When the U.S. football field–size, cigar-shaped object ‘Oumuamua entered our solar system last year, it didn’t just give us our first glimpse of an interstellar piece of rock. It also bolstered the plausibility of space rocks spreading life among the stars by ferrying microbes between distant star systems, according to a new study. “Life could potentially be exchanged over thousands of light-years,” says author Idan Ginsburg, a postdoc at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The idea, known as panspermia, has been around for centuries. Some astronomers have even speculated that life on Earth was seeded by microbes that hitched a ride on debris ejected from another life-harboring world in the solar system, perhaps on meteorites from Mars. But it seemed improbable that life could have come from interstellar space.
Take computer simulations in 2003 by planetary scientist H. Jay Melosh, now at Purdue University in Lafayette, Indiana. The analysis revealed that about a third of the meteorites shot off Earth were eventually thrown out of the solar system by Jupiter or Saturn, but that the process took millions or tens of millions of years—a long stretch for even the toughest bugs or spores to be exposed to the vacuum and radiation of space. And vanishingly few rocks would ever be captured by some distant system, Melosh found.
The outlook improves if the receiving system is a binary star, which has a more complex gravitational field than the solar system. Yet any system that’s good at capturing is also good at ejecting, meaning refugees from another system are far more likely to be tossed back out in a game of interstellar hot potato than to settle on a hospitable world.
‘Oumuamua is providing fresh hope for the idea of galactic panspermia. For the telescopic survey that found it, the Panoramic Survey Telescope and Rapid Response System, to have detected such an object in the region it had scanned, our Galaxy needs to have 1 trillion of them per cubic light-year, according to study published earlier this year. To fill space like this, every star in the Milky Way would have to eject 10 quadrillion such objects, and a few should be passing through our solar system at any given moment.
In the new study, Ginsburg, along with astrophysicists Manasvi Lingam and Abraham Loeb, also of the Harvard-Smithsonian Center for Astrophysics, calculated the chances of such objects delivering life to an alien world. A binary star such as Alpha Centauri would ensnare a few thousand rocks of ‘Oumuamua’s size every year, and our solar system might snag one a century, the team estimates in a preprint posted last week on arXiv and in a forthcoming paper in The Astronomical Journal.
The researchers then multiplied this capture rate by the number of stars an interstellar object will encounter before whatever bugs it carries all die. If the objects move, like ‘Oumuamua, at a velocity of 26 kilometers per second through interstellar space, 10 million of them will be captured somewhere in the Milky Way in a million years. “If you look at the galaxy as a whole, you expect this to happen fairly often,” Ginsburg says.
Astronomer Jason Wright of Pennsylvania State University in State College says the analysis has merit: “For reasonable numbers, this suggests that planets and asteroids are commonly exchanged between stars.”
But astronomer Ed Turner of Princeton University says the authors may be reading too much into the single example of ‘Oumuamua. “There’s no rigorous mathematical argument you can write about one event evaluated a posteriori,” he says.
And even if our galaxy is thick with ‘Oumuamuas, they are unlikely vectors of panspermia, Melosh says. ‘Oumuamua is way too big to have been ejected from an inhabited planet, he says.
Still, Loeb says more data could settle where galactic panspermia is plausible. Additional discoveries of interstellar interlopers would clarify their prevalence, and Loeb says the detection of life on other worlds would show whether it tends to cluster, as it would if it arose in one place and spread elsewhere by panspermia. If so, he says, our entire galaxy might be considered biologically interconnected, its vast distances offset by vast spans of time and the vast number of objects that set out to cross the void.
*Correction, 16 October, 10:55 a.m.: This story has been updated to note that the new study will appear in The Astronomical Journal. A link to the study has been added.