On the evening of March 28, a small automated observatory on the Andean steppe in northwestern Argentina watched a previously undetected comet drift through the solar system nearly 500 million miles away. New comets are rare enough—astronomers only add a few dozen to the official tally each year—but this particular bit of space rock came with an added bonus. Its trajectory through the solar system indicated that it might have originated elsewhere in the galaxy, which would make it only the third interstellar object ever discovered.
The following week, Vladimir Lipunov, the Russian astronomer who manages the observatory, made several more observations of the unusual comet. The first two interstellar objects discovered, ‘Oumuamua and 2I/Borizov, both had trajectories that clearly indicated they were not from our local galactic neighborhood. But the path of this new object, known as C/2020 F5, was more ambiguous. Maybe it was a local, maybe it wasn’t. The only way to tell was to gather more data.
By the time Lipunov shared his observations with the Minor Planet Center, an arm of the International Astronomical Union that is the official clearinghouse for observations of asteroids and comets, there was still enough uncertainty in the measurements to raise a few eyebrows in the professional and amateur astronomy community. Since news of the comet’s existence was made public on April 5, there have been more than 80 observations of the object. The data from additional observations has slightly reduced the chances that the comet is interstellar, but the rock’s provenance is still an open question.
“It would be cool if it turned out to be interstellar, but I’m kind of skeptical at this stage,” says Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory who studies the orbits of comets and asteroids. “Interstellar objects are unlikely, and extraordinary claims need to be backed up by clear evidence.”
NASA estimates that there are billions of comets lurking at the edge of our solar system, but we only see the small fraction of them that are kicked into a journey toward the sun by the pull of gravity. A comet’s orbit can take anywhere from a few dozen years to millennia to complete, meaning astronomers on Earth only ever see a small slice of the orbit. So how are they able to tell the difference between a local bit of space rock and an interstellar interloper? It mostly comes down to the comet’s eccentricity.
Eccentricity is astronomer-speak for how much the orbit of an object deviates from a perfect circle. Planets, asteroids, and comets bound by the sun’s gravity all have elliptical, oval-shaped orbits, which mean they have an eccentricity between 0 to 1. No matter how far away from the sun their orbit takes them, they will always get pulled back. Interstellar objects have eccentricities greater than 1, and these are called hyperbolic orbits. Rather than an oval, their trajectory looks more like a hockey stick; it will never circle back around.
Both the interstellar objects discovered so far have clocked eccentricities greater than 1: ‘Oumuamua came in at around 1.2 and 2I/Borizov registered around 3.3. But things get tricky when objects have eccentricities that are just slightly above 1. This is the case with the recently discovered comet C/2020 F5, whose eccentricity is just 1.01. Because the comet is so new, there’s still too much uncertainty in the measurement to definitively say that it is local or interstellar.
Farnocchia says that, so far, additional measurements have reduced the object’s eccentricity, reducing the chance that it might be an interstellar object. There’s also the fact that the object is moving pretty slowly relative to the solar system, which further limits the odds that it’s from elsewhere in the galaxy. “If you had something that was really interstellar, chances are the relative velocity would be much higher,” says Farnocchia.