Ahen the interstellar visitor Oumuamua crossed our solar system five years ago, he presented the astronomers with a puzzle: The object accelerated slightly on its orbit – without any apparent cause. Even renowned scientists speculated that Oumuamua might not be an artificial object, an extraterrestrial space probe. An astrochemist and a geophysicist have now found a simple explanation for the mysterious acceleration: As the two scientists report in the journal “Nature”, Oumuamua emitted hydrogen that was created by cosmic rays in interstellar space.
“Oumuamua is the first interstellar object to be identified in the solar system,” explain Jennifer Bergner of the University of California at Berkeley and Darryl Seligman of the University of Chicago. It was spotted by the Pan-STARRS automated telescope on the Hawaiian island of Maui on October 19, 2017.
It quickly became clear that it couldn’t be a typical comet of our solar system: the speed was much too high at 87 kilometers per second and the orbit was not a closed ellipse around the sun, but an open hyperbolic orbit. The astronomers from Hawaii therefore gave the celestial object the name “First messenger from afar”, Hawaiian: Oumuamua.
Astronomers had long hoped for such a discovery. Because in the formation phase of planetary systems, many small bodies – asteroids and comets – should be thrown out of their system and then traverse the space between the stars for millions of years. And every once in a while, such a lonely wanderer should traverse our solar system.
But Oumuamua surprised the researchers – because it did not resemble an asteroid or comet, as expected. At about 200 meters, it was quite small, and its brightness varied by a factor of 12. The astronomers concluded that Oumuamua must have been shaped like either a pancake or a cigar.
The biggest surprise for the scientists, however, was that the orbit of Oumuamua was not only determined by the gravitational pull of the sun, but the small body showed a small but unusual acceleration. Astronomers are familiar with such accelerations from comets that emit water vapor and dust. But Oumuamua was not “active”, there was no trace of emanating matter.
This started the speculation. Was it a “cosmic iceberg” of pure frozen hydrogen? Hydrogen could evaporate without being visible to astronomers, explaining the acceleration. But how should such objects arise? Or was it even an extraterrestrial spaceship, equipped with a solar sail for propulsion? The object was too far away and too fast to check these possibilities.
Jennifer Bergner had another idea: maybe it was a small comet made of debris and water ice – but one that had been modified by its long journey through interstellar space. During her research, the researcher came across numerous old experiments dating back to the 1970s that proved her right: high-energy cosmic rays could change water ice, produce molecular hydrogen from it – and this could remain trapped in the porous ice. As it approaches the sun, the ice changes and releases the hydrogen – which then gives the small body a gentle nudge.
Crucially, as Bergner shows along with Seligman, ‘Oumuamua is so small. Because the sunlight only works close to the surface of the object and the effect is correspondingly small. For a larger comet, the visible emission of water vapor and dust would dominate. “So our model is in agreement with comets in our solar system,” says Bergner. And it explains the properties of Oumuamua without further exotic assumptions: “It is a normal interstellar comet.”
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