Last month astronomers were thrilled by the confirmation that a second known interstellar object is flying through our solar system. Named 2I/Borisov—after its discoverer, Crimean amateur astronomer Gennady Borisov—it has already attracted huge attention. Countless observatories, from the Very Large Telescope in Chile to NASA’s Hubble Space Telescope, are studying the object, and plenty more science is on the way as 2I/Borisov approaches its peak brightness in December. “It’s been a rapid assembly of telescopes around the world,” says Michele Bannister of Queen’s University Belfast. “This, basically, is establishing a new subfield of astronomy.”
Whereas the first interstellar object, ‘Oumuamua, was found in 2017 as it was already leaving our solar system, 2I/Borisov was caught on the way in. The latter object exhibits traits of cometary activity, with dust and gas surrounding it, whereas ‘Oumuamua was more sedate, like an asteroid (although its classification is still unclear). The question now is whether or not 2I/Borisov resembles comets in our solar system, with either answer being equally thrilling. “I’m torn both ways,” Bannister says. “If it’s like the things that we have in our solar system, the processes that we see taking place are more typical than we realized. If it’s really different, then that tells us this chemistry takes place in quite a different way—in the diversity of exoplanetary systems—than we see.”
Gennady Borisov spotted the object with a homemade 0.65-meter telescope in late August. And almost immediately, other astronomers, both professional and amateur, began training their own telescopes on it. The trajectory of the comet confirmed it was unbound from our sun and therefore hailed from another star system. Some of the earliest results came from Julia de León of the Institute of Astrophysics of the Canary Islands and her colleagues, who used the Great Telescope of the Canary Islands on September 12. (They presented their findings in the non-peer-reviewed works-in-progress journal Research Notes of the AAS on September 19.) “As soon as we heard about this potential interstellar object, we decided to go for it,” de León says. By studying the light reflected by the dust emitted by the object, she and her team discovered the dust was similar in composition to that of comets in our own solar system.
A little more than a week later Alan Fitzsimmons of Queen’s University Belfast and his colleagues studied the gas ejected by 2I/Borisov. They posted their work on the preprint server arXiv.org. Using the William Herschel Telescope, also in the Canary Islands, they detected cyanide, which is common to comets orbiting our sun. “We can see that this comet, in terms of the first type of gas detected, looks a little bit like comets in our solar system,” Fitzsimmons says. “And when we look at the amount of gas we see, compared to the amount of dust particles that the comet is also ejecting, it looks pretty similar as well.”
Cyanide is one of the easiest gases to spot around such an object, but already astronomers are starting to wonder about what they might see next. At the time of these observations, the comet was beyond the orbit of Mars, nearly three times the distance between Earth and the sun, and not showing a huge amount of activity.
But astronomers expect 2I/Borisov to spew more and more material into space as it reaches its closest point to the sun, known as perihelion, on December 7, when it will be twice as far from that star as Earth. (Unfortunately, the object will not be visible to the naked eye on our planet. Astronomers even looked into the possibility that it might be visible to spacecraft elsewhere in the solar system, such as Japan’s Hayabusa2, but with no luck.) Researchers plan to look for water, carbon dioxide and carbon monoxide. “We can actually do detailed compositional studies of the surviving ices of this object,” says Bannister, who has observing time booked on the Very Large Telescope in Chile. “So this little world is being heated by incoming sunlight for the first time, potentially, in its existence. We don’t know where it formed. But at the moment, it is showing activity.”
An early attempt at measuring the cometary nucleus will come from David Jewitt of the University of California, Los Angeles, who has been awarded observing time on Hubble in mid-October. “The main science objective is to try to pick out the nucleus from the coma [the dust and gas surrounding the nucleus],” he says. Jewitt notes that getting a better handle on the size of 2I/Borisov will tell us more about other interstellar objects in the galaxy, too. “When we have some estimate [for the mass], we’ll be able to say something about the mass of these bodies spread through the whole galaxy.”
Based on some calculations, their number could be vast. Malena Rice and Greg Laughlin, both at Yale University, used observations of planetary disks around other stars to work out how these objects are ejected in the first place. They suggested that planets larger than Neptune located far from their host star, more than five times the Earth-sun distance, could be the culprits, flinging an Earth’s mass of dust grains and larger bodies into deep space early in a solar system’s life. And many of these ejecta, it seems, could be relatively close to us right now. “There should be about 850 ‘Oumuamua-sized or larger objects in the solar system at a given time,” Rice says. Although most are too faint for us to detect, this estimate suggests ‘Oumuamua and 2I/Borisov are only the beginning of the discoveries.
Upcoming telescopes such as the Large Synoptic Survey Telescope in Chile, due to switch on in the early 2020s, are expected to find many more interstellar objects, perhaps one a year. But for now, all eyes are on the comet 2I/Borisov, and it is already proving fascinating.