The European Southern Observatory (ESO) has confirmed that the object provisionally designated A/2017 U1 is, in fact, an asteroid that came from outside the Solar System. The first-ever detected interstellar traveler, now named "`Oumuamua," turns out to be different from any object seen before and has been roaming between the stars for hundreds of millions of years.
`Oumuamua, which takes its name from the Hawaiian word for "first", was first detected on October 19, 2017 by the University of Hawaii's Pan-STARRS 1 telescope on Haleakala as part of a NASA survey to identify and plot near-Earth objects. When the trajectory data was confirmed with the European Space Agency's telescope on Tenerife in the Canary Islands, it turned out that the asteroid or possible comet had its origins many light years away from the Solar System.
With `Oumuamua hurtling through the system at 95,000 km/h (59,000 mph) and already passing within 0.25 AU (23 million mi, 37 million km) of the Sun in September, there was no time to waste. The ESO Very Large Telescope (VLT) in Chile and other observatories swung into action to gather as much data as possible about the object while it was still within range.
A closer study of the trajectory confirmed that `Oumuamua was on an open-ended parabolic trajectory and was making its first and only visit to the Solar System. Meanwhile, the VLT's FORS instrument with four different filters allowed a team of astronomers, led by Karen Meech of the Institute for Astronomy in Hawaii, to discover that its brightness varied by a factor of 10. This indicated that the object was a spindle shape about 400 m (1,300 ft) long that revolves on its axis every 7.3 hours.
"This unusually large variation in brightness means that the object is highly elongated: about 10 times as long as it is wide, with a complex, convoluted shape," says Meech. "We also found that it has a dark red color, similar to objects in the outer Solar System, and confirmed that it is completely inert, without the faintest hint of dust around it."
In addition, spectrographic analysis indicated that `Oumuamua is not a comet, but an organic-rich, rocky or high-metal-content asteroid. It's dark and reddish color may be due to it's lacking water or ice and having been irradiated by cosmic rays over hundreds of millions of years.
The time frame is important because backtracking the path of `Oumuamua indicates that it came from the direction of Vega in the constellation of Lyra. However, at the speed `Oumuamua is traveling, it would have been in the neighborhood of Vega 300,000 years ago, but because stars move, Vega itself wouldn't have been there. This indicates that `Oumuamua has been in interstellar space a very, very long time.
One unexpected result of spotting `Oumuamua is the realization that there wasn't an official system for designating interstellar visitors. Initially cataloged as A/2017 U1, `Oumuamua is now called 1I/2017 U1 by the International Astronomical union, with "I" standing for Interstellar. Based on data from the object, astronomers suggest that at least one such visitor enters our system each year.
"We are continuing to observe this unique object," says Olivier Hainaut of ESO, "and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones."
The research was published in Nature (PDF).