Author: Lisa Grossman / Source: Science News
The first known interstellar visitor to the solar system is keeping astronomers guessing.
Ever since it was spotted in October 2017, major mysteries have dogged the object, known as ‘Oumuamua (SN Online: 10/27/17). Astronomers don’t know where it came from in the galaxy. And they’ve disagreed over whether ‘Oumuamua is an asteroid, a comet or something else entirely.
One of the strangest mysteries is how ‘Oumuamua sped up after it slung around the sun and fled the solar system, a motion that can’t be explained by the gravitational forces of the sun or other celestial bodies alone. The most natural explanation is that ‘Oumuamua spouts gas like a comet, which would have given the object an extra push away from the sun — except astronomers saw no signs of such outgassing.
In November, Harvard University astronomers Shmuel Bialy and Avi Loeb sparked a firestorm of media coverage when they suggested that the acceleration could be explained if ‘Oumuamua is an alien spaceship, in a paper published in Astrophysical Journal Letters. In particular, the duo suggested, the object could be a solar sail: a large flat sheet less than 1 millimeter thick that uses pushes from starlight to navigate the galaxy (SN: 9/10/11, p. 18). Loeb is part of an organization called the Breakthrough Initiative that has suggested sending solar sails to visit a nearby planet orbiting the star Proxima Centauri (SN Online: 8/25/16). Maybe some other spacefaring civilization sent a similar sail to visit us, Loeb argues.
Since then, astronomers have been kicking around other origin stories to explain ‘Oumuamua and its bizarre behavior. “Jumping to the conclusion that it has to be produced by extraterrestrial intelligence, I think we don’t have evidence for it yet,” says astronomer Amaya Moro-Martín of the Space Telescope Science Institute in Baltimore. “There are other natural explanations that can be explored.”
Here are three such possibilities.
1. Fluffy ice fractal
To get a push from starlight, an object needs to have a large surface area — to provide more surfaces for particles of light called photons to nudge — and a small mass, so that even tiny amounts of photon pressure can make a difference.
A flat sheet, such as a solar sail, isn’t the only way to harness this radiation pressure, Moro-Martín says. A fluffy, porous structure that resembles a fractal, a geometric pattern that repeats itself on smaller and larger scales, could also be propelled by light, she argues. “Physically it would be the same idea, just the geometry would be different.”
Dust particles collected in Earth’s stratosphere can have this sort of fluffy fractal form, Moro-Martín says. She also sees similar structures in computer simulations of the way planets grow up in the dusty planet-forming disks astronomers see around other stars. As ice grains in the distant, frigid regions of those disks stick together, the…
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