Oxygen is Not Enough

To point us to a habitable planet, that is.

berry GJ-1132b.jpeg
The exoplanet GJ 1132b has some of the traits needed for habitability, but is probably lifeless.

When Laura Schaefer from the Harvard-Smithsonian Center for Astrophysics and her colleagues modeled what would happen to water on an exoplanet like GJ 1132b, they found something interesting, at least to astrobiologists. The water would split into hydrogen and oxygen, with the hydrogen escaping to space and the oxygen likely building up a sizable atmosphere.

GJ 1132b is relatively close to us, just 39 light years away from Earth. But it’s not what you typically think of as an Earth-like planet, even though it is only 1.2 times larger and 1.6 times heavier than our own planet. It orbits a red dwarf star much less luminous than our Sun. And since the planet is so close, completing an orbit in just 1.6 days, the temperature at the top of its atmosphere is estimated to be about 260 °C or 500 °F. The planet’s surface is likely so hot that water would be present only in the gas phase.

Schaefer and colleagues focused on what would happen to the oxygen after it separated from the hydrogen (thanks to radiation from the host star shortly after the planet formed). The magma ocean that likely would exist on the exoplanet’s surface early in its lifetime would not absorb more than 10 percent of the freed oxygen, thus allowing the build-up of a significant oxygen atmosphere—in extreme cases up to several thousand times the density of Earth’s. This is a likely scenario, as red dwarf stars generally have high radiation fluxes (both X-ray and UV) and solar flares, especially early in their lifetimes. If the radiation fluxes were much lower, we would expect a steam atmosphere.

Since we humans live in an oxygen-rich atmosphere, our tendency is to consider that element a biomarker, or signature of life. But on GJ 1132b, the presence of oxygen would indicate that no life as we know it could exist there. Oxygen would be an anti-biomarker, so to speak, while the lack of oxygen would be more consistent with life. Even then, don’t hold your breath—life would be very unlikely to exist on a world so close to its central star.

This won’t be the last time we will hear about GJ 1132b. Since its orbital period is only 1.6 days, it’s much easier to study than most exoplanets. I’m sure many teams will be anxious to directly image the planet to evaluate whether models like those developed by Schaefer and her colleagues are correct.

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