For only the second time, astronomers believe they have detected a space rock that formed in some distant system before making the interstellar journey to fly through our own solar system. The object, a comet named C/2019 Q4 (Borisov), was recently verified by the Minor Planet Center. According to available observations of the comet, C/2019 Q4 is moving too fast, some 30.7 kilometers per second (68,700 miles per hour), to have origininated in our solar system.
The likely interstellar comet was first observed by Gennady Borisov, a Ukrainian amateur astronomer working at the Crimean Astrophysical Observatory, on August 30. The object is still inbound, and it will make its closest approach to the sun on December 7, and its closest approach to Earth—within 180 million miles—on December 29, as reported by Michael Greshko at National Geographic. Further observations by astronomers have determined that C/2019 Q4 is a comet stretching a couple miles wide, with a coma of gas and dust enveloping the object that forms when icy material is heated by the sun. (Asteroids have less icy material and do not develop comas.)
In addition to C/2019 Q4’s great speed, the object is on a hyperbolic trajectory through the solar system, meaning rather than circling the sun, it will fly in close—almost as close to the sun as Mars—and then sling back out into interstellar space. Astronomers use a measurement called eccentricity to determine how circular an object’s orbit is: An eccentricity of 0 is perfectly circular, while an eccentricity of 1 is highly elliptical, and anything greater than is hyperbolic. C/2019 Q4’s eccentricity is greater than 3.5, according to NASA JPL’s Small-Body Database.
“Based on the available observations, the orbit solution for this object has converged to the hyperbolic elements shown below, which would indicate an interstellar origin,” says a statement from the Minor Planet Center, part of the Smithsonian Astrophysical Observatory, that accompanies data about C/2019 Q4. “Further observations are clearly very desirable.”
In the coming months, as C/2019 Q4 approaches the sun and the Earth, astronomers around the world will turn their telescopes toward the object to confirm that it came from outside our solar system. The fact that the object is a comet with a hazy coma means that scientists should be able to observe its spectrum to learn more about what it is made of. Further observations will also help to determine what direction the comet came from, though tracing it back to its original solar system is highly difficult given how long the object may have been flying through space.
“We get to see one little bit of another solar system,” Michele Bannister, an astronomer at Queen’s University Belfast, tells National Geographic. “And without necessarily knowing which one it came from, that’s exciting.”
In 2017, the first interstellar object, named 'Oumuamua, was spotted on its way out of our solar system. Astronomers believe that thousands of interstellar objects may be flying through our solar system at any given time, but the objects are generally small and dim, making them difficult to spot. Future survey telescopes that pan the sky to catalogue thousands and thousands of objects could make discoveries like 'Oumuamua and C/2019 Q4 much more common. With observations of rocky and icy material that formed in other solar systems , astronomers will start to get a sense of what the planets and other celestial bodies orbiting distant stars are made of. The study of exoplanets has revealed that other solar systems can be quite different from our own, and interstellar interlopers can help reveal more about these far-off realms.
Objects with hyperbolic orbits like C/2019 Q4 sometimes form within our solar system to be tossed out by a massive object, just as Jupiter or Saturn. But the current measurements of C/2019 Q4’s trajectory suggest that such an encounter in the object’s past is impossible. Future observations of C/2019 Q4 will help determine what it is and roughly where it came from—a little bit of material that formed around another star, likely hundreds, thousands or even tens of thousands of light years away.