Over the past few decades, scientists have detected thousands of exoplanets, which are planets that orbit other stars. To follow up on these discoveries, missions are now shifting focus to capturing potential signs of life, or biosignatures, on these alien worlds.
Oxygen, an element that has been essential to life on Earth, “would represent a strong biosignature,” according to a paper published on Monday in Nature Astronomy. Led by Thomas Fauchez, a planetary scientist at NASA's Goddard Space Flight Center, the authors unveiled a new technique to detect molecular oxygen in the atmospheres of exoplanets.
Detecting signs of oxygen would not necessarily mean that extraterrestrial life is a sure thing, however, as there are plenty of non-biological processes that produce oxygen. That said, if paired with other biosignatures such as water, methane, or nitrous oxide, the presence of oxygen could significantly increase the odds of an exoplanet’s habitability.
"Oxygen is one of the most exciting molecules to detect because of its link with life, but we don't know if life is the only cause of oxygen in an atmosphere," said co-author Edward Schwieterman, an astrobiologist at UC Riverside, in a statement. "This technique will allow us to find oxygen in planets both living and dead."
The new method is based on the well-studied behavior of oxygen molecules in Earth’s atmosphere. When these molecules collide, they cause a very subtle dip in infrared radiation. The signal is so small that current observatories cannot capture it in exoplanets, but Fauchez’s team showed how NASA’s James Webb Space Telescope (JWST), due for launch in March 2021, might spot it.
"Before our work, oxygen at similar levels as on Earth was thought to be undetectable with Webb," said Fauchez in a statement. "This oxygen signal is known since the early 1980s from Earth's atmospheric studies but has never been studied for exoplanet research."
The team outlined a scenario in which JWST turned its sights to a system called TRAPPIST-1, located about 40 light years from Earth. This system is particularly tantalizing for exoplanet researchers because it contains several rocky Earth-scale planets. If these worlds have atmospheres, JWST should be able to see them as the planets pass in front of the TRAPPIST-1 host star, and perhaps even pick up the infrared signature of molecular oxygen.
It would be especially exciting to find oxygen levels that are similar to Earth in these alien skies, perhaps indicating a biological source. In contrast, an excess of oxygen could signal a “massive ocean loss” in the exoplanet’s past, the team said in the study, which would be a sign of inhabitability.
The new research lays the groundwork for scientists who hope to use JWST to probe the capacity of exoplanets to either nourish life, or prevent it from emerging. "It is important to know whether and how much dead planets generate atmospheric oxygen, so that we can better recognize when a planet is alive or not," Schwieterman said.