Is this ‘Oumuamua’s back story?

Astronomers may have a handle on where the first known interstellar object to visit our solar system came from and how it was formed.

A new scenario based on computer simulations accounts for all the observed characteristics of the elongated, cigar-shaped body known as ‘Oumuamua, they write in a paper in the journal Nature Astronomy.

And this suggests that the mysterious object may in fact be a fragment of a planetary body that passed too close to its host star before being torn apart by tidal forces.

“We showed that ‘Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, and then ejected into interstellar space,” says co-author Douglas NC Lin, from the University of California Santa Cruz, US.

‘Oumuamua – Hawaiian for “a messenger from afar arriving first” – has been the subject of great debate since it was discovered, on 19 October 2017, by the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) in Hawaii.

Different hypotheses have painted it as a wayward asteroid, a comet, and even an alien spacecraft.

Others have proposed that it was indeed the result of tidal disruption processes that can eject debris into interstellar space, and to test this Zhang and Lin ran high-resolution computer simulations to model the structural dynamics of an object flying close by a star.

They found that if the object comes close enough, the star can tear it into extremely elongated fragments that are then ejected into the interstellar space.

Thermal modelling showed that the surface of fragments resulting from the disruption of the initial body would melt at a very short distance from the star then recondense at greater distances, forming a cohesive crust that would ensure the structural stability of the elongated shape.

“Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles, which not only explains ‘Oumuamua’s surface colours and the absence of visible coma, but also elucidates the inferred dryness of the interstellar population,” Zhang says. 

“Nevertheless, some high-sublimation-temperature volatiles buried under the surface, like water ice, can remain in a condensed form.”

Observations of ‘Oumuamua showed no cometary activity, the researchers say, and only water ice is a possible outgassing source to account for its non-gravitational motion. 

If ‘Oumuamua was produced and ejected according to their scenario, they add, plenty of residual water ice could be activated during its passage through the solar system. The resulting outgassing would cause accelerations that match ‘Oumuamua’s comet-like trajectory.

“The tidal fragmentation scenario not only provides a way to form one single ‘Oumuamua, but also accounts for the vast population of asteroid-like interstellar objects,” Zhang says.

The researchers say their calculations demonstrate the efficiency of tidal forces in producing this kind of object. Possible progenitors, including long-period comets, debris disks, and even super-Earths, could be transformed into ‘Oumuamua-size pieces during stellar encounters.

Matthew Knight, co-leader of the ‘Oumuamua International Space Science Institute team, is impressed, noting that Zhang and Lin have done a remarkable job of explaining a variety of unusual properties of ‘Oumuamua with a single, coherent model”.

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Illustration of stellar tidal disruption processes. Credit: Zhang YUN

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