Orphan orb is closest object of its kind to Earth

By Tanya Lewis

Web edition: November 15, 2012
Print edition: December 15, 2012; Vol.182 #12 (p. 8)

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Rogue planet

At less than 160 light-years from Earth, the free-floating planet CFBDSIR2149 (artist’s illustration shown) is the nearest planet-like object found not orbiting a star. The planet appears blue in near-infrared wavelengths.

Credit: ESO/L. Calçada/P. Delorme/N. Risinger (skysurvey.org)/R. Saito/VVV Consortium

Not all planets are content to dutifully circle a star. A new rogue planet has been spied roaming free among a pack of young stars about 115 to 160 light-years from Earth.

It’s not a planet in the conventional sense, because it doesn’t orbit a star. Yet it’s between four and seven times the mass of Jupiter, well within planetary size range. The object appears to be a young, cold planet in a cluster of about 30 stars moving together called AB Doradus, astronomers report in the December Astronomy & Astrophysics. The free-floating planet is the closest to Earth yet discovered, scientists say.

“It’s quite a nice discovery — probably the clearest example of a planetary mass object that’s very young like this,” says astrophysicist Philip Lucas of the University of Hertfordshire in England, who was not involved with the study.

Other potential free-floating planets have been detected before, but their ages weren’t as well known. Astronomers couldn’t be sure the objects were planets and not brown dwarfs, failed stars too small to sustain fusion reactions in their cores.

The newfound object, dubbed CFBDSIR2149, lies in the southern constellation Dorado. Scientists estimate the planet is between 20 million and 200 million years old, based on the assumption that it was formed around the same time as the stars that it accompanies. Compared with the sun, which is about 4.6 billion years old, “it’s like a 1-year-old baby versus a 45-year-old man,” says astrophysicist Philippe Delorme of the Institute of Planetology and Astrophysics of Grenoble, France, who led the research team. Knowing the planet’s age helps pin down its approximate mass.

Delorme and his team were originally looking for brown dwarfs. Using the Canada France Hawaii Telescope on Mauna Kea, Hawaii, the scientists noticed a strange object with an unusual color and measured its spectrum of light — a rainbow of colors that hints at chemical makeup — which gave the first hints it might be a planet. The team confirmed its observations using a European Southern Observatory telescope in Chile. Comparing the spectrum with atmospheric models suggested the planet’s temperature is about 430° Celsius.

Researchers compared the object’s speed and trajectory with those of several moving groups of stars. They showed with 87 percent probability that the planet belongs to the AB Doradus group. Still, more measurements are needed to confirm this, says astrophysicist Ralf-Dieter Scholz of the Leibniz Institute for Astrophysics in Potsdam, Germany.

The first free-floating planets were discovered about 10 years ago in the constellation Orion. A couple of dozen have been found since, but scientists don’t know how common these planetary drifters are.

How they form also remains a mystery. They might have been planets that were kicked out of orbit around their star, or they could have formed more like brown dwarfs, from collapsing clouds of gas and dust.

Studying rogue planets might help astronomers learn more about planets in other star systems, which are much harder to image because of the starlight, says Delorme. “To me, this planet is a prototype of exoplanets we are looking for around stars.”