Life on comets

But how could life find its way onto a comet? According to Loeb, Earth and other worlds may not even need to be hit by space rocks to send microbes out into the cosmos.

In a January 2019 study published in the International Journal ofAstrobiology, Loeb and a Harvard undergraduate student, Amir Siraj, suggest that Earth-grazing comets and interstellar objects could have snagged microbes from high in our atmosphere and then carried them out into the Milky Way. Their estimates predict this could have already happened many times, depending on how high up life exists on our planet.

"We found that there could be thousands, if not tens of thousands, that could pass through the Earth's atmosphere, collect microbes, and then get kicked to another solar system," Loeb says.

Loeb and Siraj even think they may have identified the first known interstellar meteor by mining existing databases and studying known object's trajectories. However, they won't be sure until they can get someone with a U.S. government security clearance to provide them with the classified raw data. The object was likely picked up by a missile alert system, but Loeb says he's struggled to get the information they need to finish their study.

In our own system, Jupiter and the Sun also could act as a "fishing net" that permanently captures interstellar objects, instead of letting them sail through like 'Oumuamua did. Binary star systems including Proxima Centauri, our nearest neighboring star would have an easier time catching interstellar objects, according to Loeb.

But one of the biggest unknowns is still how long a lifeform could survive on an interstellar object. "The lifetime is really critical because it determines how long it can travel from the system it left," he says. "We don't have good experimental data."

Some fraction of microorganisms would likely make it below the object's icy surface, Loeb says, where they'd be sheltered from radiation. But if they can only survive there for 100,000 years, it would severely restrict panspermia's prospects. If they could instead survive tens of millions of years, then life would have a decent shot at traveling between stars.

Loeb points out that tardigrades an eight-legged micro-animal found all over the planet have endured the vacuum of space, returned to Earth, and still managed to reproduce.

"Even tiny animals are known to be very good astronauts without even a suit," he says. "Viruses and bacteria may be able to survive much longer."

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Could alien life hitchhike to Earth on space rocks from other stars? - Astronomy Magazine