New research suggests that an exoplanet's atmosphere can contain lots of oxygen — none of it created by life — if bathed in the wrong mix of ultraviolet radiation from its host star.
Fact: free oxygen shouldn't exist in Earth's atmosphere. It's orders of magnitude out of equilibrium with the other gases in the mix. But, thanks to the rise of photosynthetic organisms, its abundance started increasing some 2.2 to 2.4 billion years ago, and (thankfully) it's here to stay.
This same scenario is what many astrobiologists hope has happened on Earth-like worlds circling other stars, particularly M dwarfs. These stars are preferred for exoplanet searches because they're rampantly abundant in our galaxy, have very long stable lifetimes, and have the cool, dim, and low-mass characteristics that make finding smallish planets around them relatively easy.
Cosmic oddsmakers recently put the chance of finding an Earth-like planet around an M dwarf at nearly 50%. And the presence of oxygen in its atmosphere would be taken as a strong indication of abundant biological activity.
Well, maybe not, say Feng Tian (National Astronomical Observatories of China) and four colleagues from the U.S. and Argentina. At today's meeting of the American Astronomical Society's Division for Planetary Sciences, Tian showed how planets circling in the habitable zones of M dwarf could have lots of free oxygen but be completely lifeless.
The key, he explains, is what's coming off the host star. Earlier this year, a group led by Kevin France (University of Colorado) found that six M dwarfs studied with the Hubble Space Telescope all produce lots of ultraviolet radiation in the regions above their their otherwise subdued photospheres. They crank out a Sun's worth of potent far-ultraviolet energy — but no more than 0.1% of the Sun's near-ultraviolet rays.
That strongly skewed ratio, says Tian, makes all the difference in cooking up compounds an exoplanet's atmosphere. Specifically, the far-UV photons can break up carbon dioxide (CO2) to yield oxygen atoms and, eventually, oxygen molecules (O2) and ozone (O3). Other byproducts from reactions with hydrogen include H2O2 and HO2 (note: not H2O). But there are far too few near-UV photons to keep the reactions going in a way that would reconstitute the CO2. So ultimately the oxygen levels build up.
Oxygen is relatively easy to detect spectroscopically, and it's easy to envision future spacecraft capable of detecting it in exoplanet atmospheres. So will oxygen's presence always be a "false positive" for biologic activity? No, but as Tian cautions, "We need to know more about the ultraviolet environment before claiming the existence of life on exoplanets."