Astronomers Spy a New Planet-Like Object

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Max Planck Institute for Astronomy / National Astronomical Observatory of Japan

The August 2009 discovery image of GJ 758 B, taken with the Subaru telescope's HiCIAO instrument in the near infrared, which measures and records differences in heat

Astronomers have been finding planets around distant stars for more than a decade now, and the count is currently around 400. But the vast majority of these so-called exoplanets have been seen indirectly — by their gravitational effects or by the dimming caused when they pass in front of their parent stars. To really understand what a planet is like in detail, you have to see it directly, and that's incredibly hard to do with today's technology.

But an international team has done it. In May and August, using the powerful Subaru telescope in Hawaii, which is equipped with a new, state-of-the art planet detector, astronomers from Japan, the U.S. and Germany snapped pictures of an object they're calling GJ 758 B orbiting a sunlike star called GJ 758, about 50 light-years from Earth and between the constellations Cygnus and Lyra. Scientists have narrowed their estimate of the mass of GJ 758 B to only about 10 to 40 times the mass of Jupiter. If it were more than 13 Jupiter masses, it would probably be considered a brown dwarf, which is a kind of failed star. "We're calling it a planet-like object rather than a planet," says Michael McElwain, a postdoctoral student at Princeton who co-authored the discovery paper for Astrophysical Journal Letters published in November.

It could be a planet, though even if it isn't, there's plenty of reason to be excited. For one thing, astronomers got an image of it. The reason it's so tough to image a planet is its proximity to the blinding light of its star, which in this case is about a million times brighter. It would be like trying to see a candle burning next to the beam of a million-candlepower searchlight. Further, the blurring caused by Earth's atmosphere makes it tough to separate closely paired objects — such as a star and planet — even when they're of equal brightness.

This discovery is part of a five-year planet-hunting campaign designed to solve those problems with four keys: first, the Subaru telescope, which sits at an elevation of nearly 14,000 ft. on the summit of the extinct volcano Mauna Kea and is by itself one of the world's biggest and sharpest telescopes; second, the adaptive optics with which the telescope is equipped, which largely cancel out atmospheric blurring; third, the telescope's top-notch coronagraph filter, which blots out most starlight to remove the glare. And finally, the whole thing operates in infrared light, a type of light that renders planets especially bright and sunlike stars relatively dim. In short, says McElwain, "We're using state-of-the-art instruments on a state-of-the-art telescope."

Even with all that going for them, the discovery came as a surprise, because the instruments were still being tested out. "We were thrilled to see something so early in the process," says McElwain. What's especially thrilling is that GJ 758 B could be as close to its star as Neptune is to the sun — unusual, considering that astronomers had previously believed that large planets formed closer to or farther from their stars, but not in GJ 758 B's location. In the best-case, most optimistic scenario, McElwain says, we might be able to image a Jupiter-mass object about four times farther out than Jupiter is in our solar system.

That's not as good as the ultimate goal: to take an image of a second Earth, in an Earthlike orbit. For that, though, you really need to go into space. But the discovery of GJ 758 B is an extraordinary step, and if the project lives up to its early promise, astronomers will be learning a whole lot more about distant planets in the next few years.