Interstellar Mystery Object Now Thought to Be a Comet

By Nola Taylor Redd

When astronomers first spotted the celestial object now known as ‘Oumuamua skittering across the sky last October after it had dived around the sun, its elongated trajectory and rapid speed quickly revealed that it came from outside the solar system.

Learning anything else about our first-known interstellar visitor, however—such as whether it was an asteroid or a dim comet—proved far more challenging, as it departed our planetary vicinity as quickly as it arrived. Either classification would have important implications not only for ‘Oumuamua itself, but also for understanding how planetary systems form.

Now a team of researchers monitoring the object on its journey back to the stars say they have the answer: ‘Oumuamua is almost certainly a comet, albeit one fittingly alien from those we find orbiting the sun. Using NASA’s Hubble Space Telescope and other ground-based instruments, the team observed ‘Oumuamua’s changing position across time and plotted its outbound trajectory, finding that, remarkably, it did not follow the path they had anticipated. The result appears in the June 27 edition of Nature. “When put together, these positions showed that the motion of ‘Oumuamua was slightly different than what we expected,” says lead study author Marco Micheli of the European Space Agency’s SSA–NEO Coordination Center.

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