O'Malley-James recently co-authored a paper on this topic with Lisa Kaltenegger, director of the Carl Sagan Institute, to be published in the Monthly Notices of the Royal Astronomical Society. They looked at the ultraviolet (UV) flux of each world to work out the limits of habitability depending on their atmospheres. The thinner the atmosphere, the more damaging UV radiation makes its way to the surface and the harder it is for life to exist. Having an ozone layer like Earth may be essential.
In a previous paper from the pair, they looked at how biological fluorescence could also be an indicator of life on a world. Consider how, if you looked at Earth from afar, you might notice a lot of green light reflected by vegetation. The same could be true on an alien world with its own plant life.
“If the surface is a really bad UV environment, then maybe life on such a planet could develop biofluorescence,” said Kaltenegger. “That could be something you spot with a telescope on the surface, because when a unique flare hit the planet, it would light up in visible light that wasn’t there before… All of a sudden the planet could become green, red, or so on. It would be super cool to see.”
We’ll need a powerful telescope to see this, though. The upcoming James Webb Space Telescope (JWST), due to launch in October 2018, will be good for studying the planet’s atmosphere in infrared, but it won’t be able to see visible light like this. Instead, we may have to wait for the European Extremely Large Telescope (E-ELT) in 2024.
The type of light hitting these planets may also cause them to look vastly different to Earth. As it's mostly infrared, which is lower energy than visible light, plant life may need to be dark in order to absorb as much light as possible, rather than the bright greens we often get here.
Artist's impression of TRAPPIST-1f. NASA/JPL-Caltech