It’s been 15 years since astronomers first discovered a planet beyond the solar system orbiting a normal star. We’ve found lots of unusual exoplanets since then, but nothing where we think life could exist.
But the Kepler planets will be too far away –- hundreds or thousand of light-years — for any follow-up observations to be able to determine if they are inhabited. All we will have from the Kepler data is planet mass, diameter, orbital period, and parent star type.
The Earth clones will forever remain a blip on the exoplanet radar when it comes to determining true habitability.
But enough exoplanet research has been done so far that a cautious prediction can be made about where the first inhabited planet will be found.
The planet will orbit a nearby red dwarf star found in surveys taken within 100 light-years of Earth. Why? Because red dwarfs are much more numerous than sun-like stars and so provide many more targets. Because red dwarfs are dim, planets orbiting them will not be as swamped by starlight and so their light is easier to measure.
The planet will be in the habitable zone around a red dwarf – a sweet spot where liquid water can remain stable on a planet’s surface. The zone will be only a fraction the distance from the cool star as Earth’s habitable zone is from our hotter Sun.
For those planets with orbits tilted edge-on to Earth, detecting them will be straightforward. Astronomers will see if the star dims slightly when the planet passes in front of it, or transits.
A planet in the habitable zone of a red dwarf would complete its racetrack orbit in just two weeks. This would allow multiple transits to be observed quickly. Also, because it is so close to the red dwarf, a planet is more likely to be in an orbit aligned along our line of sight, and will be more likely to be discovered transiting.
But there is one big catch. Young red dwarfs have a petulant youth stretching over billions of years. Titanic stellar flares erupt without warning and blast out lethal doses of ultraviolet radiation. Ocean life on a planet may be safe from the UV just a few feet underwater and still extract enough light for photosynthesis. But anything living on the surface could get fried without a liberal coating of Sunscreen 2000.
But we now have a glimmer of hope for red dwarf planets. Astrobiologist Antigona Segura of the Universidad Nacional Autónoma de México (UNAM) in Mexico City, simulated how a 1985 flare from the nearby red dwarf AD Leonis would have affected a hypothetical Earth-like planet orbiting a dwarf.
He found that UV radiation actually split molecules of oxygen to create more ozone than it destroyed. The simulation made a thicker ozone layer in the planetary atmosphere such that the surface experienced no more radiation than is typical on a sunny day on Earth.
What’s more, as the dwarf settles down to a quiescent existence, there would be very little ultraviolet light and an UV filtering ozone layer would not even be needed.
To be sure, there are other oddball characteristics to worry about. Potentially habitable red dwarf planets may keep one hemisphere locked onto their star due to gravitational tidal forces. The resulting slow rotation may give them anaemic magnetic fields that do not block cosmic rays effectively.
But the best solution is to simply go looking. The light-gathering power of the James Webb Space Telescope, scheduled for launch in 2014, would be used to spectroscopically ‘sniff’ out the exoplanet’s atmosphere for chemistry that might be a by-product of organisms on the surface. If we get lucky, and these planets do develop a natural UV shield, then the discovery of an inhabited world may be no more than a decade away.
Artwork credit: Karen Wehrstein, David Agular