Scientists Do 180 Away From 'Weird' Theories on 'Oumuamua

Was it an asteroid, a comet, a chunk of a distant planet, or an alien artifact? All of these possibilities have been floated since the mysterious 'Oumuamua, our first known interstellar visitor, sped through our solar system in 2017. A 2021 study suggested what was previously thought to be a cigar-shaped comet was instead a cookie-shaped chunk of a distant icy planet, flung off some 500 million years ago. Now, a new study published Wednesday in Nature returns to the earlier theory. "We've gone through every weird, crazy, possible theory ... and then we figure out that it is just the simplest thing you could possibly imagine," co-author Darryl Seligman of Cornell University tells the Wall Street Journal.

Though 'Oumuamua experienced nongravitational acceleration like a comet, it didn't appear to have the characteristic coma or tail made from dust and gases. But Seligman and colleague Jennifer Bergner, an astrochemist at UC Berkeley, can explain that. As a water ice comet is hit with cosmic radiation, creating bubbles of hydrogen. But "water ice in its amorphous form has a fluffy structure containing pockets" where such molecules can become trapped, Bergner tells the New York Times. As the ice is warmed by the sun, "it rearranges," causing "the collapse of these pockets and the formation of channels within the ice, through which trapped gas can escape."

The researchers argue enough molecular hydrogen gas escaped to propel the tiny 'Oumuamua (estimated at a few hundred meters long, as opposed to most comets, which are measured in miles), but any dust remained trapped. There are detractors, however. Harvard astronomer Avi Loeb, who's suggested 'Oumuamua is an alien artifact, says claiming the object is a water ice comet lacking a tail "is like saying an elephant is a zebra without stripes," per the Times. Yet astronomers have detected six similar comets that appear to accelerate but lack comas or tails. "Together they reveal that there is much to be learned about the nature of small bodies in the solar system," says Bergner. (More Oumuamua stories.)