Since exoplanets are so far away, astronomers cannot look for signs of extraterrestrial life by visiting these distant worlds. Instead, they must use a cutting-edge telescope to see what’s inside the atmospheres of exoplanets. One possible indication of life is the presence of atmospheric oxygen. Now, an international team of astronomers has developed a technique to detect a signal that oxygen molecules produce when they collide.
An artist’s impression of a water-rich exoplanet with an oxygen-rich atmosphere orbiting a red dwarf star. Image credit: NASA’s Goddard Space Flight Center / Friedlander-Griswold.
“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 Dr. Edward Schwieterman, an astrobiologist in the Department of Earth and Planetary Sciences at the University of California, Riverside.
“This technique will allow us to find oxygen on habitable or uninhabitable planets.”
“Before our work, oxygen at similar levels as on Earth was thought to be undetectable with NASA’s James Webb Space Telescope,” said Dr. Thomas Fauchez, an astronomer at NASA’s Goddard Space Flight Center.
“This oxygen signal is known since the early 1980s from Earth’s atmospheric studies but has never been studied for exoplanet research.”
When oxygen molecules collide with each other, they block parts of the infrared light spectrum from being seen by a telescope.
By examining patterns in that light, astronomers can determine the composition of the planet’s atmosphere.
An artist’s impression of a dry exoplanet with an oxygen-rich atmosphere orbiting a red dwarf star. Image credit: NASA’s Goddard Space Flight Center / Friedlander-Griswold.
Dr. Schwieterman, Dr. Fauchez and their colleagues calculated how much light would be blocked by collisions of oxygen molecules.
Intriguingly, some scientists propose oxygen can also make an exoplanet appear to host life when it does not, because it can accumulate in a planet’s atmosphere without any life activity at all.
If an exoplanet is too close to its host star or receives too much star light, the atmosphere becomes very warm and saturated with water vapor from evaporating oceans.
This water could then be broken down by strong ultraviolet radiation into atomic hydrogen and oxygen.
Hydrogen, which is a light atom, escapes to space very easily, leaving the oxygen behind.
Over time, this process may cause entire oceans to be lost while building up a thick oxygen atmosphere — more even, than could be made by life.
So, abundant oxygen in an exoplanet’s atmosphere may not necessarily mean abundant life but may instead indicate a history of water loss.
“Astronomers are not yet sure how widespread this process may be on exoplanets,” Dr. Schwieterman said.
“It is important to know whether and how much uninhabitable planets generate atmospheric oxygen, so that we can better recognize when a planet is alive or not.”
The team’s paper was published in the journal Nature Astronomy.
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T.J. Fauchez et al. Sensitive probing of exoplanetary oxygen via mid-infrared collisional absorption. Nat Astron, published online January 6, 2020; doi: 10.1038/s41550-019-0977-7
