One of the more promising techniques for detecting alien life is by scanning the atmospheres of distant planets for biosignatures. But as new research shows, astrobiologists could very easily be deceived into thinking they've found extraterrestrial life when a moon is involved.

Top image: Artist's impression of the first exomoon candidate known to astronomers. Such configurations could contaminate spectral signatures, leading to the false impression that life may be present. Image: NASA/JPL-Caltech.


A few weeks ago I described 14 ways we could detect signs of alien life. One of the items listed was spectrographic analysis of exoplanetary atmospheres. This is the process of splitting the light up from a planet and trying to identify what gases are present by analyzing what's been removed or added to the light. This technique could tell us about the atmospheric composition of exoplanets — and whether or not it may host alien life. As planetary scientist Sara Seager explained to me last year,

Just like on Earth where we have satellites that look down to measure gas concentrations, we can use space telescopes to look at the atmospheres of planets far away. We're going to look for gases that essentially don't belong — gases that may be produced by life. Oxygen fills our atmosphere to 20% by volume. But without life we actually wouldn't have oxygen at all — we'd have about 10 billion times less oxygen. So plants and photosynthetic bacteria are creating oxygen in our atmosphere, and so, if aliens were to look at us from far away, and using optical wavelength telescopes — rather than radio telescopes — they would see all this excess oxygen and they would hopefully know that that it doesn't belong here — that it should be attributed to life.

This is all great and really promising, but a new study from the University of Toronto has thrown a bit of a wrench into this prospect. A problem arises, say the researchers, when another object's atmosphere — like a neighboring moon — contaminates the spectral signature in a way that delivers a false positive.


In the new paper, titled "Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets," the researchers consider a scenario where an exoplanet hosts a moon that has its own atmosphere — but that neither of the bodies hosts life. Because scientists on Earth would be largely unaware of the moon, the spectral signal would be conflated, or smudged. Together, the tarnished signature could give the false impression of a coherent biosignature.

Frustratingly, this suggests we can never be absolutely certain that a biosignature is pure and legit. Astrobiologists and astronomers now have their work cut out for them to devise a way to detect these tertiary objects, such as measuring the gravitational wobble of an exoplanet, or by improving telescopic technologies (to detect mini-eclipses, or lunar transits).