Can Life's Fingerprint Be Found On Super-Earths?

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NASA is preparing the TESS observatory (Transiting Exoplanet Survey Satellite) to follow-up on the successes of the planet-hunting Kepler observatory by identifying nearby exoplanets that pass in front of, or “transit,” their stars. A small sample of these worlds will be singled out for further scrutiny if they lie within the habitable zone of the parent star. The habitable zone is the distance from a star where temperatures on a world may allow liquid water to exist on the planetary surface.

Kepler showed us an incredible diversity among planetary systems, and that small planets like Earth greatly outnumber bloated Jupiter-class worlds. But the Kepler planets are typically over 1,000 light-years away, so understanding the environments of these worlds is technologically out of the question — at least for the foreseeable future.

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It is a reasonable prediction that the first transiting candidate planet to look for the chemical signature of life will be a world orbiting a nearby red dwarf star. There are about 90 red dwarfs within just 20 light-years of Earth, but only seven sun-like stars.

A transiting planet will allow for measuring the fraction of starlight passing through its atmosphere as well as recording the difference in light from the system when the planet passes behind its star.

TESS’ survey should at least find a few nearby transiting worlds within reach of doing a chemical inventory with NASA’s planned James Webb Space Telescope. At the very least, Webb would have a shot at providing evidence for an ocean on a planet.  This would further narrow down the candidates for more detailed studies.

Kepler’s survey found a number of super-Earths, planets several times Earth’s mass, and therefore too small to be gassy so-called ice giants like Uranus and Neptune. But what might the spectral fingerprint of a nearby super-Earth look like? And could we unequivocally deduce the planet is inhabited to everyone’s satisfaction?

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Astrobiologists are now modeling super-Earth atmospheres for planets orbiting in the habitable zone of red dwarf stars. It’s time to begin to try and understand and predict the expected signature of an alien biosphere.

A set of models developed by J. L. Grenfell of the Zentrum fur Astronomie und Astrophysik, Technische Universitat Berlin, and colleagues, start out with the assumption the planet is blanketed with life (unlike Mars where apparently nothing survives on the surface). This would likely require that the target world be several billion years old to allow for the evolution of life to expand, diversify, and significantly modify the planet’s atmosphere.

Life on any planet would use chemical reactions to extract energy, store it, and release certain gases as a byproduct of their metabolism. On Earth the biggest chemical signature of metabolism is oxygen produced by photosynthesis. Other strong signals would come from the presence of ozone and nitrous oxide. Other biotracers, carbon dioxide and methane (already detected on and exoplanets by the Hubble Space Telescope) can also be produced by non-biological processes.

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The researchers find that ozone levels can vary widely given other conditions on the super-Earths. In particular ultraviolet radiation from a red dwarf star is anemic. Without UV radiation to break apart certain gasses a red dwarf planet might have a higher concentration of biosignature gasses, as well as a smoggy sky.

The model also looks at surface gravities that could be as high as three times that of Earth’s. This might inhibit the movement of biogases from the planet’s surface into the higher stratosphere. What’s missing in the models are such critical but unknown variables are whether the planet has a shielding magnetic field, or plate tectonic for recycling atmospheric gases like carbon dioxide that otherwise might build up to trigger a runaway greenhouse effect.

The bottom line is that super-Earth atmospheres will be complicated and messy and will probably not look like anything that unequivocally settles the question of habitability. This will spur the far-future goal of building interstellar probes to visit the nearest habitable planet candidates.

“A sobering thought usually left unacknowledged is that when we finally discover biosignature gasses in may not be the triumphant 100 percent certainty,” cautions MIT astronomers Sarah Seager in a recent essay in Science magazine. She says that astrobiologists will be left with an “assigned probability” depending on the level at which the likelihood of a false positive can be estimated. “Planet habitability is planet specific,” she concludes.

Image Credits: NASA, D. Agular/CfA

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