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Exoplanets' chemicals may give false-positives for life, boffins say

Methane and Oxygen signatures light years away are no way to spot ET

Artist's impression of an exoplanet orbiting a star in the cluster Messier 67

Astronomers and exobiologists looking for spectral signatures as indicators of life might be chasing a chimera.

That's the gloomy conclusion of a study published at PNAS, which says inferring the existence of biospheres on exoplanets “might be beyond our reach in the foreseeable future”.

Since all we know about exoplanets is what their light reveals, a lot of work goes into analysing the spectrum of that light. As Science notes, an example of the kinds of chemical brews planet-watchers look for is methane and oxygen, since the only way they could both exist in abundance in the long term is if there are living creatures to renew them both.

While we can't see an exoplanet well enough to spot its plants or microbes, the light those planets reflect will show the presence of such chemicals in its spectrum.

All well and good: but the new study by University of Toronto planetary scientist Hanno Rein tosses a spanner into the search for candidates: if a planet with (for example) oxygen in its atmosphere is circled by a moon with a methane atmosphere, we wouldn't be able to tell the difference.

In fact, in most cases, Rein's study says we probably can't tell whether or not an exoplanet even has a moon. As it's put in the abstract, “Any exoplanet can host a moon that contaminates the planetary spectrum. In general, we will be unable to exclude the existence of a moon.”

“The spectral resolution of even idealised space-based spectrographs that might be achievable in the next several decades is in general insufficient to break the degeneracy,” Rein writes.

And if you're thinking that Rein is taking too hypothetical and speculative a view of things, Science reminds us that light from Earth's oxygen-rich atmosphere would be contaminated to a distant observer if we were circled by Saturn's methane-rich moon Titan.

While an exomoon may have been spotted in the Kepler data, it's not a life-habitable setup: MOA-2011-BLG-262 is about four times Jupiter's size, with a sub-Earth-mass moon. ®

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