Although Earth is the only place that harbors life that we know of, that doesn’t mean there isn’t other life out there. There is a theory that life could hitch a ride on asteroids or comets to jump from world to world – the process is known as panspermia. There is a solar system nearby called TRAPPIST-1 that hosts seven planets crowded close together. Imagine how easy it would be to spread life across those planets…especially traveling by asteroid.
Up until a recent study by Harvard University, it has been thought possible only within a planetary system like that. Scientists even sterilize spacecraft before they launch into space to prevent Earthly microbes from contaminating other worlds. The Harvard team has calculated just how likely panspermia could be throughout the Milky Way. However, they also discovered that panspermia could even work on a galactic scale. Not only have they found it is possible, but that it is highly likely that panspermia is happening, and a lot.
The Harvard team calculated that there could be as many as 10 trillion asteroid-sized objects ferrying life around the galaxy. Such objects are on the smaller side of the scale, but the team said that larger objects flying through space could be housing life too. For example, an object the size of Enceladus – a moon of Saturn that measures 500 km (310 mi) wide – could host either life or prebiotic material. The researchers estimate that as many as 100 million objects of this size, and even as many as 1,000 Earth-sized objects could be hosting and spreading life.
Idan Ginsburg, the lead author of the study, said:
The biggest worry people had for a long time with this idea was that UV radiation would just destroy life. 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, they had to focus on a more active place in the galaxy – the galactic center, where the supermassive black hole can fling objects outwards. That center is where it all begins. According to Ginsberg, bacteria-carrying asteroids could be ejected from the center of the galaxy into the far reaches of space. They could then be captured by a distant solar system, bringing life to a new world.
Ginsburg said:
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. 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.
The team had to consider many different variables and work out the likelihood of life spreading in this way to get to this conclusion. First off, panspermia would be driven by the gravity slingshot effect produced by the supermassive black hole at the center of the galaxy. Things they had to account for were different speeds of different-sized objects ejected by the black hole, how likely they were to be captured by the gravity of other stars. They also had to calculate long that journey might take, and how robust the types of life would need to be.
Ginsberg said:
With a black hole you can easily accelerate things to anywhere from 1,000 to over 10,000 kilometers-per-second. That’s fast enough to travel across the galaxy, but there is still a chance for an object like that to be captured by a solar system closer to the edge of the galaxy, so it’s possible to transfer life across vast distances in a relatively short time.
The result of their research led them to the conclusion that panspermia is quite likely happening on a galactic scale, with potentially trillions of life-bearing objects whizzing around out there. According to the researchers, the highest chances for being captured by stars 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).
Ginsburg said:
This is a seven-dimensional integral – I don’t think you could consider any more variables without getting into something like string theory. 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.
The research was published in The Astrophysical Journal Letters. Their study is the most comprehensive calculation ever of the likelihood of panspermia occurring in the Milky Way.
Ginsberg said:
We are not the first to have discussed this, but we are the first to really look into this at such a level of detail. Other researchers have mentioned the possibility of galactic panspermia, but when we did the calculations we got these very large values. That suggests that this is not only possible, it’s probable.
For now, this research remains purely hypothetical until scientists manage to observe panspermia in action, something our current technology wouldn’t be able to do. Maybe someday, but for now the best chance of proving their theory true would be to find evidence of microbes in Martian soil samples or asteroids. “We think that, hopefully, people will eventually be able to search for signs of this, and that by studying our own galaxy, it can help us understand the origins of life,” said Ginsberg.
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