Planets Close to Their Host Stars May Be Habitable, but There’s a Catch

A dusty atmosphere will increase the chances of life existing, but also make it harder to find.

Planets close to small red dwarf stars are frequently tidally locked, so that their day (the time it takes to rotate once on their axis) is as long as their year (the time it takes to orbit the star). (David A. Aguilar (Harvard CFA).)
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In a new study published in Nature Communications, Ian Boutle from the University of Exeter in the U.K. and his British colleagues showed that mineral dust can increase the habitability of Earth-like exoplanets, especially those that are tidally “locked,” meaning that they always keep the same face toward their host star.

This is a common scenario, especially for planets around M-dwarf (Red dwarf) stars, which are the most abundant spectral type in our galaxy. Because these stars are small and relatively cool, their habitable zone (the zone where water can be stably liquid on a planetary surface) is very close to the star. The same thing can apply to exo-moons: In fact, Earth’s Moon is tidally locked, and always shows the same side to Earth.

Since more than 75 percent of the stars in our galaxy are of the M dwarf type, many rocky planets in the habitable zone will be tidally locked. So will some of the planets that orbit larger stars like our Sun. Scientists used to be very skeptical that tidally locked planets can host a biosphere, because the side facing the star would be extremely hot and the opposite side extremely cold. If life existed at all, it would be located in the eternal twilight zone within a narrow region on the surface.

However, opinion started changing more than 20 years ago when an important paper by Manoy Joshi showed that tidally locked planets orbiting M dwarf stars can support atmospheres over a large range of conditions and could, in principle, be habitable. Boutle and his co-authors have now bolstered this view by their discovery that atmospheric dust would further enhance habitability. Their modeling shows that airborne dust would cool the day side and warm the night side of a tidally locked planet, thus widening the habitable area. They also found a likely feedback mechanism that would increase the amount of dust in the atmosphere and keep water closer to the surface. The net effect would be to delay the loss of water on planets at the inner edge of the habitable zone, keeping them habitable for longer periods of time.

While the prospects of life on a tidally locked planet—especially those with a lot of dust in their atmosphere—has increased, I still rate the chances of complex life being present on these planets (and moons) as low. Evolution to more complex life is spurred by a highly heterogeneous environment, and seasonal variations play a big part in causing that heterogeneity. Tidally locked planets have no seasons, no day and night cycles, and most will not be able to sustain a global biosphere. The example of Earth is instructional. Although our planet had all the environmental conditions needed to spur evolutionary progress, it still took about four billion years for the first macroscopic, complex animal life to appear.

Still, the Boutle team deserves credit for showing that tidally locked planets should not be dismissed out of hand as being unable to host life. This insight is extremely significant, given that so many of them will exist. And what if any do in fact host life? The bad news is that we may have a hard time detecting it. The same atmospheric dust that enhances habitability would likely obscure the spectral signs of airborne water vapor, oxygen, ozone, and methane. In other words, no signatures of life would be detectable from a distant telescope.

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