Chunks of rock laced with ingredients for life or early life forms could have traveled among our solar system and others much more frequently than previously thought possible, an international team of researchers including Renu Malhotra in the University of Arizona Lunar and Planetary Laboratory has discovered.
The researchers report in the journal Astrobiology that under certain conditions, there is a high probability that life came to Earth – or spread from Earth to other planets – during the solar system's infancy when Earth and its planetary neighbors orbiting other stars would have been close enough to each other to exchange lots of solid material.
The findings provide the strongest support yet for "lithopanspermia," the idea that basic life forms are distributed throughout the universe via meteorite-like planetary fragments cast forth by disruptions such as planet and asteroid collisions. Eventually, another planetary system's gravity traps these roaming rocks, which can result in a mingling that transfers any living cargo.
“We wanted to know how debris left over from the formation of our solar system can get transported from one planetary system to another,” said Renu Malhotra
, a professor of planetary science in the UA's Lunar and Planetary Laboratory
“Even today, some of these rocks leak out of the asteroid belt and hit planets,” said Malhotra. “That’s how we get meteorites. Some of them land on other planets, and some get thrown out of the solar system.”
“With this study, we wanted to find out what happens to those small rocks that are thrown out and escape the solar system. Where do they go?”
Previous research suggested that, typically, those small rocks called meteoroids leave the solar system at high speeds, making the chances of being snagged in the gravitational pull of another object highly unlikely.
“Those studies assumed a typical velocity of 5,000 meters per second or more,” said Malhotra. “They neglected the small fraction of material leaving a solar system at speeds slow enough to be captured by other planetary systems.”
Using the star cluster in which our sun was born as a model, the team conducted simulations showing that at these lower speeds, the transfer of solid material from one star's planetary system to another could have been far more likely than previously thought, explained first author Edward Belbruno, a mathematician and visiting research collaborator in the department of astrophysical sciences at Princeton University who developed the principles of weak transfer.
Weak transfer describes a low-velocity process wherein solid materials meander out of the orbit of one large object and happen into the orbit of another. In this case, the researchers factored in velocities 50 times slower than previous estimates, or about 100 meters per second.
The researchers suggest that of all the boulders cast off from our solar system and its closest neighbor, five to 12 out of 10,000 could have been captured by the other. Earlier simulations had suggested chances as slim as one in a million.
"Our work says the opposite of most previous work," Belbruno said. "It says that lithopanspermia might have been very likely, and it may be the first paper to demonstrate that. If this mechanism is true, it has implications for life in the universe as a whole. This could have happened anywhere."
The team also found that the timing of such an exchange could be compatible with the actual development of the solar system, as well as with the earliest known emergence of life on Earth.
The researchers report that the solar system and its nearest planetary-system neighbor could have swapped rocks at least 100 trillion times well before the sun struck out from its native star cluster, moving it out of range of other planetary systems. Furthermore, existing rock evidence shows that basic life forms could indeed date from the sun's birth cluster days – and have been hardy enough to survive an interstellar journey and eventual impact.
Co-author Amaya Moro-Martín
, who earned her doctorate at the UA in 2004 and is now an astronomer at the Centro de Astrobiología in Spain, said the weak transfer mechanism would have allowed large quantities of solid material to be exchanged among planetary systems, over timescales that could potentially allow the survival of microorganisms embedded in large boulders.
Star birth clusters satisfy two requirements for weak transfer, Moro-Martín said. First, the sending and receiving planetary systems must contain a massive planet that captures the passing solid matter in the weak-gravity boundary between itself and its parent star. Earth's solar system qualifies, and several other stars in the sun's birth cluster would too.
Second, both planetary systems must have low relative velocities. In the sun's stellar cluster, between 1,000 and 10,000 stars were gravitationally bound to one another for hundreds of millions of years, each with a velocity of no more than a sluggish 1 kilometer per second, Moro-Martín said.
The odds of a star capturing solid matter from another planetary system with a star similar to the sun’s mass are 15 in 10,000, the researchers report – probabilities exceeding those under the conditions proposed by previous publications by a factor of 1 billion.
To estimate the actual amount of solid matter that could have been exchanged between the sun and its nearest star neighbor, the researchers used data and models pertaining to the movement and formation of asteroids, the Kuiper Belt – the solar system's massive outer ring of asteroids – and the Oort Cloud, a hypothesized collection of comets, ice and other matter about one light year from Earth's sun widely believed to be a primary source of comets and meteorites.
The researchers used this data to conclude that during a period of 10 million to 90 million years, anywhere between 100 trillion to 30 quadrillion solid matter objects weighing more than 10 kilograms transferred between the sun and its nearest cluster neighbor. Of these, some 200 billion rocks from early Earth could have been whisked away via weak transfer.
For lithopanspermia to happen, however, microorganisms first have to survive the long, radiation-soaked journey through space. Computer simulations published previously by other researchers showed that survival times ranged from 12 million years for a boulder up to 3 centimeters (roughly 1 inch) in diameter, to 500 million years for a solid objects 2.67 meters (nearly 9 feet) across.
As for the actual transfer of life, the researchers suggest that roughly 300 million lithopanspermia events could have occurred between our solar system and the closest planetary system.
If life arose on Earth shortly after surface water was available, life would have had about 400 million years to journey from the Earth to another habitable world and vice versa before the sun’s star cluster dispersed, the researchers report. Likewise, if life had an early start in other planetary systems, life on Earth may have originated beyond our solar system.
“Our study stops when the solid matter is trapped by the second planetary system, but for lithopanspermia to be completed it actually needs to land on a terrestrial planet where life could flourish," Moro-Martín said. "The study of the probability of landing on a terrestrial planet is work that we now know is worth doing because large quantities of solid material originating from the first planetary system may be trapped by the second planetary system, waiting to land on a terrestrial planet.
"Our study does not prove lithopanspermia actually took place," Moro-Martín said, "but it indicates that it is an open possibility."
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.