For billions of years, without our knowing it, scientists say, our solar system has harbored a visitor from another star.
This interstellar immigrant — an asteroid dubbed 2015 BZ509 — lurks in the orbit of Jupiter, where it circles the sun in the opposite direction of the gas-giant planet and nearly everything else in the solar system. The twin facts that it shares an orbit with a planet and flies against the flow of traffic makes it like no other known object in our celestial neighborhood.
According to new research published Monday in the Monthly Notices of the Royal Astronomical Society, that's because it doesn't come from here. Careful analysis of the asteroid's orbit and repeated simulations of every possible origin scenario found it most likely migrated from another planetary system when the sun still inhabited the stellar nursery of its infancy 4.5 billion years ago.
“We were not at all expecting to find out it came from another star,” said Fathi Namouni, an astronomer with the Observatory of Cote d'Azur in France who co-authored the study alongside Sao Paulo State University researcher Helena Morais.
“But the general idea, which is very encouraging, is we now know the solar system did not form in isolation,” Namouni said.
It's a provocative conclusion — reached by process of elimination and without demonstrating how 2015 BZ509 could have been captured from another star, outside researchers noted.
“It's feasible what they're saying, but there are other possibilities,” said Elisa Quintana, an astrophysicist at NASA's Goddard Space Flight Center who was not involved in the new study. “We live in such a dynamical universe, it's hard to rule out just pure chaos and collisions.”
Last year, Quintana was part of a team of NASA researchers who analyzed 'Oumuamua — the first interstellar solar system visitor scientists have ever seen. That cigar-shaped space rock is tumbling past us at such high speeds it could only have originated elsewhere, researchers said, and it's already on its way back out into space. It will pass Neptune's orbit in 2022 and eventually leave the solar system en route to the constellation Pegasus, according to NASA.
By comparison, 2015 BZ509 — known to those who research it closely as “Bee Zed” — appears to be here for the long haul.
The asteroid's orbit immediately made it an outlier when it was discovered in 2014. Because basically everything in the solar system was born from the same spinning cloud of dust and gas that circled the infant sun, it takes extraordinary circumstances to knock an object into a “retrograde,” or backward, orbit. Just 95 of the nearly 800,000 known asteroids behave in this manner.
Even among that select group, Bee Zed stands out. Whereas most retrograde asteroids are thought to be unstable, a 2017 study found Bee Zed has inhabited its current position for at least a million years. It seems to be held in place by its unique relationship with Jupiter. As the three-kilometer space rock passes the gas giant — twice during every 12-year-orbit of the sun — the gravitational interactions balance each other in what's known as an “orbital resonance.” (This same phenomenon keeps Neptune and Pluto from crashing into each other, even though their orbits cross.)
Namouni and Morais sought to push this back even further in time, building a computer simulation of our solar system and populating it with a million digital “clones” of Bee Zed. Each followed slightly different orbital parameters based on astronomers' limited knowledge of Bee Zed's behavior. Of the scenarios that were stable over the lifetime of the solar system (the only statistically likely scenario that would allow humans to witness it), most placed Bee Zed in its current position since the beginning.
After considering a range of possible origin stories, the scientists concluded that the asteroid must be a migrant. In a news release from the Royal Astronomical Society, Morais explained that the sun formed in a tightly packed cluster with other newborn stars.
“The close proximity of the stars, aided by the gravitational forces of the planets, help these systems attract, remove and capture asteroids from one another,” she said.
If confirmed, the discovery could indicate that at least some components of our solar system came from elsewhere. Namouni called for further observations of the object, especially measurements of its composition that could reveal whether it carries water. The results might reveal further hints about the asteroid's origins and raise the possibility that our early environment was “enriched” by interstellar interactions.
But Quintana emphasized that this extrasolar origin story is still just one possibility of many. She could imagine scenarios in which a combination of collisions and a large disturbance of the solar system — perhaps from a passing swarm of objects, or the movement of the hypothetical “Planet Nine” — pushed Bee Zed into its current position.
Other scientists suggested the object may be an inactive comet knocked in from the Oort cloud — the bubble of icy debris that enshrouds the outer solar system. Retrograde motion is more common in comets; for example, Halley's comet circles the sun backward and mostly below the plane in which the planets move.
Whatever the answer, the questions posed by Bee Zed are worth probing further, Quintana said.
“Every time we find these weird objects in our solar system, it's just another puzzle piece that we have to fit with our idea of how planets formed,” she said.