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Top Comments: 'Oumuamua Was Likely a Comet [1]

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Date: 2023-03-26  

Artist's depiction of the interstellar object 'Oumuamua that visited our solar system in 2017. ESA/HUBBLE; NASA; ESO; M. KORNMESSER

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In October 2017, astronomers detected a fast-traveling object entering into our solar system from deep space. It was oblong, 100 - 400 meters long, and like nothing anyone had ever seen before. It was called ‘Oumuamua, Hawaiian for “a messenger from afar arriving first.” Was it an asteroid, a comet, or possibly even a space probe sent from another star system? That last speculation, though always considered highly unlikely, was bolstered by the fact that ‘Oumuamua actually sped up as it was leaving the solar system, rather than slowing down in normal response to the Sun’s gravity. Could the object have had engines powering its departure?

A new report judges that ‘Oumuamua was almost certainly a comet rather than an asteroid or a space probe devised by alien intelligence. Every aspect of its unusual behavior conforms to this interpretation. Certainly, its trajectory resembled that of a comet more than anything else, though it lacked a tail.

Their modeling shows ‘Oumuamua could have begun life as a regular water-rich comet around a nearby star, before being ejected. They found that high-energy cosmic rays that pervade the Galaxy, emitted by supernovae and other energetic events, could have turned up to 30% of the comet’s water ice into hydrogen, which could have become trapped in ‘Oumuamua’s ice as it journeyed through interstellar space. As ‘Oumuamua approached the heat of the Sun, it would have released the trapped hydrogen, giving the object its observed speed boost. But molecular hydrogen, being much less massive than the carbon monoxide or carbon dioxide ejected in typical comets, would not have had the momentum to pull much dust with it, explaining the lack of a coma or tail. “We don’t need to invoke anything superexotic to explain this behavior,” [Principal Investigator Jennifer] Bergner says.

So the lack of a tail on this comet was simply due to its release of molecular hydrogen, a molecule too light to co-eject the usual trail of cometary dust, rather than the more usual, heavier CO and CO 2 . Further, the hydrogen release could have provided the observed momentum boost. While we still don’t know any specifics of ‘Oumuamua’s origins or chemical makeup, its behavior can be explained fully without invoking any unprecedented phenomena. No aliens need apply. This interpretation may be boring compared to an alien space probe, but it’s reassuring that well studied and understood physical and chemical processes can explain a completely unexpected phenomenon such as this one.

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