No one knows where it came from, but it’s here now. And the chase is on.
Astronomers around the world are monitoring an interstellar comet hurtling through the solar system, known for the moment as C/2019 Q4. It’s the second time in less than two years that they’ve seen an object from another star swing through our cosmic neighborhood. The first time around, the discovery kicked off a worldwide sprint to inspect the object before it got away. It was mysterious enough that some astronomers even began to consider whether it was dispatched by an advanced alien civilization.
This second interstellar object was spotted in late August by Gennady Borisov, an amateur astronomer in Crimea. Borisov has a reputation for catching never-before-seen comets with his telescopes, but they’re from around here; like everything else in the solar system—the planets, the moons, a sea of asteroids—they trace an orbit around the sun. And over the past few weeks, it’s become very clear that this comet does not.
Observations with more powerful telescopes revealed that the comet is moving much faster than an object orbiting the sun at the solar system’s edges would. The clear giveaway, however, has to do with C/2019 Q4’s trajectory as it zooms through space, and in particular a measurement called eccentricity. An object that orbits its star in a perfect circle has an eccentricity of zero. No object in our solar system does that, and most eccentricities fall between zero and one. The higher the eccentricity, the more elliptical the orbit. But any higher than one, and the object isn’t going around the sun at all.
‘Oumuamua, the first known interstellar visitor, had an eccentricity of 1.2. According to the latest data, this comet clocks in at 3.7.
“This is clearly coming from outside the solar system,” says Gareth Williams, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “This isn’t going to revert into something going back into our solar system.”
Sometimes, passing by a larger object can jostle small cosmic travelers, but no planetary nudge can explain this kind of orbit, he says.
Williams is the associate director at the Minor Planet Center, the international organization in charge of confirming discoveries of new objects in the solar system—or, in this case, the stuff just passing through. The center has not yet formally bestowed C/2019 Q4 with the label of “interstellar,” but Williams and other astronomers say the evidence so far is undeniable.
Astronomers are still getting to know this new visitor from beyond. And so far, it’s nothing like the last one.
‘Oumuamua was discovered in October 2017 by a telescope in Hawaii programmed to scan the sky for icy comets and rocky asteroids. It didn’t look like that any astronomers had seen before. ‘Oumuamua is elongated and narrow, like a cigar, a decidedly unnatural shape in a universe where gravity loves to smooth things into spheres. The thing was unusual enough, in fact, that a project to search for intelligent extraterrestrial life, funded by a Russian billionaire, had a radio telescope check it for artificial signals. (It didn’t detect any.)
It took months for scientists to conclude that ‘Oumuamua is probably a comet, even though it wasn’t acting like one as it passed the sun. A comet has a tell: When it gets close enough to the sun, some of its ice melts, producing a shimmery tail of gas and dust known as a coma. ‘Oumuamua showed some signs of gas emission, but it didn’t have the unmistakable cometary haze.
C/2019 Q4 does. “The behavior has been reasonably straightforward,” says Michele Bannister, a planetary astronomer at Queen’s University Belfast. “This is the interstellar object that we expected to see first.”
The rather uncomplicated nature of C/2019 Q4 only makes ‘Oumuamua more mysterious. Astronomers have long suspected that celestial bodies from other solar systems pass through our own all the time. Before ‘Oumuamua came barreling through, they had expected the first one they found to be a clear-cut comet.
Icy objects are the most susceptible to being kicked out of their solar systems and thrust on a journey through interstellar space. The universe can be a turbulent place, especially around a young star, just ignited into existence. Cosmic dust grains around the new orb clump together until they become whole planets and moons. The jostling can send smaller bits and pieces flying deep into space, toward other stars. Astronomers predict that comets—forged at the edges, where their sun is too weak to melt them away—would probably be the first ones out.
C/2019 Q4 is holding onto a few secrets, though. The comet appears to be bigger than ‘Oumuamua, but its exact size and shape are unknown for now. From our vantage point on Earth, the comet appears close to the sun, which means astronomers can’t point their very expensive telescopes at it for very long without frying them. The fuzzy coma obscures the comet’s contours, too.
But telescopes will be able to probe C/2019 Q4 gases for clues about its composition. “This will be particularly exciting to see if the chemistry is the same as comets in our solar system,” says Karen Meech, an astronomer at the University of Hawaii’s Institute for Astronomy, who led the team that characterized ‘Oumuamua two years ago.
Astronomers will have more time with C/2019 Q4. ‘Oumuamua was detected as it was leaving the solar system, and was only observable with telescopes for several months. Borisov, the astronomer in Crimea, caught C/2019 Q4 on its way in. The comet will appear brightest in mid-December, when it makes its closest approach to the sun. Unless the comet breaks apart, the biggest, most powerful telescopes will be able to monitor it until October of next year. “Early observations will help us decide better what additional observations are needed to best understand it, so we should be able to learn a lot more, without the immediate rush that we had with the previous one,” says Rob Weryk, an astronomer at the University of Hawaii, the first to lay eyes on ‘Oumuamua in the telescope data.
After that, C/2019 Q4 will fade from view for good, moving on to the next leg of its journey. The comet may have traveled for hundreds of millions of years before reaching our star, and it may be as long until it reaches the next one. Interstellar visitors are just that; onetime guests, unwilling to overstay their welcome, as thrilling as their company might be for the hosts.