By The Washington Post

Published: Monday, November 19, 2012, 5:34 p.m.
Updated: Monday, November 19, 2012

There's an orphan planet roaming our galactic neighborhood.

It's a globe of gas about the size of Jupiter, astronomers say. And it's out there by its lonesome, untethered to any star, drifting about 100 light-years from Earth. (In astronomical terms, that's close.) Astronomers have spied lonely planets before. But this newest object, spotted near the southern constellation Dorado, is the closest to Earth yet found.

Unobscured by starlight, the new planet — it has no name, just a catalog number — provides a perfect opportunity for astronomers to learn about the mysterious class of “substellar objects.” Such rogue bodies might number in the billions in our galaxy alone.

“There could really be a lot of them,” said Christian Veillet, former director of the Canada-France-Hawaii Telescope, who studied the lonely planet. “But it's a big challenge in terms of observing them.”

That's because these drifting bodies are dark. With no home star, they reflect no starlight, nor do they generate any. But, like an iron pulled from a fire, the youngest of these objects still glow with the heat of their creation.

In 2009, astronomers in Hawaii spied such a heat signal with an infrared camera. A team at the Paranal Observatory in Chile then swung a big telescope around to take a peek.

They detected a planetlike object, estimated to be as big around as Jupiter but perhaps four to seven times as huge. Instruments sensed ammonia, methane and water vapor in the object's atmosphere — typical of Saturn, Jupiter, Neptune and Uranus, the gas-giant planets in our own solar system.

By watching the object's motion, astronomer Jonathan Gagne of the University of Montreal concluded that the planet is probably part of the AB Doradus group, a loose collection of 30 stars that formed from the same cloud of galactic gas. That cloud must have broken off a small puff that coalesced into the lonely planet, Gagne and his colleagues surmise.

Connecting the planet to the star group was key to determining that it is young, just 50 million to 120 million years old, Gagne said. (Our solar system, in contrast, is 4.5 billion years old.) Its youth, in turn, was crucial for classifying the object as a planetlike body rather than a brown dwarf, an object almost large enough to ignite and become a star. To be a brown dwarf, the object would have to be much older, Veillet said.

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