Dennis Overbye

Astronomers eventually concluded that the object was about 1,000 feet long and cigar-shaped, evoking the image of a rocket cylinder. But further analysis revealed that it was speeding up as it exited the solar system, evidence that gas boiling off the surface of the icy body was giving it a boost. That meant it was a comet after all.

But when Dr. Seligman and Dr. Laughlin tried to simulate the comet’s behavior based on calculations of how much energy it would have received from the sun, they found that water ice, the main constituent of ordinary comets, did not provide enough oomph to explain the comet’s acceleration.

“Frozen hydrogen does, however, offer a compelling mechanism for acceleration,” Dr. Seligman said. Being more volatile, it could easily supply the necessary energy boost.

It wouldn’t take much, he noted. The subliming hydrogen would add about 200 feet per second to the velocity of a cosmic ice ball that was already speeding along at 40 miles per second.

“This speed increment would be a big deal on the autobahn, but it’s a rounding error on the cosmic racetrack,” Dr. Seligman said.

Dr. Laughlin added in an email, “As Oumuamua passed close to the Sun and received its warmth, melting hydrogen would have rapidly boiled off the icy surface, providing the observed acceleration and also winnowing Oumuamua down to its weird, elongated shape, much as a bar of soap becomes a thin sliver after many uses in the shower.”

But that version of events would require rewriting Oumuamua’s origin story yet again. Forget about alien planet systems or even aliens. Solid hydrogen can exist only at temperatures below 6 degrees Kelvin, or 6 degrees Celsius above absolute zero. To find temperatures cold enough to freeze hydrogen out of the interstellar gas, you would have to go inside the coldest, densest lumps, called prestellar cores, in big, dark molecular clouds.

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