An intriguing asteroid was spotted traveling backwards around Jupiter back in 2015. Now a team of researchers think it could have formed around another star.
An asteroid found traveling backwards along Jupiter´s orbit has been puzzling astronomers since it was discovered in 2015. Most asteroids move around the Sun in the same direction as planets do, a motion inherited when the solar system formed from a swirling cloud of dust and gas. But asteroid 2015 BZ509 (or BZ for short) goes the wrong way, in a retrograde motion around the Sun. Now, a team of astronomers think they know why: It might have come from outside the solar system.
BZ’s motion is co-orbital with Jupiter, meaning that it travels around the Sun in the same orbital space — but in the opposite direction. It comes close with Jupiter twice every orbit, and also with the roughly 6,000 Trojan asteroids that inhabit Jupiter’s orbit. Yet it avoids crashing into any of them because its orbit is inclined and slightly off-center. The asteroid receives a slight gravitational nudge every time it approaches Jupiter, which helps keep things stable.
But, how did it get there and when?
Fathi Namouni (Côte d’Azur Observatory, France) and Helena Morais (Universidade Estadual Paulista, Brazil), think that the only way to explain BZ’s kamikaze behavior is assuming that came from outside the solar system. Just like Oumuamua, the interstellar object that zoomed across the solar system in 2017, they claim that BZ could have come into our neighborhood in a similar way, only Jupiter’s gravity captured it for a longer stay. That event could have happened as far back as 4.5 billion years ago.
Using computer simulations, the team has shown that BZ’s current orbit could be stable for the age of the solar system. They simulated 1 million “clones” of the object, tracking the evolution of their orbits over time. BZ’s orbit isn’t known with a lot of precision — it was only observed during a span of 300 days through the Large Binocular Telescope on Mt. Graham, Arizona. The simulated orbits include small variations to make up for that uncertainty.
This animation shows the asteroid's peculiar retrograde, co-orbital motion around the Sun.
By tracking evolution of the system backwards in time, the researchers have shown that 27 out of the million clones are stable beyond 4.5 billion years. Their results appear in the May 21st Monthly Notices of the Royal Astronomical Society Letters.
The key question is how to interpret these results. Namouni and Morais argue that these 27 long-term stable show that BZ could have maintained its current orbit since the solar system’s beginnings.
“We apply what is known in astronomy as the Copernican principle, which stipulates that systems are never observed at preferred epochs,” says Namouni. In other words, we’re not observing BZ just at the right time after it was captured. “This translates into: When we have unstable orbits and stable ones that live for the age of the solar system, we have to take the stable ones.” This approach is commonly used to determine configurations of exoplanetary systems — by assuming that they’ve existed as we observe them for a long time, scientists can remove any solutions that lead to a short-lived system.
But other researchers aren’t convinced. The median lifetime of all the clones in the simulation is 7 million years, notes Bill Bottke (Southwest Research Institute). “That is not a long time by solar system standards. It likely suggests the object was captured onto this orbit over more recent times.”
Bottke suggests that other explanations might be more suitable. For example, this body could be a captured comet; those objects belonging to the same family as Halley’s Comet often have retrograde orbits.
Namouni & Morais don’t try to provide a scenario of how the capture of an interstellar object could have happened. The intent of their work was simply to explore the orbit’s stability and determine if the object could have originated in the solar system.
Still, the capture from interstellar space remains difficult to prove. “Interstellar objects come in at high speed, and therefore they tend to escape,” says Alessandro Morbidelli (Côte d’Azur Observatory, France). “So, altogether this looks implausible to me.”
The upside is that the hypothesis is partly testable, says Renu Malhotra (University of Arizona). If they are correct about the long-term stability of this object’s orbit, then there should be many more objects in similar orbits. “Ongoing observational searches should be able to find them. If lots are indeed found, it would bolster the ‘long-term stable’ hypothesis,” Malhotra says. “Finding such objects could help us test theories about the early history of the solar system!”