Nearly all of the objects that pepper our solar system revolve around the sun in one direction, opposite the hands on your wristwatch. A small percentage, however—less than one-hundredth of one percent—circle the sun in the direction opposing the normal stellar commute.
Strikingly, according to a new study published last week in Monthly Notices of the Royal Astronomical Society: Letters, a backward-orbiting asteroid named
2015 BZ509—nicknamed Bee-Zed—made itself at home in our solar system’s earliest incarnation. This, at least, according to a simulation performed by Helena Morais, PhD, a researcher at Brazil’s São Paulo State University, and Fathi Namouni, PhD, a CNRS researcher at the Côte d’Azur Observatory in Southwestern France.
In October 2017, ‘Oumuamua’, an oblong interstellar object, entered our solar system’s interior, and then it shot back out whence it came—that is, towards the solar system’s exterior and, beyond it, interstellar space. But, it was only a visitor to our star system. Bee-Zed, the new research suggests, was here from the get-go of our solar system’s life.
Drs. Morais and Namouni have studied various aspects of celestial dynamics during their careers, but, only recently, in 2012, did their attention turn to the study of retrograde Centaurs—the clockwise-orbiting asteroids that are observed between the orbits of Jupiter and Neptune, explain Drs. Morais and Namouni. Their unusual orbits and unknown origins made these asteroids prime targets for scientific inquiry. “Such extreme orbits must have witnessed violent histories in order to have been reversed,” says Dr. Namouni.
Drs. Morais and Namouni’s work in this area—that is, on retrograde Centaurs—is foundational to the study of these celestial bodies. Their work includes the retrograde coorbital resonance—equal orbital frequencies but opposite orbital motion, says Dr. Morais. This stable resonance allows Bee-Zed to remain in its shared orbit with Jupiter, in a temporary “trap.” Jupiter pulls on the celestial rock in one direction on first pass, then in the opposite direction on another, over the course of 12 years. These interactions cancel each other out, leaving Bee-Zed’s orbit intact.
Discovered in 2015, Bee-Zed’s shared orbit with Jupiter’s, old, chaotic path around the sun suggested a recent origin for Bee-Zed and a tough origin story to trace back to the formation of the planets 4 billion years ago. In 2017, though, researchers conducting a 100-clone simulation found that Bee-Zed clones had stable orbits lasting over 1 million years, an age 100 times greater than Drs. Morais and Namouni found for other Centaurs in 2012, which suggested a different origin from that of other Centaurs. In a step further, Drs. Morais and Namouni decided to use supercomputing to conduct a massive simulation.
Beating The Chaos
The retrograde Centaurs’ orbits between the giant planets of the solar system are home to ‘orbital chaos’. Unlike the dramatic, fictionalized take on the branch of mathematics portrayed by Ashton Kutcher in the movie, The Butterfly Effect, chaos simply means that changing the initial conditions of say, the orbit of a hunk of rock in space, yields drastically different outcomes over long timescales. In this case, slightly changing the initial position of the asteroid based on small variations in measurements drastically alters its final position over a massive ‘spread’ of possibilities if given enough time. Accounting for all of these possible outcomes remained elusive until modern supercomputing was up to the task. “Scientists used to study such orbits just for short time spans. “There was no way to beat the chaos inherent to their motion between the giant planets of the solar system,” says Dr. Namouni.
Drs. Morais and Namouni created a virtual sample of 1 million asteroid clones that covered the known error bars on the real asteroid’s orbit, then, using the power of supercomputers, they simulated the virtual asteroids’ motions backward in time and observed how the virtual space rocks spread across the solar system’s lifetime of 4.5 billion years. “The spread is caused by chaos, but because the sample is huge, we cover all possibilities to which the chaotic system can lead. Further, we can go and see if there are special locations where clones congregate implying a stronger likelihood for the asteroid’s original location,” says Dr. Namouni.
If you arrange the ages of all the simulated asteroids from least to greatest, the age in the middle of that set is 6.48 million. 99.9% of the clones were destroyed by the sun, collided with a planet, or they were shot into outer space. However, 46 of these clones remained at the simulation’s conclusion at 4.5 billion years, and 60% of these were similar to Bee-Zed’s current orbit.
