Finding alien life, be it microbes or Vulcans, would revolutionize our understanding of our place in the universe, not only because we would no longer be alone in the galaxy, but also because it may help us figure out the origins of life on Earth.
Panspermia is the theory that the seeds of life somehow came to our planet from another world. The idea is controversial at best—most biologists would tell you that it just pushes the problem back a step, because we still wouldn't know what sparked life in the first place. And so far, there’s little reason to think life on other planets should be anything like what we see on Earth.
Now Henry Lin and Abraham Loeb of Harvard University say that if we do see evidence of alien life, the distribution of inhabited planets would be a “smoking gun” for panspermia. According to their model, if life arises on a few planets and spreads through space to others, inhabited planets ought to form a clumpy pattern around the galaxy, with voids between roughly spherical regions. This bubble pattern appears no matter how the distribution happens, whether its aliens traveling by spaceship or comets carrying life’s building blocks.
“It’s not that different from an epidemic,” says Lin, an undergraduate with the Harvard-Smithsonian Center for Astrophysics and lead author of the study, which was accepted by the Astrophysical Journal. “If there’s a virus, you have a good idea that one of your neighbors will have a virus too. If the Earth is seeding life, or vice versa, there’s a good chance immediate neighbors will also have signs of life.”
We've already found almost 2,000 exoplanets, and the next generation of planet-hunting telescopes should be able to search their atmospheres for telltale signs of life. That's when Lin and Loeb's model would come into play.
In an ideal case, Earth is sitting near the edge of a bubble of inhabited worlds. Astronomers looking at life-bearing planets from Earth should then see the nearest living worlds concentrated on one side of the sky. It wouldn’t take that many exoplanets to confirm the distribution—only about 25 will do, Lin and Loeb say.
One of the more popular ways to check whether panspermia is valid has been to look for the building blocks of life—or something actually living—on comets. But the sheer number of comets in our solar system alone means that life-bearing ones could be lost in the crowd, making it hard to definitively test the notion. With this new model, if inhabited planets are randomly distributed, then scientists can be far more confident that panspermia doesn’t work, Lin says.
But while the statistical argument is an elegant one, the visibility of the bubbles depends in part on how fast life spreads. Our Milky Way galaxy is billions of years old, and stars have had a lot of time to move around. The sun, for example, takes a quarter of a billion years to complete an orbit around the galactic center, and it’s made some 20 such orbits over the last five billion years. If it was surrounded by a cluster of other star systems when life started here, they’ve long since scattered.
If panspermia happens relatively fast, on time scales of 100 million years or so, then the bubbles would grow quickly and be dispersed as the stars on the outer edges fell behind those closer to the galactic center. The broken-up bubbles would form new ones, and while they’d be smaller, they would still be detectable, Lin and Loeb write. If life spreads very slowly, then the bubbles will be much harder to see.
Lin also acknowledges that alien life doesn’t need to resemble anything like that on Earth, and that could be another strike against panspermia. We only have one example of a biosphere, and our bias is to look for creatures that also breathe oxygen, for example, and live in the habitable zones of stars. But scientists can think of possible life-forms based on radically different chemistries.
For his part, Lin says astrobiology is an exciting field precisely because it allows for this kind of speculation. “Most of the papers like this are going to be wrong,” he says.