Of the nearly 2000 exoplanets discovered to date, only about 10 have been seen directly, because they are so faint compared with the bright stars they orbit. Now, an instrument designed for direct imaging has found its first new exoplanet: a Jupiter-like world 100 light-years away in the constellation Eridanus. It is the faintest and least massive exoplanet directly imaged so far, and the first to show an atmosphere rich in methane, similar to the giant planets in our own solar system. “This spectrum really looks like a planet,” says Bruce Macintosh, a physicist at Stanford University in Palo Alto, California, and the principal investigator of the Gemini Planet Imager (GPI), which made the discovery.
The planet, known as 51 Eri b, is quite young—only about 20 million years old. It is roughly twice the mass of Jupiter and orbits more than twice as far from its star than Jupiter does from the sun, the team reports online today in Science. According to Macintosh, astronomers had expected to see strong evidence for methane in the spectra of giant Jupiter-like exoplanets but until now had detected only traces. Many other direct-imaged planets have spectra more like those of small, cool stars, Macintosh says, whereas 51 Eri b shows strong signals for both water vapor and methane. “Since the atmosphere of 51 Eri b is also methane rich, it signifies that this planet is well on its way to becoming a cousin of our own familiar Jupiter," says GPI team member Mark Marley, an astrophysicist at NASA’s Ames Research Center in Mountain View, California. Macintosh agrees. “It’s really the most Jupiter-ish thing ever imaged directly,” he says.
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