The Earth is the only place we know for sure harbors life, but given how big the observable universe is, the odds that we're completely alone are astronomical. It's been suggested in the past that life could hitch a ride on asteroids or comets to jump from world to world – a process known as panspermia – and now, a Harvard team has calculated just how likely that scenario would be throughout the Milky Way. In short: extremely.
Panspermia is commonly thought to be possible within a planetary system. After all, scientists go to great lengths to sterilize spacecraft, to prevent Earthly microbes from contaminating other worlds. Asteroids could do the same thing naturally, especially in a system like TRAPPIST-1, a nearby neighbor that hosts seven planets crowded close together. But could panspermia work on a galactic scale?
According to the Harvard team, it could. The researchers calculated that there could be as many as 10 trillion asteroid-sized objects ferrying life around the galaxy. Larger objects housing life would be rarer but still pretty common: the team estimates as many as 100 million objects the size of Enceladus – a moon of Saturn that measures 500 km (310 mi) wide – and roughly 1,000 Earth-sized worlds could host either life or prebiotic material.
"The biggest worry people had for a long time with this idea was that UV radiation would just destroy life," says Idan Ginsburg, lead author of the study. "But it turns out if you're shielded, even just a few inches, by rock or ice, that's enough protection. There are even more complex life forms, like tardigrades, that can survive in space – they simply go into hibernation. So we know that microbes on a planet can survive being ejected into space; they can survive in space and, in theory, survive re-entry to be transplanted from one planet to another."
But to calculate the chances of this happening, we'd need to look beyond our quiet corner of the cosmos towards more active places. And few places are as active as the galactic center, where the supermassive black hole can fling objects outwards.
"Our solar system is fairly stable, but there are other places – especially in the center of the galaxy – where things are much more dynamic, and objects can be and do get kicked out all the time," says Ginsburg. "Planets, planetesimals, comets, moons, asteroids – all should be plentiful in the galactic center, so the galactic center can act like a dandelion and seed these objects out to the rest of the galaxy."
To come to those conclusions, the team had to consider many different variables, and work out the likelihood of life spreading in this way. They accounted for different speeds of different-sized objects ejected by the black hole, how likely they were to be captured by the gravity of other stars, how long that journey might take, and how hardy different types of life would be.
"This is a seven-dimensional integral – I don't think you could consider any more variables without getting into something like string theory," says Ginsburg. "This is not just a thought experiment, it was incredibly mathematically detailed – we took the mathematics, the physics, and the biology together and put together a clear picture of how this might work."
In the end, the team found that panspermia is quite likely happening on a galactic scale, with potentially trillions of life-bearing objects whizzing around out there. The highest chances for being captured by stars, according to the researchers, were objects traveling between 10 and 100 km (6.2 to 62 mi) per second, but it could still happen at speeds of over 1,000 km/s (621 mi/s).
Of course, this is purely hypothetical at the moment, but it's compelling research. The ultimate goal would be to observe panspermia in action, but with current technology that seems like it might be a long way off. For now, astronomers could start by finding evidence of microbes in Martian soil samples or asteroids.
The research was published in the Astrophysical Journal Letters.
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