The astronomers operating the Pan-STARRS1 telescope on the island of Maui were not expecting to hit cosmic paydirt on Oct. 19, 2017—but they did. On what was otherwise an ordinary night of skygazing, they suddenly spotted what is easily the oddest comet ever detected. Its high speed—87 km per second (54 mi. per second)—and highly elliptical angle indicated that it originated from deep space, the first known interstellar object ever to enter our solar system. It was cigar shaped and, as comets go, tiny—just 115 m (377 ft.) long and 19 m (62 ft.) wide.
Most important, the comet—which was dubbed Oumuamua (Hawaiian for “a messenger from afar arriving first”)—actually accelerated during the latter part of its transit, more than the gravitational influence of the sun could explain. That left even sober scientists to speculate that the object might actually be an alien spacecraft, speeding up under its own power during its barnstorming of our solar system.
In the years since Oumuamua’s discovery, most people have put aside the E.T. talk, but no one has yet explained how, in fact, the object defied traditional cometary physics and hit the gas as it was leaving our solar system. Now, at last, a new paper in Nature might have the answer—and it has everything to do with molecular hydrogen.
Every comet that passes through our solar system speeds up on the way out. For one thing, as it swings around the far side of the sun, the solar gravity gives it a sort of whipcrack push. What’s more, dust on the surface of the comet outgasses due to solar heating, providing a natural jet that adds even greater acceleration. But Oumuamua was too small to have any surface dust—denying it the glowing halo, or coma, that circles the body of common comets as well as creating the comet’s characteristic tail.
“When astronomers looked for common signatures of outgassing activity, they couldn’t find them [on Oumuamua],” says Jenny Bergner, professor of chemistry at the University of California, Berkeley, and lead author of the new paper. And while Oumuamua did pick up some energy from the sun’s gravitational push, calculations showed its increase in speed was too great to be explained by that factor alone.
“The most mysterious thing about Oumuamua was this very significant non-gravitational acceleration,” says Darryl Seligman, a post-doc in astronomy at Cornell University and the co-author of the paper
One possible answer, Seligman says, was what’s known as the Yarkovsky effect, a phenomenon by which small bodies like asteroids—or tiny comets like Oumuamua—absorb photons from the sun and re-radiate them in a kind of propulsive plume. But this effect too was too small to account for the degree of Oumuamua’s acceleration.
That left three possible explanations: propulsion provided by nitrogen, carbon monoxide, or molecular hydrogen (H2). All three of these gasses are present in comets and all three are what is known as hypervolatiles. “They really want to be in the gas phase all the time,” says Bergner, “but sometimes they can be frozen.”
When Oumuamua was in deep space, the hypervolatiles were frozen indeed, exposed to temperatures as low as -269ºC (-450ºF). In theory, as the comet approached the sun, those hypervolatiles could have warmed up and outgassed as plumes, giving Oumuamua the push it needed to explain its extra-gravitational acceleration. The same process would take place on other comets, but they are too large to be affected much by such a subtle nudge. A tiny comet like Oumuamua would be a different matter
In the course of their work, Bergner and Seligman ran computer models to determine both the overall composition of the comet and its so-called “budget” of hypervolatiles—how much carbon monoxide, nitrogen, and H2 would be present. They also conducted thermal modeling to estimate how the change in temperature from deep space to the warmer inner solar system would affect those materials. On the whole, they determined that the quantities of carbon monoxide and nitrogen onboard Oumuamua would be too low to explain the outgassing and acceleration. But H2 would be a different matter.
Oumuamua, like most comets, is rich in water. Before the comet entered the solar system, the extreme cold of deep space would cause the water to freeze into ice in what is known as an amorphous state. Rather than the solid, crystalline structure of ordinary ice, amorphous ice is porous, dotted by pockets. Exposure to deep space would have a second effect on the ice too—with cosmic radiation causing some of the H2 in the H2O molecules to break away. That H2 would collect in the pores of the amorphous ice, like fuel in tiny fuel tanks. When Oumuamua entered the inner solar system, it warmed up just enough for the ice to convert to its crystalline state, essentially closing the pockets and forcing the H2 out of the comet, providing the propulsive push that explained the acceleration.
“When the water matrix has enough energy, it rearranges to a more stable and more compact configuration,” says Bergner. “In the process, you lose those pores and the hydrogen can escape through the surface.”
So question answered, problem solved and, alas, no alien spacecraft in the mix. Bergner, Seligman, and other astronomers will be looking for similar small and dark comets when the National Science Foundation’s Vera C. Rubin Observatory goes into operation in Chile’s Atacama desert in 2025, with a specific charge to spend part of its observation time looking for hydrogen outgassing from comets. Before 2017, astronomers did not even know that a species of comet like Oumuamua existed. Now, thanks to the Rubin Observatory and the astronomers who will make use of it, we’ll learn more about their behavior, composition, population, and more.
“The main takeaway is that Oumuamua is consistent with being a standard interstellar comet that experienced heavy processing [in space],” said Bergner in a statement that accompanied the release of the paper. If one Oumuamua exists, many more should be found.
Write to Jeffrey Kluger at jeffrey.kluger@time.com.