In a first, scientists spotted an ‘interstellar visitor’ in our solar system

On Oct. 19, researchers at the University of Hawaii Institute for Astronomy spotted a strange guest in the night sky: a quarter-mile-wide hunk of space rock hurtling through our solar system, the first “interstellar visitor” ever observed by scientists.

“Spacecraft, like Stardust, have captured interstellar dust,” says Meenakshi Wadhwa, director of the Center for Meteorite Studies at Arizona State University. “And there’s actually interstellar dust in meteorites, too. But this is the first object that’s macro in size.”

As Wadhwa explains, the rock — affectionately named “A/2017 U1” — was unusual not just for its speed, but for its trajectory.

“It has a hyperbolic orbit, which seems to suggest that it came from another star system altogether,” she says. “When you think of all the planets and asteroids and comets that formed within our own solar system, they all have these elliptical orbits that are closed loops, which means that these objects are gravitationally bound to the sun,” she adds. In contrast, an object with a hyperbolic orbit “basically indicates that the object is not gravitationally bound to the sun.”

As for where the rock hails from, scientists still aren’t quite certain. According to Wadhwa, it was spotted coming from the direction of the star Vega, in the constellation Lyra. But as she points out, “Ultimately, we really don’t know where it originated and how long it’s been traversing interstellar space.”

The space rock crossed the ecliptic plane inside of Mercury’s orbit on Sept. 2, according to a NASA press release. By Oct. 14, it had plunged below Earth’s orbit. Then — changing course under the Sun’s gravitational influence — the rock whizzed above the ecliptic plane again, this time heading towards the constellation Pegasus at a clip of 27 miles per second.

Wadhwa says that while scientists weren’t able to get samples from the strange visitor, spectrum analysis suggests it could look similar to some objects in our own solar system — the Kuiper belt, for example.

“It has a reddish color to it, but that information is very, very limited,” she says. “And I would love to have a sample of it in my laboratory, of course, but that’s not going to happen any time soon.”

But scientists may get another chance to study interstellar visitors like this one. While this is the first such object that scientists have spotted in our solar system, Wadhwa says it’s surprising that we haven’t seen more like it.

“We think that the probability of objects being ejected from other solar systems is actually quite high, just like we believe that … there’s asteroids and cometary objects being ejected from our own solar system,” she explains. “Particularly during the initial phases of solar system formation when the giant planets were forming, interaction of the gravity of these giant planets with some of these objects basically did throw a lot of material out into interstellar space.

“So I would have expected to have seen these types of objects more often, actually, and probably we will in the future. It is my hope that we will.”

This article is based on an interview that aired on PRI’s Science Friday with Ira Flatow.