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Ultraviolet light key to life search, too?

Too little UV light, and life might not ever start. Too much, in the form dramatic UV flares from stars, and the atmospheres of orbiting planets might undergo damage.

Artist’s concept of an other-worldly ocean on a distant exoplanet, under the light of a red sun, via CfA.

Our sun emits its own unique balance of “light,” a combination of different forms of radiation across most of the electromagnetic spectrum. It emits visible light, of course, and our eyes are most sensitive to that form of the sun’s radiation. And it also emits in the ultraviolet, which is the form of radiation we’re trying to block when we apply sunscreen. But some stars emit light primarily in the ultraviolet, or UV, part of the spectrum. Recent research from Harvard astronomers suggests that UV light might have played a critical role in the emergence of life on Earth. These astronomers believe it could be a key for where to look for life elsewhere in the universe, too.

Their study was published this summer in the peer-reviewed Astrophysical Journal and is available online.

Sukrit Ranjan of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts led the new study. It suggests that red dwarf stars – by far the most common sort of stars, and thought by some to be the best star systems in which to search for life – might not emit enough UV light to kick-start the biological processes most familiar to our planet. The statement from the Harvard astronomers pointed out:

For example, certain levels of UV might be necessary for the formation of ribonucleic acid, a molecule necessary for all forms of known life.

Ranjan further explained:

It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match. Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.

Red dwarf stars are smaller and less massive than our sun. Recently discovered planetary systems with potential habitable zones – zones at distances from their stars where liquid water can exist – include red dwarfs such as Proxima Centauri, TRAPPIST-1, and LHS 1140.

These scientists used computer models and the known properties of red dwarfs to estimate that the surface of rocky planets in the potentially habitable zones around red dwarfs would experience 100 to 1,000 times less of the ultraviolet light that may be important to the emergence of life than the young Earth would have. They said that chemistry that depends on UV light might shut down at such low levels, and even if it does proceed, it could operate at a much slower rate than on the young Earth, possibly delaying the advent of life.

And there’s another wrinkle in the question of life on exoplanets, related to UV radiation. Previous studies have shown that the red dwarf stars in systems such as TRAPPIST-1 might erupt with dramatic flares in UV. If the flares deliver too much energy, they might severely damage surrounding planets’ atmosphere, inhibiting life.

Ironically, these UV flares might also provide enough energy to compensate for the lower levels of UV light steadily produced by the star. Co-author Robin Wordsworth of the Harvard School of Engineering and Applied Science said:

It may be a matter of finding the sweet spot. There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.

Another co-author, Dimitar Sasselov, also of the CfA said:

We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life. Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.

Read more about this new study from CfA

Bottom line: Scientists at Harvard studied the role of ultraviolet, or UV, radiation in the possibility of life on other planets.

Deborah Byrd