SSPL / Getty Images This artist's concept shows a brown dwarf surrounded by a swirling disk of planet-building dust. NASA's Spitzer Space Telescope spotted such a disk around a surprisingly low-mass brown dwarf, or "failed star."

(Correction appended Nov. 14, 2012)

Just 20 years ago, astronomers imagined that planets beyond the Solar System would be more or less like the ones we know: small, rocky worlds like Mercury, Venus, Earth and Mars orbiting relatively close to their stars, and big, gassy ones like Jupiter, Saturn, Uranus and Neptune, farther away. (Even then, Pluto was recognized as an oddball, though it hadn’t been demoted yet.) Then the first actual exoplanet was discovered, and it turned out to be a big, gaseous world orbiting ridiculously close to its star. Dozens of others very much like it soon turned up, and the astronomers’ preconceptions were abruptly laid to rest.

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But at least these so-called “hot Jupiters” actually orbited a star. Not so for a new planet just reported in the journal Astronomy and Astrophysics. The object, known only as CFBDSIR2149, appears to be a planet from four to seven times as massive as Jupiter, floating along with a cluster of stars known as the AB Doradus Moving Group — but tethered to no one star in particular.

That’s the only reason the planet was spotted at all, in fact. If it were orbiting a star, the parent sun’s bright glare would make even a huge planet tough to discern. It would be like trying to see a candle sitting next to a searchlight. The team of French and Canadian astronomers who made the discovery weren’t looking for planets in any case. They were looking for brown dwarfs, objects too big to be classified as planets, but too small to ignite the nuclear reactions that would qualify them as full-blown stars.

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But when CFBDSIR2149 showed up in the Canada-France-Hawaii telescope on Mauna Kea, says co-discoverer Etienne Artigau, of the University of Montreal, “we saw that it was very red compared with the typical brown dwarf.” That meant it was relatively cool. It could still be a brown dwarf, but it would have to be billions of years old to have lost so much of its internal heat. If the object were very young, its temperature ruled it out as a brown dwarf at all. In general, says Artigau, “it would not be a trivial thing to distinguish an old, massive object from a young, small one.”

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In this case, however, there was an extra clue: careful measurements with the European Southern Observatory’s New Technology Telescope showed that CFBDSIR2149 is moving across the sky in the same direction and speed as the AB Doradus group of stars. It’s possible that this is pure coincidence. The odds are 87%, however, that it is indeed part of the group. And since astronomers know the group itself is between 50 and 120 million years old, that means CFBDSIR2149 is a young planet.

It’s not the first free-floating planet ever found, the scientists hasten to make clear. Astronomers have found indirect hints that many more may be roaming the galaxy, and theorists have no trouble explaining how such a thing could happen. CFBDSIR2149 might have formed originally as part of a solar system and then been sling-shotted out in a close encounter with another massive planet. Or it might have formed directly from a small collapsing cloud of gas and dust.

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But while rogue planets like CFBDSIR2149 may be common, this one is uncommon in a very important way: it’s close to Earth, at a mere 100 to 120 light years from us. At that distance, it’s amenable to detailed study — and indeed, astronomers have already detected the signature of methane and water vapor in its atmosphere.

That’s not the only bit of good luck here. Moving groups like AB Doradus are made up of stars that formed together, but they eventually drift apart. Our own Sun very likely went through a stage like this in fact, but there’s no way to prove it. “After a few hundred million years,” says co-author Jonathan Gagné, a grad student at the University of Montreal, “the stars are so dispersed that we cannot tell to which group they belonged.”

If astronomers had found CFBDSIR2149 just a little bit later (in the cosmic scheme of things, anyway), they might therefore never have figured out that it was a planet at all. As so often happens, science and serendipity joined hands — and new knowledge resulted.

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(An earlier version of this story said that CFBDSIR2149 is 120,000 light years from Earth. it’s in fact 100 to 120 light years away.)