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Are we alone?

By Hamish Johnston

Last week I had aliens on my mind as I looked at whether it would be possible for next-generation telescopes to spy signs of life on distant exoplanets.

The answer — at least according to — Enric PallĂ© and colleagues — is yes.

But what should astrobiologists be looking for?

An international team of scientists has addressed that very question in a paper to be published in Astrobiology. You can read a preprint here.

Instead of focussing on planet-star systems like the Earth and Sun — which would be difficult to study, even with next-generation telescopes — the paper suggests we should look at “super-Earths”. These are planets up to ten-times the mass of Earth and therefore easier to find and study.

These super-Earths should be in the “habitable zone” — orbiting neither too near to, nor too far from, their stars. The limitations of next-generation instruments narrows this further to super-Earths orbiting relatively close to dim stars, rather than far from brighter stars.

Once a candidate has been lined up, scientists could try to look at visible and infrared light that has passed through the planet’s atmosphere for signs of chemicals associated with life. More precisely, this involves looking for combinations of chemical species that wouldn’t be expected to coexist without the help of life — oxygen, water and carbon dioxide (or methane) for example.

Astronomers will also have to work out the temperature of the super-Earth to understand chemical processes on the exoplanet. Its radius is another crucial parameter because this is related to plate tectonics and the existence of continents.

If we have a particularly good view of the exoplanet, we might even see spectral features associated with plant life. The existence of seas, continents or ice sheets could be revealed in differences in emitted light as the planet rotates.

So, when will astrobiologists spot the first signs of life on a habitable exoplanet? Many are confident that it will happen sometime after 2014, when NASA’s James Webb Telescope is up and running.

That is, if alien life doesn’t find us first!

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