Is there a second Earth?

Seven worlds orbit a small star around 40 light years from Earth. For astrobiologists, the TRAPPIST-1 system is something of a test lab for what may be the biggest question of all: could there be life out there?

“The system is extremely intriguing,” says Eva-Maria Ahrer, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. Several of the seven Earth-sized planets are situated in the so-called habitable zone – the region where, in theory, liquid water could exist, and therefore life.

Decoding atmospheres using light

Researchers at the institute study the atmospheres of such distant planets. Drawing on data such as that gained from observations with the James Webb Telescope, they analyse which gases can be detected – and whether any of them are biosignatures indicating past or present life. To do this, they examine starlight passing through a planet’s atmosphere. By analysing brightness and colour, they can determine which gases it contains.

“In the case of larger planets, we can often clearly detect water vapour or carbon dioxide,” says Ahrer. But for small, Earth-like planets such as those in the TRAPPIST-1 system, the task is much more difficult. Their atmospheres are thin and the signals are weak, often pushing measurements to the limits of what is technically possible.

Can life exist on other planets?

Then there are complications caused by the stars themselves. TRAPPIST-1 is a so-called red dwarf – small and relatively cool, but highly active. Radiation and stellar flares can distort measurements and may also alter or even strip away planetary atmospheres. “The star is currently one of the biggest uncertainties when it comes to interpretation,” says Ahrer.

That uncertainty is precisely what makes the system so interesting scientifically. After all, TRAPPIST-1 differs fundamentally from our own solar system. “It’s fascinating to explore whether life could exist on planets orbiting a star that’s not at all like the Sun,” says Ahrer. 

Being a habitable zone alone is not enough

So far, however, the findings are sobering. “At present, there is no planet in another system for which we have convincing evidence of life-friendly conditions,” says MPIA spokesperson Markus Nielbock, adding that the habitable zone itself is only a rough indicator. “This criterion is not particularly meaningful,” he says. Even Mars is situated in such a zone – and it is still hostile to life.

What will matter most is how quickly observation methods continue to improve, and how precisely atmospheres can be analysed in future. “With ever more refined measurement techniques, significant progress in the search for a ‘second Earth’ is possible over the coming decades,” says Nielbock.