Astronomers Spot First-Ever Space Rock from Another Star

Scientific American:

For the first time ever, an asteroid or comet from another star has been caught hurtling through our solar system, astronomers announced late Thursday. Provisionally designated A/2017 U1, the object appears to be less than a half-kilometer in diameter and is traveling at just over 40 kilometers per second—faster than humanity’s speediest outbound space probes. Because this is the first object of its type to be found, there are as yet no official rules for naming it, and its discoverers have balked at suggesting anything besides “Interstellar.” Whatever one might call it, though, it is presently racing away from the sun and has sparked a stampede of astronomers rushing to observe it before it fades entirely from view in the darkness of interstellar space.

“All we can say right now is this was something that was tossed out of another star system,” says Karen Meech, an astronomer at the University of Hawaii. Meech is helping coordinate a global observation campaign, which presently includes nights on large telescopes in Chile and Hawaii, as well as five orbits of the Hubble Space Telescope. “Everyone is trying to get time to look at this thing on big telescopes right now, urgently, within the next few days,” she explains.

Exactly what those telescopes might see is anyone’s guess. Astronomers will first and foremost attempt to pin down the object’s exact size (based on its brightness), as well as its shape and spin rate (based on how its brightness fluctuates). They will also seek to measure A/2017 U1’s color, and perhaps even its spectrum—the subtler details of its emitted and absorbed light. Taken together, all that information could show what exactly A/2017 U1 is made of—whether it is mostly rock, or ice, for instance—and potentially reveal more about its history and origins, somewhere out there among the stars.

Most likely, Meech says, the object is an outcast from another star system: a space rock flung out during the star’s tempestuous youth when it was surrounded by freshly-formed giant planets embedded in a disk of debris. “One of those planets hefted its weight, and tossed this thing and a lot of other stuff around,” she says. “So now we have a piece of another planetary system flying by Earth, flying through our solar system, that we briefly have a chance to study.”