A new paper suggests that there may be 19 asteroids — all part of the Centaur family — that didn’t come from our own solar system at all. If this were proven true, it would mean we have ancient material formed around an alien star in orbit in our own solar system.
Centaurs are some of the most interesting space rocks in our solar system. A Centaur is formally defined as a small body that orbits between Jupiter and Neptune, and that crosses the orbit of at least one giant planet. They sit outside the range of the Jupiter trojans but are much closer to the sun than either the scattered disc or Kuiper Belt. The fact that they cross the orbit of at least one giant planet means that Centaurs do not have stable orbits beyond a time scale of a few million years.
Centaur orbits, as compared to the outer planets, Kuiper Belt objects, and the scattered disc. Image by Wikipedia, CC BY-SA 3.0
Centaurs are called such because they demonstrate the characteristics of both asteroids and comets. The first Centaur discovered, 2060 Chiron, was found in 1977. Initially, it brightened by 75 percent and developed a cometary coma, with a tail detected in 1993. But Chiron is approximately 220km in diameter, much larger than a typical comet nucleus, and it may have a ring system — something never observed around a comet. What do you call a half-comet, half-asteroid? A comeroid Centaur, obviously.
The fact that a Centaur’s orbit around the sun isn’t stable is one of their defining characteristics, which is where this new paper comes in. According to the researchers, there are 19 Centaurs whose orbits and inclination within our solar system can only be explained if they did not originate here at all. Objects in unstable orbits around the sun follow one of three outcomes: They assume stable orbits around either a planet or our sun, they fall into the gravity well of either a planet or the sun, or they are ejected back into the interstellar medium.
Prior to this study, only one potential extra-solar Centaur had been identified, 514107 Kaʻepaokaʻawela. The idea that there might be 19 such objects out of a total population of between 44,000 – >10M is not crazy, nor is the idea that our solar system might have captured such asteroids in the relatively recent past. We have recently detected two comets with an interstellar origin — comet Borisov and the cigar-shaped ‘Oumuamua.
The authors, Namouni and Morais, analyzed the orbits of Centaurs with particularly tilted orbits relative to the plane of the ecliptic (the plane in which the major planets and asteroids all move). They found 19 objects whose orbits cannot be explained if they began life as objects that formed within our solar system. If they’re captured space rocks, on the other hand, their orbits are perfectly explainable. It is not clear that all 19 objects are from the same ejection event, either.
Namouni and Morais do not claim that their gravitational interaction simulation proves that Centaurs are extrasolar captures. But if they are, it would mean there are pieces of another solar system caught within our own. Such material might date back to the Sun’s formation (if it was captured near the beginning of our solar system) or it could have been captured within the past few million years. The dinosaurs, for all their many accomplishments, were absolutely terrible at leaving astronomical records.
Again, there’s precedent for this kind of theory. We have already located two “stellar siblings” of Sol — HD 162826 (110 LY distant, F8V star, 4.5B years old) and HD 186302 (184 LY distant, G2V star, 4.57B years old). Our sun is a G2V star thought to have ignited some 4.6B years ago. It’s clear that there was a great deal of exchange going on at that point in time; the protoplanetary disk of the solar system was clearly seeded with heavy metals by a supernova that went off nearby while our planets were just forming. The explosion wasn’t close enough to scatter our disc, but it left a telltale signature we can still pick up.
The coolest thing about these findings is that it means we could gather information from asteroids that didn’t originate here. It doesn’t really matter if they represent a 4.5B-year-old capture from a long-lost stellar sibling or if these Centaurs are random space rocks like Borisov or ‘Oumuamua. No matter where they came from or how long ago we formed, we’d learn a tremendous amount about conditions elsewhere in the galaxy — and we could learn them in a matter of a few decades, rather than the tens of thousands of years currently required to send an unmanned probe to another solar system.
That’s an excellent reason to focus some probes on these Centaurs. Any time there’s a chance of gathering information in decades that would otherwise take tens of thousands of years, it’s worth exploring.
Feature image by NASA
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