Life finds a way 鈥� perhaps even across the stars. It may be possible for organisms to travel all over the galaxy by hitching a ride on a fast-moving rock in a phenomenon called galactic panspermia. In this way, just a few inhabited worlds could spread life throughout the Milky Way.
In October 2017, astronomers spotted the first interstellar object we have ever seen come through our solar system, called 鈥極umuamua. That was the first concrete proof that rocks can be tossed out of orbit from distant stellar systems and make it intact to our solar system.
Of course, it is not enough for a space rock to travel between the stars. In order to transfer life, it must also be captured by the sun鈥檚 gravity and eventually smash into a planet.
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Interstellar billiards
Now, Idan Ginsburg, Manasvi Lingam and Avi Loeb at Harvard University have calculated just how often these banished rocks might be captured into a new stellar system, and how likely any life stuck on such interstellar projectiles would be to survive.
鈥淚t鈥檚 like billiards,鈥� says Ginsburg. 鈥淵ou hit the cue ball and it hits the other balls, and beside just transferring momentum it also spreads life, and then life spreads across the whole table, which is the galaxy.鈥�
The researchers found that up to 100 million life-bearing objects with a radius of 200 kilometres聽鈥� about half the size of Saturn鈥檚 moon Enceladus聽鈥� could have been captured in stellar systems around the Milky Way. Even about 1000 Earth-sized objects could have been captured in this way, they say.
Smaller objects are much more likely to make the journey between stars, but the smaller they get, the harder it is for microbes to take shelter from the punishing space environment in the interior of the rock.
鈥淚t鈥檚 a very dangerous ride, but you can think of the microbes as tiny astronauts sitting in a natural spacecraft,鈥� says Loeb. 鈥淚 would actually be thrilled to be a microbe sitting in a rock that makes it across the Milky Way.鈥�
Once one of these life-laden rocks is captured into orbit around a new star, it can smash into a planet, dropping off its passengers. The same process on a smaller scale could also spread life to other planets in the system.
Many Mars rocks have been blown off the planet by impacts and ended up on Earth, leading some researchers to speculate that life on our planet could have come from Mars. It is even possible, if unlikely, that life on Earth began with interstellar microbes, says Loeb.
However, Ed Turner at Princeton University says the researchers may have overestimated the likelihood that these captured objects carry life. And many of these objects would not be chipped from larger, habitable planetary bodies, so if they have life it must have evolved there on its own.
鈥淥nly a tiny fraction of the objects that would be captured would plausibly carry life,鈥� he says. 鈥淚f that somehow were not the case and a lot of them carried life, then life is very common and you probably don鈥檛 need panspermia anyway.鈥�
Our space-faring descendants may even be able to test this idea. If life in different places around the galaxy is varied and diverse, that would be an indication that it聽arose independently on each world. However, if there are groups of stellar systems with similar life on their planets, says聽Loeb, this could mean that microbes really are travelling between the stars.
Reference:听
This article appeared in print under the headline 鈥淎lien life could ride space rocks to travel the stars鈥�
