Could life planet hop in the TRAPPIST-1 system?
The set of seven tightly packed worlds could be perfectly positioned for microbial migration
Living on one of the seven planets that orbit the nearby star TRAPPIST-1 would be very different from living on Earth. The seven are packed close around their parent star, with even the most distant of them, TRAPPIST-1h, completing an orbit every three weeks. From several of the planets, it would be common to see the others loom larger in the sky than our full Moon does, easily close enough to pick out the details of continents or the fine structure of storms.
A paper by Harvard’s Manasvi Lingam and Avi Loeb argues that such proximity may have more profound consequences than just making these worlds excellent for backyard observers. They revive the old fringe idea of panspermia – the idea that life may not have started on Earth but have travelled here from elsewhere in the cosmos – and look at the possibility of microbes emigrating from one world to the next.
This interplanetary version of panspermia is sometimes discussed in our Solar System too. We have meteorites here on Earth that have come from Mars, and there are presumably a few lumps of Earth sitting on the Red Planet too. Given this exchange of material, any life on Mars may not have formed independently from Earth. It may just have been transported there. Equally, it’s possible, and some would argue probable, that life on Earth got its start elsewhere.
Lingam and Loeb point out that this sort of transfer is much more likely when planets are closer together. The numbers are quite startling: if an impact on TRAPPIST-1e throws 1,000 pieces of debris into space, then as many as 30 are likely to hit TRAPPIST-1f. Similar calculations can be made for each of the other planet pairs, with similar results; hitchhiking bacteria would clearly have a great time in this system.
It might not just be bacteria, either. These high rates of transfer make it much more likely, the authors say, that the molecular building blocks of life – whatever they are – can be transferred between the worlds. If life gets started as a form of complex chemistry, then maybe the initial stages of that process can be shared amongst worlds.
This, it seems to me, is much less convincing. I suspect life’s start depends more on local conditions than on the availability of molecular ingredients. The truth is that without a much clearer understanding of how life gets going, and what’s needed, it’s hard to go much beyond these back of the envelope, probabilistic calculations.
For the authors though, that’s exactly the point. They look ahead to the next generation of telescopes, which might be capable of detecting the signs of life in the atmospheres of TRAPPIST-1’s planets. Such observations would be a test of the idea of panspermia. If it plays an important role then we should expect life on several of the star’s planets (or none at all). It’s a grand, though plausible, experiment – a beautiful test of what was once considered a crazy idea.
“If an impact on TRAPPIST-1e throws 1,000 pieces of debris into space, then as many as 30 are likely to hit TRAPPIST-1f”
If panspermia is a real influence, life should exist on several of these worlds, not just one
CHRIS LINTOTT is an astrophysicist and co-presenter of The Sky at Night on BBC TV. He is also the director of the Zooniverse project.