The discovery by Dr. Fitzsimmons’ team marked the first time that astronomers had detected gas emitted by an interstellar object. The most remarkable thing about the comet, they found, was how unremarkable it appears. “Overall, we find the gas, dust and nuclear properties for the first active Interstellar Object are similar to normal Solar system comets,” Dr. Fitzsimmons and his co-authors wrote in a paper submitted to Astrophysical Journal Letters.
That is to say, nature seems to work the same out there as it does here.
Likewise, it seems, for the origin of these interstellar wanderers. Astronomers theorize that comets are fragments of ice left over when planets form in the chilly outer realms of planetary systems. Subsequent encounters with large planets like Jupiter can toss these cosmic icebergs willy-nilly toward their parent stars or even outward into interstellar space, as evidenced by the intrusions of the comets Oumuamua and Borisov.
However, none of the 4,000 or so known or suspected exoplanets orbit in the icy regions around their stars, where this process could happen. So Malena Rice, a graduate student at Yale, and her adviser, Gregory Laughlin, went looking for more. They found evidence for what they call “hidden planets” in the form of gaps in icy debris disks surrounding about 20 stars observed by the Atacama Large Millimeter Array radio telescope in Chile.
The gaps, they found, are consistent with having been carved by planets that orbit at least as far from their stars as Jupiter is from the sun, and that have masses between Neptune’s and Jupiter’s. About half the young stars in the Milky Way galaxy could have such disks and gaps and be spewing comets outward, the researchers concluded in a paper to be published in Astrophysical Journal Letters. They wrote the paper in July, just before Borisov was discovered.
“The timing has been incredibly exciting,” Ms. Rice said in an email. “Right as we proposed this idea, a new interstellar object that nicely fit our theory fell right into our laps!”
She added, “It’s very likely that interstellar objects have been passing through the solar system regularly, and we just haven’t had sensitive enough telescopes to see them.”
If they are right, several of these gifts from the outer universe should become visible to astronomers every year once the Large Synoptic Survey Telescope in Chile, which will be able to survey the entire sky every three days, in essence making a movie of the universe, when it begins operation in 2022.