How Astronomers May Hunt for Life on Alien Planets

Measuring Exoplanets' Chemical Signatures — This chart explains how astronomers measure the signatures of chemicals in the atmospheres of exoplanets. (Image credit: NASA/JPL-Caltech)

Any sulfurous molecules that astronomers spot on alien worlds might be a way to reveal whether or not those distant planets host life, researchers suggest.

On Earth, microbes can live off the energy available in sulfurous molecules that volcanoes release, essentially "breathing" these compounds the way humans breathe oxygen. If a similar kind of metabolism evolved on an extrasolar planet, the sulfurous molecules detected in the atmosphere of that world might help reveal the presence of alien life, said researcher Renyu Hu, a doctoral student in planetary science at MIT.

To see what telltale signs any sulfur-dependent life might generate, Hu and his colleagues modeled Earth-sized planets in the habitable zones of sun-like stars — that is, areas where worlds could have liquid water on their surfaces. These simulated planets possessed nitrogen-based atmospheres like Earth but 1,000 times more sulfur.

Sulfur-dependent life on Earth releases hydrogen sulfide as waste. The researchers found these microbes could increase hydrogen sulfide levels by nearly 10 times what they would be on a planet without such life. [Graphic: Sky Full of Alien Planets]

Volcanoes on Earth can release huge amounts of hydrogen sulfide and other gases into the atmosphere. (Image credit: USGS)

From interstellar distances, it would be hard to distinguish hydrogen sulfide (H2S) from water (H2O) on the surfaces of exoplanets. However, the researchers calculate that extra atmospheric hydrogen sulfide would in turn cause more pure sulfur aerosols to form in the air, which astronomers could detect based on their distinctive spectra or fingerprint in the visible and infrared wavelengths.

"Hydrogen sulfide emissions from the surface would have a large impact on the atmospheric composition of a planet," Hu said.

Habitable zones for different star types. Our solar system is used for comparison. (Image credit: Astrobiology Magazine)

Still, no Earth-sized planets have been discovered yet in the habitable zones of sun-like stars. "Characterization of the atmospheres of exoplanets has been confined to close-in planets so far," Hu said.

Also, Hu cautioned that hydrogen sulfide is not a conclusive signature of life. "We need to test our assumptions thoroughly," he said. "It may be, for instance, that volcanism could produce tremendous amounts of that gas."

Hydrogen sulfide is not the only biosignature gas the researchers are investigating.

"We want to study as many as possible — look at many, many gases in Earth's atmosphere and see if they can be biosignatures as well," Hu said.

Hu, with his colleagues Sara Seager and William Baines, detailed their findings May 26 at the American Astronomical Society meeting in Boston.

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.