The cigar-shaped 'Oumuamua, the first interstellar object in recorded human history to whizz through the Solar System, is a comet after all, a pair of astronomers declared in research published in Nature on Wednesday.

In 2017, [1]'Oumuamua captured the imagination of scientists and space fans with its peculiar characteristics. It had a nobbly, rocky surface, and was elongated and flat unlike anything they had seen before. The object looked like it might be an asteroid, but behaved more like a comet.

As it travelled through the Solar System, astronomers were puzzled over how it was accelerating using non-gravitational forces. Some people even suggested the first-known foreign visitor, whose name is a Hawaiian word meaning "a messenger from afar" might be an alien spaceship. But new evidence suggests 'Oumuamua was just an icy comet after all.

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Jennifer Bergner, assistant professor of astrochemistry at the University of Berkeley, and Darryl Seligman, an postdoctoral fellow studying theoretical and computational planetary science at Cornell University, and co-authors of the Nature [3]study , believe they have cracked the mystery by figuring out 'Oumuamua was powered by the outgassing of hydrogen gas as it was heated by the Sun.

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"A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result," Bergner [6]said in a statement. "Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen? Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?".

They found old research papers which showed that cosmic rays bombarding ice could produce molecular hydrogen (H2) that was trapped within the ice. In Oumuamua's case, as it absorbed solar radiation, its ice produced hydrogen to propel it along its path, but the gas remained trapped underneath its surface so it didn't have a dust coma like typical comets do.

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"Even if there was dust in the ice matrix, you're not sublimating the ice, you're just rearranging the ice and then letting H2 get released. So, the dust isn't even going to come out," Seligman said.

"For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn't necessarily expect that to be a detectable effect," Bergner added. "But because 'Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration."

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[11]NASA spots first evidence of an active volcano on Venus – in a big pile of CD-ROMs

Planetary systems like the Solar System also contain comets and asteroids. These space rocks can be expelled due to gravitational interactions with other objects, and end up travelling through our Solar System. Rogue objects like 'Oumuamua provide rare opportunities to study planet formation in distant worlds if astronomers could study them in more detail.

"The comets and asteroids in the solar system have arguably taught us more about planet formation than what we've learned from the actual planets in the solar system," Seligman said. "I think that the interstellar comets could arguably tell us more about extrasolar planets than the extrasolar planets we are trying to get measurements of today."

A second interstellar visitor, 2I/Borisov, was [12]spotted passing by in 2019. Astronomers immediately realized it was a comet due to its visible coma created by water vaporized from its surface as it was warmed by the Sun. ®

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[1] https://www.theregister.com/2018/06/28/oumuamua_cigar_comet/

[2] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2ZByFtI-WGHrgo4RtyybeDQAAAIc&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

[3] https://www.nature.com/articles/s41586-022-05687-w

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[6] https://news.berkeley.edu/2023/03/22/surprisingly-simple-explanation-for-alien-comet-oumuamuas-weird-orbit/

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[8] https://www.theregister.com/2023/03/23/japanese_private_moonshot_hakutor/

[9] https://www.theregister.com/2023/03/22/space_dust_alien_life_research/

[10] https://www.theregister.com/2023/03/20/earth_light_pollution/

[11] https://www.theregister.com/2023/03/16/nasa_venus_volcano/

[12] https://www.theregister.com/2020/04/28/comet_2i_borisov_water/

[13] https://whitepapers.theregister.com/