to have any atmosphere freezing and end up on the surface. Any atmosphere on the hot side, eventually migrates to the cold side, then freezes and falls to the ground. Many seem so unrealistically hopeful of "earth like" worlds, that they accept minimal data, or even stretch the meaning to extremes. One NASA scientist called a tidally-locked planet, very close to its star, and 10x the size of earth, as "earth like". I have even seen on NASA scientists say that he was 100% that life was on a particular extra-solar planet. Later, it was realized that the planet didn't even exist. Even rare suns, like our own, are called common by ignoring the unusual stability it has.
vblancer
Bob you assume their star would be like our Sun. There are hundreds of billions of stars that are much weaker than our Sun and in that case you may NEED to live "sunny side up"! Things like distance and solar flares are all over the spectrum of possibilities. It may even be that only the cool side would be habitable along with the ring around the planet's surface that could be called a "twilight ring". The possibilities are endless so do not assume that to be habitable it has to look life Earth or our star Sol. Things like planets much further from a hotter star or closer to a cooler star. Many possibilities. Hard part will be getting there!!
Gillouz
Hi We have Venus here that have a revolution periode of 225 days and turn around the sun in 583 days.... and it does not seem's like it is a good place to live
Bob
Remember, every star no matter how relatively cool, is one large fusion nuclear reactor. Nearby planets would be subject to extreme radiation. Even our earth would not be easily habitable without its strong magnetic field to shield it.
Ralf Biernacki
@cadcoke5: If you reread the article several times slowly, you will come to understand that the main point of what it says is that atmospheric circulation can prevent the atmosphere from freezing out on the night side. Atmosphere doesn't "migrate". It circulates. It transports heat energy when it circulates, from one place to another. It is heated by the star on the daylight side. When it moves to the night side, it heats the surface of the planet there. Think of it as a warm wind blowing over cold ground. The ground gets warmer. Air-freezing temperature is really low. It is easy for the warm wind to heat the ground just a little, so that air doesn't freeze anymore. The scientists in the article did calculations to see if this is enough. They think that, for many planets, it is.
Douglas Bennett Rogers
Freeman Dyson's Project Orion would get your descendants there without much new technology but a lot of money.