The mysteries surrounding ‘Oumuamua, our solar system’s first known interstellar visitor, are many: How did it get here, and from where? What gave rise to its extremely elongated (circa 250 meters long), potentially cigar-like shape, which so starkly distinguishes it from any natural object ever seen orbiting the sun? And most of all, what caused it to “hit the gas” after it swooped by our star, accelerating away like a passenger-filled car that accidentally entered a bad neighborhood?
The most obvious explanation for ‘Oumuamua’s properties and behavior—particularly its anomalous acceleration—is that it is a comet from another star system, albeit a decidedly weird one. In this scenario, ‘Oumuamua would have been ejected from its home system by a gravitational interaction with a large planet, perhaps gaining its shape from the associated wrenching forces and subsequent eons of exposure to cosmic radiation. Its speedy departure from our inner solar system, then, would be due to its briefly spouting plumes of gas from its icy, light-warmed surface after its close passage by our sun. This is the explanation preferred by European Space Agency scientist Marco Micheli, University of Hawaii astronomer Karen Meech and their colleagues, who first reported ‘Oumuamua’s anomalous acceleration.
But observations of the object with the Spitzer Space Telescope failed to detect any signs of a comet-like tail that would be expected from such outgassing, and provisional estimates for the frequency of interstellar comets had earlier suggested we would be exceedingly lucky to ever witness an object as small as ‘Oumuamua passing through our system. Roman Rafikov, an astrophysicist at the University of Cambridge, dealt the idea another blow in a peer-reviewed study published last year. Rafikov calculated that essentially any outgassing sufficient to account for ‘Oumuamua’s observed acceleration would also dramatically increase the object’s spin—something difficult to reconcile with data that suggested the object has an approximately four-hour rotational period.
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