Wayward comet ‘Oumuamua, the first interstellar object to visit our solar system, has been an object of fascination since it was observed in 2017, including its intriguing acceleration as it receded from the sun.
Hypotheses have been put forward as a result of its unpredictable behaviour, including fleeting speculation that it could be an alien spacecraft. A new study offers a more realistic explanation – that ‘Oumuamua’s velocity was due to the release of hydrogen gas as the comet warmed in sunlight.
Oumuamua (pronounced oh-MOO-uh-MOO-uh) lacks the gas and dust tail typical of many comets. It was previously described as cigar-shaped, but is now thought to look like a rock pie. It is smaller than originally estimated and is now estimated to be about 115 meters by 111 metres, with a thickness of 19 metres.
According to the researchers, ‘Oumuamua, like many other comets, appears to have originated in the form of a so-called protoplanet – a small body formed in the early stages of planet formation – and was essentially a large icy space rock.
After the comet was somehow ejected from its solar system, the comet’s chemistry changed as it was bombarded by high-energy radiation as it traveled through interstellar space. This converted some of the comet’s ice – frozen water – into hydrogen gas trapped in the rest of the ice.
Oumuamua then warmed as it traveled through our inner solar system, rearranging the comet’s ice structure and releasing trapped hydrogen gas. The release of this hydrogen in a process called outgassing will not cause a visible tail.
The key discovery is that “Oumuamua may have started as an icy, water-rich planet much like comets in the solar system,” said astrophysicist Jenny Bergner of the Institute for Astrochemistry Jenny Bergner. This model could explain ‘Oumuamua’s strange behavior without resorting to exotic physics or chemistry. University of California, Berkeley, lead author of the study published this week in the journal Nature.
“The simplest explanation, which is exactly what we would expect for an interstellar comet, fits all the data without fine-tuning,” said study co-author Daryl Seligman, a postdoctoral researcher in planetary sciences at Cornell University.
Oumuamua, whose name in Hawaiian means a messenger coming from far away, was first detected by the University of Hawaii’s Pan-STARRS1 telescope.
“We don’t know where it came from, but it probably traveled through interstellar space for less than 100 million years. It had a reddish color that matches the colors of many small bodies in the solar system. It has now passed Neptune on its way out of the solar system,” Bergner said.
A second interstellar object, Comet 2I/Borisov, was discovered in 2019 when visiting our solar system. This seemed like an ordinary comet.
These alien intruders may be more common than previously known. According to the researchers, one to two interstellar objects can be detected in our solar system every year once a new astronomical observatory now being built in Chile is scheduled to open next year.
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