When Oumuamua was initially spotted last October by the Pan-STARRS1 telescope on Haleakala, scientists weren’t sure what it was.
With no evidence of gas emission or a dusty environment, which are typical characteristics of comets, Oumuamua was classified as the first interstellar asteroid.
“An asteroid is basically a rocky remnant of the process of building planets that presumably doesn’t have many volatile materials or ices, whereas a comet has a lot of ice and they develop these beautiful tails and clouds of gas and dust as they come close to the sun,” said Karen Meech, an astronomer with the University of Hawaii’s Institute for Astronomy. “We called this one an asteroid, because when we observed it, there was no tail, no gas or no dust.”
Following the initial discovery, astronomers continued to make high-precision measurements of the rocky, cigar-shaped object using ground-based facilities like Canada France Hawaii Telescope on Maunakea, as well as the Hubble Space Telescope. The final images were taken with Hubble in January before the object became too faint to observe as it sped away on its outbound orbit.
Months of analysis led scientists to change their mind. A paper published Wednesday announced their new assessment: Oumuamua is not an asteroid, but a comet, albeit an unusual one.
“The big discovery that was very surprising to us is that it’s accelerating its orbit, so instead of being controlled just by the gravity of the sun, it was getting an extra push,” Meech explained. “We examined many, many different hypotheses, and the only one that stood up was that it has to be a comet that is ejecting gas closer to the sun, and that’s pushing it away from the sun so it’s actually accelerating.
“Oumuamua is small – no more than a half a mile long – and it could have been releasing a small amount of relatively large dust for it to have escaped detection,” she added. “What’s exciting about latest result is that although we didn’t see any gas or dust, we could infer that chemical composition may be different from comets in our solar system, so that’s exciting to me, because we have a first hint that other chemistry in other star systems can be different.”
Scientists say, unfortunately, the data they’ve gathered doesn’t provide all the information needed to truly answer all their questions.
“It’s moving out of solar system. Right now, it’s farther than the planet Jupiter. It’s moving away never to be seen again,” Meech said. “To really understand Oumuamua, we would need to send a space probe to it. This is actually possible, but it would be very expensive and take a long time to get there, so it isn’t practical this time. We just have to be ready for the next one.”
Oumuamua means “scout from the distant past” in Hawaiian. Experts say one in 10 of these interstellar visitors drop by annually, but they move so fast, scientists have never been able to study one until now.
“This is the first time we’ve ever seen anything like this. Astronomers have been predicting this for decades, so it was very exciting to see the first one come through,” Meech said. “I think the Pan-STARRS in particular is going to start to see a lot more of these with the second telescope now starting its survey, so it’s an exciting time to be an astronomer.”