Finding life outside our Solar System may be more challenging than we thought, following the identification of a mechanism that could mask its presence. It is hoped that future generations of telescopes will be able to study nearby planets' atmospheres for biologically produced gases, but many of the best prospects may be very hard to study.
Most of the stars in the galaxy, and particularly in the neighborhood of the Sun, are red dwarfs. Consequently, if we want to search for life that's close enough for us to get a good look at it, most of our opportunities will be orbiting small red stars, including the recently discovered Proxima Centauri b and Trappist 1 systems.
Since these stars are so faint, nearby planets need to have short orbits to be warm enough to support life. Almost any planet in such a close orbit will become “tidally locked”, keeping one side forever towards its star, just as the Moon always keeps the same face towards Earth. Life would probably only be possible in a narrow zone, where there is enough light to grow, but not enough to overheat everything. Dr Ludmila Carone of the Max Plank Institute has spent years designing models of the atmospheres of tidally locked planets.
While some doubt whether such a planet could support life, rather than being sterilized by the frequent ultraviolet outbursts red dwarfs are prone to, Carone has raised the possibility that even if life exists, its signatures may be disguised.
Most proposals for detecting life around other stars rest on the fact that certain gases, including ozone, are too reactive to last long. Their presence would signal a constant source of replenishment, most likely biological.
Carone's most recent work, published in the Monthly Notices of the Royal Astronomical Society, observes a hitch to the idea of finding ozone on a tidally locked world.
Such a planet would have very strong winds since one side would always be hotter than the other. Assuming relatively simple atmospheres, Carone modeled the winds on four known red dwarfs: Proxima Centauri b, TRAPPIST-1b and -1d, and GJ 667 Cf.
Carone and colleagues found that on planets with orbital periods less than 25 days, aerosols and gases like ozone would be swept into a belt around the equator, where they might be hard to detect. "Absence of traces of ozone in future observations does not have to mean there is no oxygen at all. It might be found in different places than on Earth, or it might be very well hidden,” Carone said in a statement.
This concentration may restrict the habitable zone even further, by exposing much of the planet to harmful ultraviolet radiation.