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‘Oumuamua, which entered our Solar System from interstellar space in 2017, may be one of many remnants of a comet, asteroid, or small planet ripped apart by another star.

ESO/M. Kornmesser

How the first visitor to our Solar System may have formed—no alien technology required

When ‘Oumuamua swooped into our Solar System in 2017, the object stirred up excitement. The strange shape and trajectory of this first known visitor from interstellar space prompted even some serious scientists to suggest it might be an alien probe. But a new study arrives at a much more mundane explanation.

‘Oumuamua—“scout” or “messenger” in Hawaiian—is about 100 meters long, or slightly longer than a U.S. football field, and at least six times longer than it is wide. The object also didn’t follow a path shaped only by the Sun’s gravitational attraction, which suggests ‘Oumuamua was releasing gas as a comet might, even though observations hint that the object doesn’t have the icy surface expected of a comet.

In a new study, Yun Zhang, an astrophysicist at the Cote d’Azur Observatory, and a colleague used computer simulations to understand how ‘Oumuamua got its strange shape and trajectory by looking at what happens to various orbiting objects if they strayed, Icarus-like, too close to their own sun. For example, if an asteroid, a small rocky body that in our Solar System is often nothing more than a loosely packed pile of rubble, passed within 60,000 kilometers of its parent star, it would be stretched and then pulled apart by strong tides, creating a large number of tumbling, elongated fragments. Some of these would be ejected from the solar system into interstellar space, the researchers report today in Nature Astronomy.

If ‘Oumuamua’s parent body was instead a comet, it would suffer a similar fate. Strong gravitational tides would rip the comet apart, and much of the ice on its surface would be baked away by the close call, Zhang says. But some volatile ices, including water and carbon dioxide, would survive at depths of 10 to 50 centimeters below the object’s rocky surface.

If such an object later swooped past a larger and warmer star such as our Sun, those ices could evaporate and slowly but steadily spew into space. If those emissions were uneven, they would act as tiny booster rockets and cause the sort of trajectory anomalies that astronomers have observed for ‘Oumuamua. And if the remnants of an asteroid carried small amounts of water beneath its surface, its emissions also could result in a weird trajectory like ‘Oumuamua has, Zhang notes.

Even a planet 10 times larger than Earth could be torn apart if it passed within 40,000 kilometers of a red dwarf, the team’s simulations show. About half of the oddly shaped, water-bearing shrapnel from such an event could escape the star and eventually pass through other solar systems.

Zhang and her colleague have put together “a compelling analysis,” says Matthew Knight, an astronomer at the United States Naval Academy. Several of these general scenarios to explain ‘Oumuamua’s origins have been floating around for a while, he notes, “but these guys are the first to have actually run the numbers.”

*Correction, 13 April, 4:20 p.m.: Two erroneous distances mentioned in the original story have been corrected.