By most estimates, our galaxy is a bustling place. Given its hundreds of billions of stars—each accompanied by whirling planets, asteroids, comets, and more—one would expect objects to occasionally be flung into interstellar space. The first of them was found in 2017: officially called 1I/2017 U1 but better known as ‘Oumuamua, it was discovered by chance as it swooped past our sun on an outbound trajectory that rapidly took it beyond the reach of Earth’s best telescopes. Now astronomers think they have found another outcast wandering the space between the stars. But this time, it may linger slightly longer, offering unprecedented scientific opportunities.
The object was originally labelled gb00234 but has now been provisionally designated C/2019 Q4 (Borisov) by the Minor Planet Center (MPC) at the Center for Astrophysics at Harvard University and the Smithsonian Institution. It was discovered on August 30 by Gennady Borisov, using a homemade telescope in Crimea. Borisov, an amateur astronomer, is one of many hobbyists who scour the skies for undiscovered comets orbiting the sun in our solar system.
“Half a year ago, I made a new telescope. And with the help of it, I discovered this unique comet,” Borisov says. “I was excited about the comet. And of course, I was very surprised by the uniqueness. I would be glad if such an interesting space object gets my name.”
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Ever since ‘Oumuamua, amateur and professional comet hunters have had their eyes peeled for another object “unbound” from our star, meaning something moving fast enough to slip the sun’s gravitational clutches. And the faint point of light in Borisov’s images, streaking sunward through the space between Jupiter and Mars, was moving fast indeed.
After Borisov’s initial discovery, astronomers used bigger telescopes—including at observatories in Hawaii and on the Canary Islands—to get additional images of C/2019 Q4, refining its orbital characteristics with computer models all the while. The latest data suggests the object is several kilometers across and hurtling into the inner solar system at about 30 kilometers a second. Its projected path is highly eccentric, describing less a circle and more an open curve. Taken together, the speed and trajectory strongly suggest C/2019 Q4 is not bound to our sun. It will make its closest approach to that star on December 8, at a distance of 1.7 astronomical units, or AU (one AU is the average distance between Earth and the sun). On December 28 it will make its closest approach to Earth, coming within about two AU of our planet.
After only having weeks to study ‘Oumuamua on the outbound leg of its one-way trip around the sun, astronomers are ecstatic at the prospects of having more time to scrutinize a visitor from another star. Plans are already emerging for a cavalcade of observations as C/2019 Q4 continues its journey through our solar system. “It’s going to be around for a comparatively long time,” says Matthew Payne of the MPC. “It should be visible for many, many months—possibly for the good portion of a year.”
One of the reasons 2019 Q4 is more visible besides being on its way into the solar system is that it is much brighter than ‘Oumuamua—about six times by current estimates. This brightness, coupled with what looks to be a visible “tail” of outgassed material extending from the object, suggests it is an interstellar comet—whereas the tailless ‘Oumuamua was something far less active, perhaps an asteroid (or something much stranger). If all these details are confirmed, C/2019 Q4 would become the first known interstellar comet, and that finding would have huge implications for what we could learn from it. Comets are primordial relics, recording the “starting conditions” of planetary systems, when worlds first begin to coalesce around newborn stars.
“Because it’s a real comet, we’ll be able to get spectra of the coma [the atmosphere of gas and dust surrounding it], what molecules are outgassing from it, the relevant abundances of different elements like carbon and oxygen—and really get a detailed survey of a planetary building block from an alien solar system,” says Gregory Laughlin of Yale University. Laughlin notes we could even detect signatures of water on the object. “There might be a chance to track it back to its location of origin and the kind of system that might have given rise to it,” he adds.
The fact that this finding was made by an amateur astronomer is particularly noteworthy, too. Since the detection of ‘Oumuamua, we have seen no great change in our observational techniques, so C/2019 Q4’s discovery was a bit of a “fluke,” according to Laughlin. Most astronomers are waiting for upcoming facilities such as the Large Synoptic Survey Telescope in Chile, which is expected to switch on in the early 2020s and should be capable of finding many more interstellar objects (at least one is thought to be in our solar system at any given time). On this occasion, however, it is the hobbyists who deserve plaudits. “The amateur astronomer community is very, very good these days,” says Marshall Eubanks of the company Space Initiatives. And they don’t get a lot of the recognition.”
Team members on the European Space Agency’s Comet Interceptor mission will be watching these developments closely. Due to launch in 2028, their spacecraft is designed to loiter in space, waiting to possibly rocket toward an interstellar object if a suitable target is found. Colin Snodgrass of the University of Edinburgh, deputy lead on the mission, notes that by the time it launches, C/2019 Q4 will be “farther away than we expect Comet Interceptor to be able to target.” But he adds the discovery of a second interstellar object within two years bodes well for Comet Interceptor. “The primary mission is still to visit a comet from our own Oort cloud, but everyone does still hope that we get lucky and get to visit [an interstellar object]!” he says.
C/2019 Q4’s status as truly interstellar still needs to be definitively determined—a decision that will be made by the International Astronomical Union after further observations have been made. “If they say it’s interstellar, it’s interstellar,” says Michele Bannister of Queen’s University Belfast. But Bannister notes the observations so far are “pretty solid,” and already astronomers are clamoring for observing time on the planet’s most powerful telescopes to study it in more detail. The object is currently too close to the sun for telescopes such as Hubble to safely look at it, but come October,all eyes will be trained on this enigmatic visitor trundling past, with all the secrets of another solar system onboard. “A lot of very exciting opportunities are going to arrive,” Laughlin says.