When astronomers first spotted the celestial object now known as 'Oumuamua skittering across the sky last October after it had dived around the sun, its elongated trajectory and rapid speed quickly revealed that it came from outside the solar system.

Learning anything else about our first-known interstellar visitor, however—such as whether it was an asteroid or a dim comet—proved far more challenging, as it departed our planetary vicinity as quickly as it arrived. Either classification would have important implications not only for 'Oumuamua itself, but also for understanding how planetary systems form.

Now a team of researchers monitoring the object on its journey back to the stars say they have the answer: 'Oumuamua is almost certainly a comet, albeit one fittingly alien from those we find orbiting the sun. Using NASA's Hubble Space Telescope and other ground-based instruments, the team observed 'Oumuamua’s changing position across time and plotted its outbound trajectory, finding that, remarkably, it did not follow the path they had anticipated. The result appears in the June 27 edition of Nature. “When put together, these positions showed that the motion of 'Oumuamua was slightly different than what we expected,” says lead study author Marco Micheli of the European Space Agency’s SSA–NEO Coordination Center.

'Oumuamua’s motion, it turns out, was changing ever-so-slightly over time—suggesting some force other than the sun’s steadily diminishing gravitational pull was acting on it. The strange push was small, about two million times weaker than the pull of gravity on Earth’s surface and about 1,000 times smaller than the effect of the sun’s gravity, Micheli says. Even so, over time that tiny push made big changes: At the distance of Jupiter, the team’s measurements show, 'Oumuamua's position was shifted from expectations by approximately the width of the giant planet. But what was nudging the mysterious visitor?

To find out, Micheli and his colleagues first simulated its journey through the solar system, accounting for gravitational shoves from all eight planets, Pluto, the moon and the largest bodies in the Asteroid Belt. They also investigated other possibilities such as the influence of “radiation pressure” from sunlight, tweaked rotational rates from uneven solar heating of 'Oumuamua’s surface or potential collisions with other objects that could have affected the visitor’s trajectory. None of these explained the observed changes. “There are a whole bunch of potential reasons that the comet could get nudged,” says cometary scientist and study co-author Karen Meech, a planetary scientist at the University of Hawai'i at Mnoa. “We systematically ruled them all out. The only one that’s physically plausible that remains is outgassing.” In other words, 'Oumuamua’s shift was self-induced, caused by the rocketlike effect of streams of gas shooting out of sunlight-warmed ice at or near its surface. Such a phenomenon is regularly observed in ordinary comets that pass near the sun—just as 'Oumuamua had.

A Tale of No Tail

Understanding what this all means requires a quick history lesson: Our solar system was a violent, chaotic place when it was young. In the first few hundreds of millions of years—an eyeblink in astronomy timescales—a wealth of material is thought to have been kicked out of the solar system as a result of gravitational interactions among the giant planets and other, smaller bodies. Most of those outcasts were rich in icy, cometlike material from the outer solar system rather than rocky asteroids. If our solar system is typical (and there are as of yet few reasons to believe it is not), then most young planetary systems should suffer from similar violence and the space between the stars should be littered with more ejected comets than asteroids. Comets would thus be the default emissaries from other stars.

But as telescopes around the world turned their attention to 'Oumuamua, it became clear the visitor showed no signs of cometary activity. It lacked the streaming tail of a comet or any signs of ice and gas emerging after sizzling so close to the sun. Some astronomers suggested the comet had been fried by interstellar radiation, forming a crust of material that shielded the lighter ices beneath from the sun’s evaporative warmth.

So, if Micheli and Meech's data is sound, why didn’t earlier observations detect any gas—or, for that matter, the associated shift in 'Oumuamua’s motion? One of the biggest reasons is because the emission of gas—and the resulting change in motion—was very small. “[The push] was so little that it could not have been seen in our observations,” Micheli says—particularly, according to Meech, given the brevity of 'Oumuamua’s close encounter with Earth and the object’s inherent dimness. Within a week of discovery 'Oumuamua had receded so far from our planet, it was 10 times dimmer than when first spotted; after a month it was a hundred times darker still. That made some observations difficult, if not impossible, Meech says.

When astronomers study comets, they hunt for cyanide, which when excited by starlight emits a distinct, telltale blue glow readily detectable with advanced telescopes. The compound is stirred into the comet at its formation, a fingerprint of the early planetary system. If 'Oumuamua streamed cyanide gas, however, it was below the detection limits of current instruments. That null result suggests 'Oumuamua must have a cyanide to water ratio at least 15 times lower than our solar system's most cyanide-depleted comets—further proof the object was truly not born in our solar system. “It would not surprise me that a different solar system would have a totally different environment, and that we might find cyanide depletion,” says cometary scientist Matthew Knight of the University of Maryland, College Park, who was not part of the study team.

A dearth of small, light-reflecting dust particles on its surface could also account for 'Oumuamua’s cloaked cometary emission. Knight says the object’s apparent lack of small dust could have come about in two different ways: Either it could have passed by its star multiple times in its home system, in which case stellar winds would eventually blow away its smallest dust particles; or the dust could have slowly eroded away from eons of exposure to cosmic radiation during its long sojourn in interstellar space. He doubts the first explanation, simply because most of the objects ejected from a planetary system are done so early on, before they can be so intensely baked by their stars. Although it is still possible to eject things from the solar system today, a late-breaking outcast would be rare in contrast to the wealth of material ejected in the early years. Statistically speaking, 'Oumuamua should not have had enough time to shed its small particles before leaving its home system. The second option—the gradual erosion of dust from cosmic radiation—is the explanation Meech and others find most likely.

'Oumuamua is long gone, forever faded from the view of even the world’s current best telescopes. But astronomers are now busying themselves preparing for the next visitor. Meech says the completion in the 2020s of several planned next-generation observatories will allow for more detailed examinations of the solar systems’ next interlopers.

“Now that we know from direct experience how interstellar objects behave, we hope that the next time an object like this is discovered we will be able to get even more detailed observations,” Micheli says. “Hopefully the next one may also remain observable for a bit longer, giving us more time to study its motion and its composition in greater detail.”