The problem is, astronomers never saw a coma, the tail of dust, trailing behind ‘Oumuamua. “That has been something that has bothered us all along,” Meech says.
There might be an explanation, the astronomers say. ‘Oumuamua is much smaller than comets in our solar system, so it may not require a significant ejection of gas to alter its trajectory. The less gas a comet emits, the fewer dust particles get disturbed. And perhaps, Meech says, during the long journey between stars, interstellar radiation eroded the surface of ‘Oumuamua, reducing the amount of dust coating the object.
‘Oumuamua may also be coated in large dust particles—about 100 times larger than particles found on comets in our solar system. Larger particles are more difficult to detect in optical wavelengths, which Hubble is designed to see.
Coma or no coma, the results are convincing for comet researchers like Jessica Agarwal, a scientist at the Max Planck Institute for Solar System Research. The study authors “have very thoroughly investigated a range of conceivable processes that could explain non-gravitational acceleration of [‘Oumuamua],” Agarwal said. “The conclusion that outgassing is causing the non-gravitational acceleration seems solid to me, and in that sense I find the description as a miniature comet appropriate.”
The telescope survey that found ‘Oumuamua has been running for only a few years, and astronomers hope it can spot more distant visitors soon. Some estimate that there is at least one interstellar object between the Earth and the sun at any given time. If they’re anything like ‘Oumuamua, it may not be easy to detect them, says Andy Rivkin, a planetary astronomer at Johns Hopkins Applied Physics Laboratory who studies asteroids. “If it’s without a coma or tail, they’d be much harder to find than if it did have a coma or tail.”
Eight months after their historic discovery, astronomers are now done observing ‘Oumuamau in real time. At the time of this writing, the comet is about 595 million miles from Earth. The sunlight the space rock can reflect is about 5 billion times fainter than the naked eye can see, and about 13 times fainter than the Hubble Space Telescope, Meech said.
But the story of the mysterious space rock is far from over. Each new piece of information about ‘Oumuamua and its properties is another clue about its origins. “We’d still like to trace back to its home solar system,” Meech said. Astronomers are currently using observational data and computer simulations to run the clock backward and trace ‘Oumuamua’s path. The discovery of outgassing, however, throws a big wrench into those plans. Comets only release gases when they approach stars and get warmed up. The amount of gas varies, which means it’s difficult to predict the resulting acceleration.
In the meantime, Weryk says he is more vigilant about his daily searches of telescope data in Hawaii. “Every time we get a [near-Earth object] candidate now—that probably ends up being a normal asteroid—I actually think, oh, is it possible this could be like ‘Oumuamua?” he says.