Late last year, astronomers detected the first known interstellar asteroid, dubbed 'Oumuamua. New research suggests these exotic objects are more abundant than we thought, an observation that boosts the panspermia hypothesis - the idea that asteroids seeded life on Earth. At the same time, the presence of so many foreign objects in our Solar System could also change the way we search for extraterrestrial life.
The cigar-shaped 'Oumuamua (pronounced "oh-moo-ah-moo-ah" and meaning "a messenger from afar arriving first") may have caught astronomers off guard when it zipped through our Solar System last November, but they have been studying it at a fevered pitch ever since. Scientists are trying to understand where it came from and what it's made of and see if it can tell us anything about how planets form and what distant star systems might look like. Some scientists even speculated that 'Oumuamua might be a spaceship or some kind of probe, but that doesn't appear to be the case.
Another important implication of the discovery is that it's challenging our notions of how common such interstellar objects are, both in the Milky Way galaxy and our Solar System. New research by Manasavi Lingam, a postdoc at the Harvard Institute for Theory and Computation, and Abraham Loeb, a researcher at the Harvard-Smithsonian Center for Astrophysics, suggests our Solar System is acting like a celestial "fishing net," capturing these objects with surprising frequency. Their new study, which is currently being considered for publication in The Astrophysical Journal, suggests there may be as many as several thousand interstellar asteroids in our Solar System at this very moment.
Lingam and Loeb used a computer model to calculate the rate at which our Solar System has been able to capture objects like 'Oumuamua over time, using the gravitational strength of the Sun and Jupiter as the metaphorical fishing net (the so-called three-body interaction model). They did the same for another star system, the binary stars Alpha Centauri A and B, to compare the results and to get a sense of the conditions elsewhere (binaries are exceptionally common in our galaxy). The hypothetical frequency of interstellar objects was pulled from a recent study by researchers at the Institute for Astronomy at the University of Hawaii, who revised their estimates in the wake of 'Oumuamua.
Lingam and Loeb's model suggests thousands of interstellar objects are inside our Solar System at any given time, some potentially as large as a few dozen kilometers in size. In the case of Alpha Centauri A and B, this binary system has the potential to capture Earth-sized objects - meaning it could snatch rogue planets drifting through interstellar space.
The researchers also modelled the rate at which such interstellar objects may have collided with Earth - an important implication for the panspermia hypothesis. Or more specifically, the lithopanspermia hypothesis, whereby a chunk of material originating from a different planet, whether inside or outside the Solar System, delivers the seeds of life during a collision with another planet. These "seeds" could be bits of RNA/DNA, some kind of alien extremophile, or even just the organic, biochemical prerequisites required for life (a process known as pseudo-panspermia). Scientists aren't entirely sure if these seeds can survive the rigors of space, atmospheric entry, and the ensuing collision, but life and biological matter appear to be surprisingly durable and resilient (see here, here).
Incredibly, Lingam and Loeb's model showed that around 400 interstellar objects with a radius of 325 feet (100 meters), and around 10 objects nearly a kilometer in size, could have struck Earth prior to abiogenesis - the moment when life first emerged on our planet some 3.8 billion years ago. "Hence, this opens up the possibility that life could have been transferred to the Earth by means of lithopanspermia," conclude the authors in the study. The "seeding" of life could've happened in one of two ways, either through a direct impact with Earth, or by "glaciopanspermia" whereby an asteroid strikes another planet or planetesimal (e.g. Ceres) in our Solar System, and it spreads to Earth from there.
"The 'seeds' of life can be protected by means of shielding, for example by being buried deep inside the rock or ice," Lingam told Gizmodo. "There have been several laboratory studies that suggest interplanetary panspermia - even over 10 million-year timescales - may be feasible, and hence interstellar panspermia, which may involve billion-year timescales, could also be possible."
In addition to boosting the panspermia hypothesis, the new study also offers a new strategy for searching for extraterrestrial life. Instead of using our telescopes to scan planets in distant star systems, Lingam and Loeb say we should examine the interstellar objects captured by our Solar System.
"The presence of thousands of interstellar objects means that a third route is open to exploring objects outside the Solar system," said Lingam. The two other routes being the use of telescopes to study planets remotely (which is feasible) and sending interstellar probes (something we're not even close to doing). The third route - detecting and exploring interstellar asteroids within our Solar System - is also feasible with current technologies. And should we want to explore the millions of objects in our Solar System's outer Oort Cloud, we could construct a solar sail.
The trick, however, will be to tell which asteroids are native and which are foreign.
"The situation resembles a family dinner," Loeb told Gizmodo. "You sit down for dinner and look around the table and you assume that everyone is a member of the family. But occasionally you realise that there is a guest who is not related to the family. Then you can learn about the outside world by examining that person."
He says the simplest way to discern between the two is to examine the abundance ratio of Oxygen isotopes in the water vapour that produces cometary tails, which can be done through high-resolution spectroscopy. "After identifying a trapped interstellar object, we could launch a probe that will search on its surface for signatures of primitive life or artifacts of a technological civilisation," said Loeb.
The researchers added that it should be easier to detect smaller objects at closer distances using reflected sunlight, but that larger objects at greater distances could also be detected by scanning for their heat signatures.
Of course, there's lots we don't know about interstellar asteroids and how abundant they really are, whether on a galactic or local scale. The researchers are using an estimate to power their models, and scientists have only ever discovered one such object. We also don't know if panspermia is actually a thing, or whether such a process might actually work. Intense radiation in space could fry any RNA/DNA that may be hitching a ride aboard an asteroid, and any life-giving material could be destroyed on impact. What's more, we have no reason to believe that life didn't originate spontaneously on Earth.
But until we have definitive answers to all these questions, it's fair to speculate and explore the possibilities.
This paper is currently being considered for publication in The Astrophysical Journal, but a pre-print can be found at arXiv.