We now know what caused comet ‘Oumuamua’s strange orbit

Remember what feels like decades ago, when we were visited by a comet from another solar system in 2019? Interstellar comet ‘Oumuamua captured headlines when its cigar-shaped body was spotted following an unusual orbit through our solar system, and subsequent research suggested it might once have been part of a Pluto-like planet and was possibly pancake-shaped.

One thing particularly puzzled astronomers, though, because the comet was accelerating away from the sun in a path that seemed strange. Now, researchers say they have an explanation for its unusual pathway, and it isn’t aliens — it’s a natural phenomenon called outgassing.

An artist’s depiction of the interstellar comet ‘Oumuamua, as it warmed up in its approach to the sun and outgassed hydrogen (white mist), which slightly altered its orbit. The comet, which is most likely pancake-shaped, is the first known object other than dust grains to visit our solar system from another star. NASA, ESA and Joseph Olmsted and Frank Summers of STScI

Comets are primarily made of dust, rock, and ice, and as they approach the sun they warm up and give off water and dust particles (which form comets’ distinctive tails). We are used to observing average-sized comets which eject gases, giving the comet a tiny kick and changing its trajectory ever so slightly.

But ‘Oumuamua was much smaller than your typical comet, at just over 100 meters across, and when it was first observed, it didn’t have a tail and didn’t seem to be ejecting water. So how could its trajectory be different from the typical elliptical orbit created by gravity?

The lead researcher, an astrochemist, worked with an astronomer on the idea that the hydrogen being outgassed by the comet might have been trapped within the comet’s own ice.

“A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result. 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?” researcher Jennifer Bergner said in a statement. “Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?”

The pair found that the trapping effect did happen, but only in a thin shell around a comet’s outer body. Most comets we observe are much bigger than ‘Oumuamua, at a few miles across, so the effect is almost invisible. But in tiny ‘Oumuamua, the researchers found that the effect would be significant enough to alter its orbit.

“What’s beautiful about Jenny’s idea is that it’s exactly what should happen to interstellar comets,” the other researcher, Darryl Seligman, said. “We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation.”

The research is published in the journal Nature.

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