An Alien Visitor Turns Out to Be a Comet

Oumuamua, we never knew you.

It came from outer space, zooming through the solar system at 50 miles a second last October trailing mystery and dust: a lazily spinning reddish cigar-shaped rock — a cosmic stogie — named in honor of the Hawaiian-based Pan-Starrs telescope which found it.

It was an interstellar something, but what, exactly? Some astronomers turned radio telescopes on it just in case it was an alien spaceship, but it was silent.

Astronomers had long considered that interstellar debris might invade the solar system from time to time, in the form of icy chunks spit from the rocky disks forming faraway planets, that is to say, as interstellar comets. And they would come in on weird orbits like Oumuamua did.

But Oumuamua never lit up like a comet on its passage past the sun and through our realm, and so astronomers concluded that it was an alien asteroid, a dim, weird oblong rock, not quite like anything in our own system.

Now, however, the same team that discovered Oumuamua has concluded that it was a comet after all. Or as the headline in a news release announcing their paper in Nature said, “Sometimes a cigar-shaped ‘comet’ is just a comet.”

The key to this conclusion comes from an analysis of Oumuamua’s trajectory as measured by a variety of telescopes, including the Hubble Space Telescope, by Marco Micheli of the European Space Agency’s SSA-NEO Coordination Centre, in Frascati, Italy. He was a member of the original team that discovered Oumuamua. Dr. Micheli found that the gravity of the sun and planets was not the only force acting on the little wanderer. Something else was pushing the object away from the sun.

Such “non-gravitational” forces caused by the outgassing of gas and dust are characteristic of comets.

Unlike asteroids, which are mostly rock, comets are sometimes called “dirty snowballs,” conglomerations of various kinds of ices along with rock and dust. When they get close to the sun, these ices vaporize, carrying gas and dust into a cloud around the comet nucleus. The gas shoots up in little geysers or jets, that act like the thrusters on a spacecraft, giving the comet a little kick this way and that.

Astronomers have discovered a passing rock from another star — the first interstellar asteroid.Published On

As Dr. Micheli explained in an email, these jets are typically on the sunward side of the comet. So as a typical comet approaches the sun, the jets cause a braking effect so the comet falls a bit more slowly than it would if only gravity was pulling it toward the sun.

The astronomers only observed Oumuamua on the outward part of its journey, when it was going away from the sun, however. As Dr. Micheli said, “the object is receding from the sun and slowing down, and the outgassing effect acts in the opposite direction, and speeds it up a bit.”

That is exactly what Oumuamua seems to have done, according to Dr. Micheli’s analysis. “Exploring a variety of possible explanations for the detected non-gravitational acceleration, we find out-gassing to be the most physically plausible explanation,” Dr. Micheli and his colleagues write in Nature.

That explanation comes, however, with a caveat, namely that Oumuamua behaves like other comet nuclei we already know and love when it heats up.

Even so, reconciling the observations — particularly the lack of dust or water in vicinity of the suspected comet — suggests that Oumuamua must be a bit different, in its chemical composition and in the sizes of its dust grains, either because of where it came from or what happened to it over the eons along the way to us.

“This work shows that while Oumuamua looks familiar, there are differences that relate to its birth in a solar system far from our own,” Dr. Micheli wrote.

Some of those differences might be the result of its evolution as Oumuamua wandered through interstellar space, said Karen Meech of the University of Hawaii, leader of the team, including Dr. Micheli, that discovered the comet. Another part of it might just be the chemistry of where it came from. If we get more of these, she said, “we could get a look at chemical processes in another solar system.”

And where that was is even more mysterious now than it was before. All these non-gravitational forces will make it harder to figure out where Oumuamua came from and where it is going.

Calling the whole Oumuamua experience “wildly exciting.” Dr. Meech said, “I can’t wait for the next one that comes along.”

A version of this article appears in print on , on Page D2 of the New York edition with the headline: False Alarm: As Space Aliens Go, Oumuamua Is Kind of a Dud. Order Reprints | Today’s Paper | Subscribe