‘Oumuamua might be a shard of a broken planet

The solar system’s first recorded interstellar visitor has a new proposed origin story. The enigmatic celestial object known as ‘Oumuamua might be a shard of a planet ripped apart by its star’s gravity, researchers suggest April 13 in Nature Astronomy.

Ever since ‘Oumuamua showed up in our solar system in 2017, astronomers have struggled to explain its origin, suggesting that it might be a wayward asteroid, a comet or even an alien spacecraft (SN: 2/27/19).

Searching for other explanations, astronomers Yun Zhang at Côte d’Azur Observatory in Nice, France and Douglas Lin at the University of California, Santa Cruz developed computer simulations in which planetary bodies got too close to their parent stars.

In these simulations, objects ranging in size from comets to rocky planets orbit relatively lightweight stars. The scientists found that if these bodies repeatedly come within about 600,000 kilometers of their star — nearly 80 times as close as Mercury gets to our sun — then the star’s gravity shreds the objects, and flings the fragments into interstellar space.

If ‘Oumuamua were such a fragment, that might explain its odd shape, its tumbling motion and why it sped up as it left the solar system (SN: 6/27/18). The simulated planetary shards tumble and tend to be cigar-shaped, similar to ‘Oumuamua. And such a fragment could retain water ice deep within, the researchers say. Buried ice could have produced water vapor as ‘Oumuamua got heated by our sun. If that gas escaped through porous rock, it could have acted as a natural rocket, giving our fleeting visitor a little nudge as it rounded the sun.

This origin story is a plausible one that links ‘Oumuamua’s strange properties to planet formation throughout the galaxy, says Gregory Laughlin, an astronomer at Yale University who was not involved with this study.

However, Harvard astronomer Avi Loeb sees a plot hole in that narrative. To account for the estimated number of ‘Oumuamua-like objects that must be whizzing around the galaxy, “one needs each star to produce roughly a quadrillion such objects.” But planetary shredding should instead be rare, requiring the bodies to pass through a tiny sliver of space around a star, he says.

And that’s assuming a doomed object doesn’t just evaporate rather than break apart. “The statistics of such events makes the proposed scenario unlikely,” Loeb says.