Strange interstellar object could be a shard from a dead planet

'Oumuamua, the first object known to have come into our solar system from beyond it, might have gotten its oblong shape when a planet was shredded by a faraway star.

By Nadia Drake
Published 14 Apr 2020, 14:40 BST
This artist’s impression shows the first interstellar object known to pass through the solar system: 'Oumuamua. ...

This artist’s impression shows the first interstellar object known to pass through the solar system: 'Oumuamua. The unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawaii. Subsequent observations from observatories around the world show that it is unlike anything normally found in the Solar System.

Photograph by Eso, M. Kornmesser

An unusual visitor cruised through the solar system in late 2017—a small, rocky object that did not hail from Earth’s neighbourhood, but was born in a star system far, far away.

The interstellar visitor—named ‘Oumuamua by the team that discovered it, which roughly translates from Hawaiian as “a messenger from afar arriving first”—posed more than a few puzzles. First spotted by the Pan-STARRS project at the Haleakalā Observatory in Maui, the tumbling object accelerated in ways that could not be explained by gravity alone. And based on the light it reflected, ‘Oumuamua appeared to be an elongated, cigar-shaped object—a shape unlike anything seen in our own solar system.

New computer simulations reveal a possible origin story for this strange interstellar object: A world was ripped to pieces by its home star, leaving behind a wake of long, thin fragments. Some of these fragments would have been launched into interstellar space, and millions—perhaps billions—of years later, ‘Oumuamua reached our solar system. The simulations point to three possible types of home systems for ‘Oumuamua, and the work explains both the elongated shape and curious motion of the interstellar visitor.

“’Oumuamua provides a lot of problems to explain its origin,” says Yun Zhang, a researcher at the Côte d’Azur Observatory in France and lead author of a study on the simulations published today in Nature Astronomy. “Before our study, no solution can produce such an elongated shape.”

A mystery from beyond

Astronomers have always suspected that interstellar objects were traipsing through our solar system—it was just a matter of time until we spotted one. But they guessed those objects would look more like the recently discovered interstellar comet Borisov. A disintegrating world with an icy halo, Borisov looks like the frozen objects of the outer solar system.

“With Borisov, we get exactly what we expect an interstellar visitor would act like and do. Everything about it is completely ordinary,” says Greg Laughlin, a professor of astronomy at Yale University. “And that’s a startling contrast with ‘Oumuamua, where literally nothing about ‘Oumuamua was ordinary.”

Instead of icy and comet-like, ‘Oumuamua was rocky and dry, more like an asteroid. It was too tiny and dim to observe the surface directly, so astronomers inferred its shape based on how it reflected light as it tumbled. Its strange, elongated form immediately provoked speculation about its origins, and as astronomers continued to observe ‘Oumuamua’s passage, they noticed curious accelerations that were ascribed to water vapour erupting from beneath its surface.

As late as last year, ‘Oumuamua’s origin was still a mystery, but, “all these puzzles can be solved by our scenario,” Zhang says.

How to make a space cigar

Zhang and her colleague Doug Lin of the University of California, Santa Cruz, considered the possibility that ‘Oumuamua came from a system with planets or smaller planetesimals orbiting a small, dense, central star—something with enough gravity to destroy worlds that come too close without burning them to a crisp first.

The team plotted the trajectories of three types of objects orbiting these stars: half-mile-wide planetesimals, frozen objects similar to comets, and larger planets such as super-Earths.

Zhang and Lin found that if any of these objects come within about 220,000 miles of their host star, they will be spun up, stretched out, and shredded by the star’s gravity—a process that astronomers call tidal disruption. The smaller the object, the closer it has to get to be ripped apart. Depending on the composition of the parent world, some of the fragments could be tumbling, extremely elongated objects like ‘Oumuamua. And due to the violence of this process, many of the shards would launch into interstellar space, never to return.

“Nature doesn’t make a lot of shard-like objects,” Laughlin says. “So the fact that tidal disruption does naturally do that makes it a very compelling idea to explore, and they’ve done a very complete and careful job of exploring that option.”

The simulations also suggest that when the parent worlds are ripped apart, the star’s warmth melts the fragments and vaporises any water near the surface. But pockets of ice buried deep inside the fragments survive, which could explain the hypothesised jets of water vapour that pushed ‘Oumuamua when it neared our sun.

When these planetary fragments melt and re-solidify, the rock becomes stronger and forms a tough outer crust, like melted and cooled chocolate. “It’s harder to take apart again after the surface freezes,” Zhang says. This tempering process could explain why ‘Oumuamua didn’t crumble completely when it passed our sun, unlike Borisov, which recently broke apart on its way out of the solar system.

Cigar shape, or pancake?

The simulations do a great job of explaining how objects like ‘Oumuamua may form, says Michele Bannister of New Zealand’s University of Canterbury, who also studies interstellar objects. “I think some of the mechanisms they’re looking at are more likely than others,” she says, noting that a comet makes the most sense as a parent body for ‘Oumuamua, while a super-Earth-sized planet isn’t as convincing.

But both she and Laughlin introduce a new wrinkle to the mystery: They’re skeptical that ‘Oumuamua is actually a cigar-shaped object, pointing to a paper published last summer that revisited the original observations of the object. The newer analysis concludes that ‘Oumuamua may actually have a pancake-like shape—a shape that Bannister likens to an overstuffed pita, similar to an object in the outer solar system called MU69, or Arrokoth.

“We flew [the spacecraft] New Horizons past Arrokoth and what do we find? We find two overstuffed pita breads stuck together,” Bannister says. “It’s interesting and suggestive.”

If ‘Oumuamua is not a stretched-out, rocky fragment, then its true origin remains shrouded.

“If you assume that the object was cigar-shaped, this is a very plausible end-to-end description of what it was,” Laughlin says. “But if it is the case that ‘Oumuamua is a pancake-shaped thing, then that puts the whole thing back into the pure mystery zone.”

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