We still don’t know where the first interstellar asteroid came from

ISO: A home for a stray space rock. Astronomers are tracking the motions of stars to figure out which one sent an alien asteroid speeding past Earth in October — but they may never find the rock’s true origins.

Officially named ‘Oumuamua, the asteroid was spotted by the Pan-STARRS 1 telescope in Hawaii on October 18 (SN: 11/25/17, p. 14). Its inbound speed of about 25.5 kilometers per second and its slingshot route around the sun pegged it as the first interstellar vagabond ever seen.

That was the easy part. Now astronomers are trying to trace ‘Oumuamua’s path backwards to a particular star or group of stars, without much success. Several papers published online since the asteroid’s flyby peg it as a relatively youthful vagabond, but where it came from is all over the map.

“From my perspective, this object has been hopping stars since it became a free-floating object,” says Simon Portegies Zwart of the Leiden Observatory in the Netherlands. He and his colleagues used star data from the Gaia spacecraft, which released a preliminary catalog of millions of stellar motions in 2016 (SN: 10/15/16, p.16), to calculate the positions of nearby stars in the past. The team found that about 1.3 million years ago, ‘Oumuamua passed within about half a light-year of an unremarkable star called TYC4742-1027-1, it reports on arXiv.org on November 13.

But ‘Oumuamua moved so quickly that the asteroid simply grazed the star on its way here, Portegies Zwart concluded. There’s no reason to think that star was the first the asteroid visited, either, he says.

Two other groups, Fabo Feng and Hugh R. A. Jones of the University of Hertfordshire in England, and a second team led by Eric Gaidos of the University of Hawaii at Manoa, looked at ‘Oumuamua’s speed for clues.

Stars born in the same cluster maintain similar speeds for up to hundreds of millions of years, even after moving apart. That similarity lets astronomers identify families of young stars, called associations, streaming across the galaxy. Any stray asteroids ejected from those stars’ neighborhoods should start out with that same speed.

Both Gaidos and Feng calculated that ‘Oumuamua’s speed is eerily similar to some of those stellar families, although the teams chose different streams. Gaidos and colleagues named the Carina/Columba associations, which are around 40 million years old, as ‘Oumuamua’s likely family of origin, the team reports in the Nov. 8 Research Notes of the AAS. In paper posted at arXiv.org November 27, Feng and Jones chose the Local association, whose stars are around 100 million years old. Both teams agree that ‘Oumuamua is probably young — a fraction of the age of the 4.5-billion-year-old solar system.

There’s one way to test that theory, says Eric Mamajek of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who posted a paper at arXiv.org on November 20 suggesting ‘Oumuamua was most recently a free-floating object in interstellar space. Just find a way to catch up to the asteroid and bring a piece of it home. Measuring the rock’s radioactive elements would give its age and point to one star stream or the other — or neither.

Unfortunately, catching up is not practical — ‘Oumuamua is now fleeing the solar system at about 50 kilometers per second. But thousands of other interstellar interlopers should be zooming through the solar system all the time, Portegies Zwart calculated. With future telescopes, like the upcoming Large Synoptic Survey Telescope, astronomers expect to see many more.

“When we have five or 10 of these interstellar asteroids, that will tell us a lot,” Mamajek says.