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Scientists spotted a strange object flying through the solar system last year. It looked skinny,r18日活ポルノ映画痴漢電車視聴 like a cigar, and may have been a quarter-mile in length.

They named it `Oumuamua, Hawaiian for "messenger from afar," and after analyzing its trajectory, were sure it arrived from a distant solar system.

At first, scientists suspected it was a comet. But after the European Southern Observatory found no wake of dust and gas trailing behind it -- the telltale signs of traveling comets -- the International Astronomical Union classified it as a rocky asteroid. But that's not the end of the story.

Now, scientists have reversed course again. A new study published Wednesday in the journal Naturemakes clear beyond little doubt that `Oumuamua is truly a comet, not an asteroid. (And not that we should have to say this, but it's almost certainly not an alien ship.)

The scientists behind the study found that `Oumuamua's curious path through our solar system could not be explained by the gravitational pulls of the sun, planets, and large asteroids alone. Instead, the object had to have been propelled around as gas and dust ejected from it, like a spaceship using its thrusters to move in a certain direction.

"The gravitational effect of the planets and the large asteroids can all be modeled very well, because we know the positions and masses of them," Marco Micheli, a European Space Agency planetary scientist and the study's lead author, said via email.

"We took all of that into account in our analysis of the trajectory, and we noticed that an additional force was needed to explain the observations."

This additional force, said Micheli, is caused by the comet's outgassing as the icy body travels near the sun. The sunlight heats the surface, the ice turns to gas, and is then ejected into space.

"The comet itself therefore feels a push in the opposite direction," said Micheli.

To watch `Oumuamua's path through our solar system, Micheli and his team used measurements from the Earth-orbiting Hubble Space Telescope, as well as the European Southern Observatory's Very Large Telescope, situated high up in Chile's Atacama Desert.

Even before the researchers completed their analysis, however, some astronomers suspected `Oumuamua wasn't an asteroid.

Astronomers believe that the rockier bodies in solar systems -- like Earth, Mars, and the asteroids -- form in the in the inner solar system, whereas icy bodies, like comets, inhabit the outer realms. The reason is simple: It's simply too hot for the icy bodies to form near the sun, and they melt away, Thomas Barclay, an astrophysicist at NASA's Goddard Space Flight Center, said in an interview.

The icy comets, which live on the edges of solar systems, are much more likely to be tossed out, into the interstellar abyss.

But if `Oumuamua were an asteroid, scientists' understanding of the basic tenants of solar systems would have been challenged.

"There would be a fundamental misunderstanding in how stars and planets form," said Barclay, who had no involvement with the study.

Astronomers had been waiting to see an interstellar visitor, like `Oumuamua, so when they finally got their opportunity, "the chance of the first one we see being an asteroid were astonishingly small," said Barclay.

The fact that `Oumuamua is a comet confirms scientists' theories of planet and solar system formation, which holds that comets are thrown out of solar systems during the formation process as big masses of rock and ice are moving around.

"The discovery of 'Oumuamua, and the evidenceof it being cometary, nicely supports these models," said Micheli.

So, there's little doubt 'Oumuamua is a comet, even if it doesn't have a highly-visible tail of ejecting material. Not all comets do, said Barclay.

"It’s not at all that surprising that this is cometary even though it doesn’t look cometary," he said.