Life from other star systems

But it’s one thing for worlds within a solar system to cross-pollinate. It’s quite another for alien life to survive the light-years-long journey from one star to another. These extreme distances have pushed many astronomers to reject the idea of panspermia between star systems. The trip would require life to sit on an asteroid for perhaps hundreds of millions or even billions of years while it's bombarded with cosmic radiation. Survival would depend on whether these microorganisms could get below the surface before they're destroyed.

However, the recent discoveries of two interstellar visitors have brought new life to the idea.

In 2017, astronomers in Hawaii using the world’s top asteroid-hunting telescope discovered something totally new: a fast-moving object on an orbit that approached our Sun just once. It was the first known interstellar object to visit our solar system. And this space rock, named ‘Oumuamua, has some truly alien properties.

Astronomers had always expected to find interstellar comets. Their orbits at the outer edges of star systems should make it easy for a passing star to give them a gravitational bump that knocks them loose. However, the best estimates suggested astronomers wouldn't start finding interstellar comets until bigger and better telescopes were built. Even more confusing, 'Oumuamua looked like an asteroid. For starters, it lacked a distinct coma — the fuzzy envelope of sublimated ices created when comets get close to the Sun. The cigar-shaped object is also 10 times longer than it is wide. That’s far more elongated than anything seen in our own solar system.

"In my book, it's still very peculiar," Loeb says. "It’s a very weird object."

And while astronomers are still debating if ‘Oumuamua was a comet or an asteroid, the space rock did leave a few clues about its composition. For one, it’s dense and rocky, perhaps even metallic. But there’s also evidence that it’s icy — a significant find because it implies those ices weren’t baked off over the eons.

Astronomers can’t say what star ‘Oumuamua came from, let alone if that system had Earth-like exoplanets. But based on its trajectory, the space rock must have been flung from its home star system at least several hundred thousand years ago, but it also could have circled the Milky Way several times on a course that’s lasted billions of years.