ESO/M. KORNMESSER

Mysterious interstellar visitor is a comet, flight path reveals

‘Oumuamua, the first interstellar object observed visiting our solar system, had a bit of an identity crisis when it was discovered late last year. The cigar-shape space rock was thought to be a comet, but it was reclassified as an asteroid after astronomers failed to detect telltale gas emissions. Now, it turns out they may have been right the first time.

As ‘Oumuamua speeds away from us, scientists have continued to watch it with space and ground telescopes, allowing them to refine its trajectory. They’ve found that ‘Oumuamua’s path makes sense only if some extra force—in addition to the gravity of the sun, planets, and other large solar system bodies—is giving it a push. After running the numbers, scientists ruled out a boost from solar radiation, a collision with another object, and several other possibilities.  

Instead, the most convincing explanation was water vapor and gases jetting from ‘Oumuamua, the researchers write today in Nature. The amount of gas needed to produce this additional propulsion is lower than astronomers are able to detect, explaining why it had previously gone unnoticed.

Based on these findings, ‘Oumuamua appears to be a comet after all—but with some unusual features. For example,‘Oumuamua should be spewing out dust like other comets, but so far none has been observed. This suggests it has some unique physical characteristics such as large dust grains, which wouldn’t be detected by standard optical measures, the authors say. Such unusual features may very well be a relic of the far-away solar system it was formed in—or of its long journey to ours.