A study led by a Princeton scientist has provided the strongest evidence yet that life on earth could have arrived on a rock plunging from outer space.

Princeton University researcher Edward Belbruno and his colleagues published an article in the journal Astrophysics theorizing that earth could have been bombarded with about 3 billion times as much rubble from other planets than was previously believed. The paper gives a major boost to one hypothesis about how the earliest microorganisms came to exist on the hostile environment of the young earth. With more interstellar debris comes a greater chance the earth was “seeded” with life from afar.

The idea of lithopanspermia, or the distribution of life throughout the universe via rocks, goes back to at least 1874. Scientists have long studied the possibility that meteors, ejected by volcanoes or celestial impacts from planets where life existed, bore the elements of life or even simple organisms over the gulf of space, and may have jump-started the formation of life on Earth.

For lithopanspermia to be true, Earth would have to have been hit by countless meteors originating from distant planets, and those meteors would have had to contain at least the chemical elements of life, if not life itself. Until now, it seemed unlikely that the first part of this equation was plausible.

That’s because previous theories held that most rocks escaping any given solar system would be traveling at rocket-like speeds of 18,000 miles per hour. Something that fast would have a miniscule chance of striking the Earth.

Therefore, scientists believed that even if the solar system had been close to another star in its early days, only tens of large rocks from the other star could have possibly come into the system from its neighbor, with maybe one or two hitting the Earth. Although scientists agree that Earth was pulverized with rocks billions of years ago in a time known as the “Heavy Bombardment,” it was thought most of these meteors came from inside the solar system.

But what if the ejection from another star was more of a slow lob than a cannon shot? Slow rocks would be drawn into the solar system by gravity, where fast rocks would zoom past.

Belbruno, working with colleagues from the University of Arizona and the Centro de Astrobiología in Spain, explored the possibility that rocks could be traded between planets and star systems at far slower speeds of about 100 meters per second, about the speed of a racecar, via a mechanism called “weak transfer.”

Belbruno first described objects in a “weak stability boundary” in 1986, when he was a young mathematician working for NASA’S Jet Propulsion Lab, devising spacecraft trajectories.

How his weak transfer idea came to be accepted by the scientific community is a story in itself.

Belbruno recalled that at that time he came up with it, his job wasn’t going well. Belbruno had invented a new way to steer NASA space missions. He reasoned that if a ship could be positioned in a slow orbit around a celestial object such that it was on the verge of drifting off into space, it would only take a tiny nudge with an engine burning a negligible amount of fuel to send the craft off to a new destination – the moon, Mars, Jupiter, or anywhere, really.

He figured if the trajectory was right, it was even possible for the nudge not to come from the spaceship’s engines, but from the subtle gravitational tug of a distant planet or moon. In a field where every pound of fuel lifted into space costs thousands of dollars, Belbruno had just discovered how to get a free ride, surfing on the tides of chaos.

But Belbruno’s colleagues were not impressed, he remembered, partly because such a meandering journey through space would be extremely slow, even if it worked. The traditional method for traveling to the moon was to go virtually straight at it, then use rockets to slow down. The direct path from the earth to the moon took days, but Belbruno’s method took two years.

It took a crisis to make everyone realize that Belbruno’s “weak transfer” method not only worked, but was useful.

“In 1990, I was sort of politely asked to leave,” Belbruno said. “On the particular day I decided not to pursue it anymore, that’s when some engineer knocked on my door and said the Japanese had one of their small spacecraft go on a transfer [to the moon] and they lost it. The mothership was going around the earth and had hardly any fuel, and they wanted to rescue it … it was at that moment that a flash of insight hit me, and I said, ‘I know how to rescue that spacecraft.’”

Computer simulations showed it would work, but Belbruno was as surprised as anyone when they tried it, and it was a success.

The rescue of the Hiten space probe in 1991 was a turning point in Belbruno’s career and the idea of weak transfer.

“If that particular guy did not knock on my door that day with this proposal, I would not be here talking to you right now. My work would have ended because I would have left JPL without having done any useful work to show for four years except computer results,” Belbruno said.

After that, Belbruno studied the implications of weak transfer on celestial objects rather than spacecraft.

In 2004, Belbruno had a conversation with Princeton Astrophysics chair David Spergel that set off an eight-year research project. Belbruno credits Spergel with making him realize that if a spacecraft or a comet could change orbits with so little energy, so too could a smallish rock, and that this might have implications for lithopanspermia.

Belbruno at first thought making the calculations would be easy, and he would be done with the paper fairly quickly. But he soon realized he needed to supplement his expertise in orbital mechanics with detailed knowledge of the conditions that would have been needed for lithopanspermia to occur. He enlisted the help of Amaya Moro-Martín, an astronomer at CAB and a Princeton visiting research collaborator in astrophysical sciences and Renu Malhotra, a professor of planetary sciences at Arizona, who became co-authors of the paper.

The team, working over eight years, fleshed out the details of how slow space rocks could have traversed the void between the stars.

Because the speeds of weak transfer are so low, the process would only work with star systems in very close proximity, and right now, the solar system is isolated, with its closest neighbor, Proxima Centauri, being 4.2 light years away. This distance is much too far for any rocks to be exchanged via weak transfer.

It is thought that the sun was once part of a cluster of stars packed relatively close together. The paper notes that since the earliest evidence for life on earth is as old as 3.8 billion years ago, when the earth was 718 million years old, and the latest possible time for the sun leaving its stellar nursery was 300 million years later, it is conceivable that billions of rocks hit the Earth during that time.

The researchers decided to further investigate the slow transfer scenario by running a computer simulation. Princeton graduate student Dmitry Svransky ran a Monte Carlo simulation — calculating millions of trajectories at once — that supported Belbruno’s hypothesis.

The paper concludes that overall, about 300 million possible lithopanspermia events could have occurred between Earth and a planet in a nearby star system, noting the possibility that Earth seeded another planet as well as the other way around.

The evidence is compelling enough that Belbruno believes it’s a likely possibility.

“Based on my experience doing this research, I believe now that is likely that the raw materials necessary for life to have evolved on the earth came from rocks which were ejected from other planets,” he said.

The paper, “Chaotic Exchange of Solid Material between Planetary Systems: Implications for Lithopanspermia,” was published Sept. 12 by Astrobiology, and was supported by grants from NASA, the National Science Foundation and the Ministry of Science and Innovation in Spain.