Dr. Namouni says that, as opposed to a model, the recent study was a hi-res simulation. “To put it in a colorful way, we kind of invented the equivalent of CT scan to study the origin of centaurs for which, until now, only classical projective radiography was used,” says Dr. Namouni.
The co-orbital asteroids of Jupiter, also known as the ‘Trojan asteroids’. The prograde
asteroids are shown in white, and 2015 BZ509 (with a trail, shown in green) appears later.
The planets and asteroids have been enlarged for visibility.
Facing The Critics
Hal Levison, a former colleague of Dr. Namouni’s during their participation in NASA’s Cassini imaging initiative, cautioned against making definitive conclusions in light of the study, for now, given the lack of a dynamic modeling approach, and that Morais and Namouni’s conclusions rest in the assumption that no other possibilities exist.
Dr. Namouni agrees that building a model may be a next step, but notes that the predictive power of the simulation is lost in the process. “Levison is asking us to build a model—or a theory—and show how to capture Bee-Zed. A model by definition has an artificial component…on the other hand, our method is quite simple in principle, we just follow a real asteroid—with no artificial setting—in order to find where it came.”
Making The Interstellar Case Stronger. While Drs. Morais and Namouni aren’t averse to modeling, they say a stronger case for Bee-Zed can be made in two other ways. “One is to find other asteroids like Bee-Zed near Jupiter, a family of sorts. The second way is to know the composition of Bee-Zed by observing the asteroid,” says Dr. Namouni.
Ramon Brasser, PhD, a researcher at the Tokyo Institute of Technology, who wasn’t involved with the new research, helped clarify the points of criticism, as well as the merits of Drs. Morais and Namouni’s assertions. “When simulating both forward and backward in time, Jupiter will eject the majority of these objects to interstellar space. This is a well-known outcome, and it is this outcome that led the authors to conclude that the object came from outer space,” he says. “It is that fact that Levison was arguing against, but Fathi is right when he says that to test those other sources requires a model and certain assumptions.”
Levison’s colleague, Bill Botkke, of the Southwest Research Institute, also told National Geographic that “short term solutions” to Bee-Zed’s origins make more sense, since the simulation found a median clone lifetime of roughly 7 million years. Dr. Basser interprets Botkke’s statements to mean that the authors cannot rule out recent capture of Bee-Zed onto its current orbit in the past 7 million years. “Such a capture is a valid hypothesis because we don’t, and never will know, when Bee-Zed came to be on its current orbit. We only know, based on Morais Namouni’s work, it could be as long as 4.5 billion years ago,” says Dr. Brasser.
One area on which all parties to the study seem to be in agreement: Bee-Zed couldn’t have been produced by collisions. “The planet forming community around the world has never produced retrograde orbits by colliding planetesimals,” says Dr. Namouni.
Origin Story: Start at The Source
“Since the motion is chaotic, it cannot be easily predicted and thus a statistical study such as this one is needed,” says Dr. Basser. However, the simulation does not tell you from where the object originated, he says.
Dr. Brasser says that, to be sure about Bee-Zed’s origins, use modeling, similar to a 2012 he conducted. If they want to determine an interstellar origin, he says, bombard the inner solar system with interstellar objects whose initial distribution is computed a priori, then have these fly past the giant planets and see if any of them get captured—in simple terms, a “flyby model” of capture based on nearby planets, explains Dr. Basser. “This would be the logical way to do this: start at the source, not at the destination,” he says.
Bee-Zed is like a tree that hides in a forest, says Dr. Namouni, and, he says, the results of the simulation bolster the notion that the solar system did not form in isolation from the rest of the galaxy. “The sun in its original star cluster captures comets and asteroids, and perhaps even planets, as some suggest the hypothetical planet 9—previously known as planet x in the 1980s—could itself have been captured that way,” he says. The process is symmetric in that other stars must have captured solar system bodies too, explains Dr. Namouni.
“Once we knowBee-Zed’s composition and that of similar space rocks, we can understand the influence of the sun’s stellar environment on its planets,” says Dr. Namouni. Knowing Bee-Zed’s composition may reveal the origins of earth’s water—a yet-unsolved mystery—along with the origins of organic material and water in the other planets, he says.
Michael Riviello serves as the Interim Editorial Director for Modern Scientific.