Faint ‘young Jupiter’ spotted with methane atmosphere


A newly discovered exoplanet that weighs just twice as much as Jupiter is believed to be the lowest-mass exoplanet ever directly imaged using a space telescope instrument, according to a new study published in the latest edition of the journal Science.

Known as 51 Eridani b, this new world is the first exoplanet to be discovered using the Gemini Planet Imager (GPI), a new imaging instrument perched on top of the Gemini South Telescope in Chile that was initially deployed in 2013. It is classified as a young Jupiter, is a million times less bright than its central star, and has a rather unique atmosphere, the study explains.

“Of all the directly imaged planets so far, this is the first one where we’ve gotten a spectrum that shows methane,” Bruce Macintosh, the head of the GPI team, a professor of physics at Stanford University and a member of the Kavli Institute for Particle Astrophysics and Cosmology, said to redOrbit via email. “Methane is very common in Jupiter’s atmosphere (and all other giant planets seen so far), but has been really hard to see in extrasolar planets.”

Why has it been so difficult to see methane in exoplanets?

Macintosh believes it is likely because they tend to be either “too hot” or “too cloudy”. Conversely, 51 Eridani b “has cooled off enough that the clouds are breaking up and the methane is stable. It’s exactly what we were looking for.”

Furthermore, the faint nature of the planet, combined with its relatively close proximity to the star it orbits, makes it “the most Jupiter-like planet ever imaged – except, of course, that it’s so young, which is why we can see it,” he added. The new planet is believed to be just 20 million years old – “still warm from energy released when it formed,” the Stanford professor noted.

Unlike Kepler, GPI can directly detect exoplanets

As mentioned above, 51 Eridani b is the first exoplanet discovered by the Gemini Planet Imager, an instrument designed to find and analyze faint young planets that orbit around bright stars. While NASA’s planet-hunting Kepler mission has successfully found thousands of never-before-seen alien worlds, the two tools use different techniques to find new planets.

Kepler searches for a loss of starlight as a planet transits, or passes in front of, its central star. GPI, on the other hand, was designed to search directly for light from the planet itself. Kepler looks for the shadow of a planet, while GPI seeks out their glow, using a process called direct imaging to find planets of lower mass with a closer proximity to their stars.

“GPI was designed from the beginning just to see these planets,” Macintosh said. “We have special masks to block the light from the star and let us separate the planet from the star, and a very advanced adaptive optics system that can do a extremely good job fixing the turbulence in the earth’s atmosphere and the slight imperfections in the telescope that would hide a planet.”

“The planet light is fed into a spectrograph specifically designed for analyzing what we see in planets, so it’s nice to see it doing what it’s supposed to do,” he said, adding that he and his colleagues were currently looking at 600 young stars near the sun in the hopes that they will be able to find and study more planets.

“The big question, of course, is whether earth-like planets are common,” he concluded. “We can’t image those directly, but if we can understand how the giant planets form, that might provide clues to how smaller planets also form, and help us understand if, for example, all the Kepler ‘super-earths’ are really nice rocky planets, or ‘mini-neptunes’, or something else even weirder… it’s very clear that our solar system just isn’t typical in any way.”

(Image credit: Danielle Futselaar and Franck Marchis, SETI Institute)