How Much Ultraviolet Light is Best for Life?

Artist's impression of an Earth-like planet before life or an ozone layer has formed. Credit: W. Henning

As our stock of known planets beyond the solar system grows, astronomers are increasingly focusing on which represent the best prospects for life. The latest in planetary computer modeling examines how the ultraviolet (UV) light from different sorts of stars affects the chances of life developing and thriving on nearby worlds. The results have been published in The Astrophysical Journal.

"Depending on the intensity, ultraviolet radiation can be both useful and harmful to the origin of life," says Dr. Lisa Kaltenegger, associate professor of astronomy at Cornell University and first author on the study. 

With a new generation of space telescopes about to provide us with access to vastly more information about exoplanets, establishing the relationship between UV light and the possibility of life on nearby planets is becoming all the more pertinent.

"With the next generation of missions, we expect to observe a wide diversity of extrasolar planets," said lead author Sarah Rugheimer, a Cornell research associate at the Carl Sagan Institute. "We're going to see all kinds of planets in all kinds of stages in their own evolution, but we wanted to take four kinds of epochs from Earth history, as samples of what we might see.”

The four models chosen were a lifeless world similar to Earth 3.9 billion years ago, a planet experiencing the first atmospheric oxygen and the process of biosynthesis, a world similar to the one where multicellular life appeared 800 million years ago and, lastly, a world similar to our own today.

The modeling is much more complex than simply exposing the planet to a fixed amount of radiation. "It's not just the amount of ultraviolet radiation, but also the specific types of ultraviolet radiation which will impact biology," said Rugheimer. "We consider which wavelengths are most damaging to DNA and other biomolecules.”

Very hot stars have been excluded, since they burn out much too quickly for multicellular life to evolve. Those that were considered in the study range from F-type, substantially hotter than the sun, to M type red dwarfs.

Unsurprisingly, the models suggest that cool stars, which were recently found to host many of the galaxy's planets, produce less radiation that affect life forms. What was less expected was that, by the time oxygen had appeared, less UV radiation reached the ground from bright F-type stars. This is because planets circling them have conditions conducive to a dense ozone layer.

The authors conclude that before life began, an Earth-like planet orbiting the brightest star in their study received "6 times the biologically effective radiation as around the early Sun and 3520 times the modern Earth-Sun levels.” At the other extreme, a planet orbiting an M3.5 type star, like the red dwarf GJ 581, received "300 times less biologically effective radiation, about 2 times modern-Earth sun levels,” even before life emerged to produce an ozone layer.

The work is just one piece of the puzzle in helping us recognize the best places to look for life. Last month, both Kaltenegger and Rugheimer were part of a larger study that explored the likely surface conditions of Earth-like planets around a range of stars, arguing, “Our quest to observe and characterize biological signatures on rocky planets must consider the star-planet system as a whole, including the interaction between the stellar irradiance and the exoplanetary atmosphere.”

Finding the sweet spot, or spots, for UV radiation received by a planet could be a big step towards doing just that.

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