Exoplanets: where to look for signs of life

If there is life out there, and it’s easy to think there is, if we can catch a glimpse (blurry and shy) of a star’s light, in the atmosphere of a distant planet, maybe we can catch also a sign of life. But bio-signatures are really the key of alien life?
“Signs of life in the courtyards and houses at dusk, the lights remind the celestial mechanics,” sings Franco Battiato. But the signs of (alien) life on an exoplanet, in a galaxy far far away, where are they?
If there is life out there, and it’s easy to think there is, catch an illuminated glimpse (blurry and shy) by the light of a star, the exo-planetarian atmosphere of another Earth. Strong defense of Sara Seager, slayer of exoplanets in force at the Massachusetts Institute of Technology, and the physicist William Bains on the columns of Advanced Science: the atmosphere, the atmosphere first!


Credit: NASA / JPL

The footprint of extraterrestrial life is in the gases that make up the sky for extrasolar planets. If the metabolism of organisms can, as we know, to alter the components of an atmosphere, a spectroscopy could be decisive. A spectrum of an alien atmosphere may reveal the presence of molecules of carbon dioxide, water, ozone, methane, ammonia. We have to go hunting bio-footprints more and more.

In the last twenty years we have discovered hundreds of new planets. The dream of another world that can harbor life in some remote point of the universe has an irresistible charm. Researchers and astrophysicists are looking for planets similar to our size, temperature, and possibly within the so-called habitable zone, the region around a star where we can find liquid water on the surface.
The water and life can also be located on Super-Earths orbiting outside the habitable zone, at distances ten times higher than those that separate the Earth from its star, the sun. As long as the atmospheres of these worlds contain hydrogen gas in sufficiency, and then a powerful greenhouse, they are able to keep the heat within the atmosphere and create a mild climate despite little radiation received at the surface… Likewise arid planets and closer to their parent stars may need a smaller amount of water to create life, given the high humidity.
As to the atmosphere: each molecule absorbs light in a different way. That’s how astronomers, observing how the atmosphere of an extrasolar planet absorbs light of its star, can identify which molecules are composed of the skies of other worlds. So: no more talk, let’s concentrate on atmospheres.
The main problem, to date, remains our objective technological inability to determine the characteristics of a planet. Beyond mass, radius, and quantity of light received, there are no means to analyze comprehensively the atmosphere, or the surface or the geology of alien planets. The James Webb Space Telescope NASA certainly can tell us more about these distant worlds.
Meanwhile we can meditate: are there any reasons of doubt for those who consider premature, if not completely superfluous, to question the concept of the habitable zone where it stands? As long as there are technologies available and efficient, the concept of new habitable planets remains very much possible. But we will be more and more careful with their atmospheres.

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