Like a hit-and-run driver who races from the scene of a crash, the interstellar guest known as ’Oumuamua has bolted out of the solar system, leaving confusion in its wake. Early measurements seemed to indicate that it was an asteroid — a dry rock much like those found orbiting between Mars and Jupiter. Then by this past summer, astronomers largely came around to the conclusion that it was instead a comet — an icy body knocked out of the distant reaches of a far-off planetary system.
Now a new analysis has found inconsistencies in this conclusion, suggesting that ’Oumuamua may not be a comet after all. Whether it’s actually a comet or an asteroid, one thing is clear: ’Oumuamua is not quite like anything seen before.
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Other astronomers quickly joined in the hunt, measuring everything they could. (One team even trained radio telescopes on it to check whether it might be transmitting extraterrestrial broadcasts. It was not.) By last December, a team of astronomers published ’Oumuamua’s electromagnetic spectra, which can be used to probe what an object is made of. The researchers found that ices with organic material similar to those seen in comets in our solar system lurked just below ’Oumuamua’s surface; that ice could have survived a long interstellar journey.
They also looked at ’Oumuamua’s rotation. Many asteroids tend to spin around their long axis like an expertly thrown football. ’Oumuamua, by contrast, tumbled like an errant pass by Charlie Brown.
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Yet comets have tails. And ’Oumuamua, if it was indeed made out of icy rock and propelled by jets of gas as it passed by the sun, should have displayed a tail that would settle the question of its origin. Yet no tail was ever found.
Now in a new study that is currently under peer review, Roman Rafikov, an astrophysicist at the University of Cambridge, argues that the same forces that appeared to have accelerated ’Oumuamua — the same forces that should have also produced a tail — would have also affected its spin. In particular, the acceleration would have torqued ’Oumuamua to such a degree that it would have spun apart, breaking up into smaller pieces. If ’Oumuamua were a comet, he argues, it would not have survived.
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