Could life planet hop in the TRAPPIST-1 sys­tem?

The set of seven tightly packed worlds could be per­fectly po­si­tioned for mi­cro­bial mi­gra­tion

Sky at Night Magazine - - BULLETIN - CHRIS LIN­TOTT was read­ing… En­hanced in­ter­plan­e­tary pansper­mia in the TRAPPIST-1 sys­tem by Manasvi Lingam and Abra­ham Loeb. Read it on­line at

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.

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