Scanning atmospheres for signs of extraterrestrial life

Giada Arney thinks that life likely exists somewhere besides Earth. Arney, a third-year Ph.D. student in astronomy and astrobiology at the University of Washington, gave a talk at November’s meeting of the Seattle Astronomical Society about the search for the origins of life in the universe.

“Some of us like me who are astrobiologists think it’s likely that life has arisen elsewhere in the cosmos and perhaps elsewhere in our own solar system,” Arney said. “But so far the only evidence we have for life that actually exists is on this singular planet.”

Ph.D. candidate Giada Arney is planetarium coordinator at
the University of Washington, and used the facility, refurbished
a couple of years ago, to illustrate her talk about astrobiology.
This shot of Earth was part of the talk. Photo: Greg Scheiderer.

It’s hard to extrapolate from a single data point, but Arney is on the case. The educated guess that there’s some form of life out there stems from the fact that the raw materials are all over the place. Asteroids, for example, are loaded with water and carbon molecules—and much more.

“We’ve looked at the composition of various types of very carbon-rich asteroids, and we’ve looked at the specific types of carbon molecules that exist in those asteroids,” Arney said. “We found sugars, we found amino acids—the building blocks of proteins in our cells. We found nucleic acids, the building blocks of our DNA.”

“What this suggests is that these building blocks of life are easy for nature to synthesize and they’re cosmically common,” she said.

On top of that, Arney said study of the interstellar medium reveals lots of sugars and alcohols. This had me thinking, “Well, what else do you need?!” Arney said that the significance of these is that they’re the building blocks for amino acids. Astrobiologists have yet to pinpoint amino acids in the interstellar medium—it’s exceedingly difficult to pick out their spectral fingerprints—but Arney bets they’re there.

“This suggests that this complex carbon chemistry, that at least life on Earth requires, is cosmically abundant,” she concluded.

Arney’s research bailiwick is planetary atmospheres, and that’s where astrobiologists are going to look for evidence of life on other planets. There are plenty of potential places to investigate. Arney said that around eight percent of low-mass stars have an Earth-size planet in their habitable zones. She said a recent analysis of Kepler data that put this figure at close to 20 percent came in too high because of what she feels is an overly generous definition of the zone. Even eight percent, though, gives scientists a lot of planets to explore. The ultimate test will involve direct imaging and spectroscopy of the exoplanets’ atmospheres, something we can’t really do yet.

“Once direct-imaging missions become possible, we’re going to look for gases like water vapor and oxygen in the atmospheres of exoplanets,” Arney said. “Maybe that will give us evidence for life on these planets.”

The effort will require use of another rare element: cash.

“It will be a very expensive mission because it’s going to require a very big telescope,” she said, bigger even than Hubble or Webb. “You need to collect a lot of photons in order to measure the spectra of an exoplanet to have a high enough signal-to-noise ratio to be able to confidently say, ‘Hey, there’s oxygen in this planet’s atmosphere.'”

Arney expects life is out there.

“Microbial life is probably common, but the general consensus in the astrobiology community is that complex life and certainly intelligent life is probably remarkably rare,” she concluded.

The search continues.