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Gemini imager finds its first young Jupiter-like planet

Kimberly Cartier, Exoplanet Correspondent
Aug 15, 2015, 16:39 UTC

Sen—The Gemini Planet Imager Exoplanet Survey has discovered a young Jovian planet orbiting in the triple star system 51 Eridani. The planet, 51 Eri b, was discovered and observed using the new Gemini Planet Imager on the Gemini South telescope in Cerro Pachon, Chile. Lying in the constellation of Eridanus, the River, it is the first exoplanet discovered by the Gemini Planet Imager Exoplanet Survey.

“51 Eri b is the first [exoplanet] that’s cold enough and close enough to the star that it could have indeed formed right where it is the ‘old-fashioned way’,” says Bruce Macintosh, lead author on the discovery, in a statement. “This planet really could have formed the same way Jupiter did—this whole planetary system could be a lot like ours.”

The Gemini Planet Imager (GPI) directly observes exoplanets in near-infrared wavelengths by first blocking out the starlight using an advanced type of shield called a coronagraph. The coronagraph is designed to block out as much of the starlight as possible from the inner region around the star, leaving light from other nearby sources intact. 

Using the coronagraph coupled with adaptive optics to correct for the effects of our atmosphere, the GPI team was able directly to detect light from 51 Eri b. The planet emits infrared light one million times fainter than the host star and is only visible once the light from the host star is obscured by the coronagraph.

GPI, like all direct imaging instruments, is most sensitive to planets that orbit farther away from their stars and those are have hot effective temperatures. At larger planetary orbital distances, the coronagraph is better able to remove the starlight from the area around the planet. Likewise, hotter planets emit more infrared light and are more easily seen against any residual starlight. Since planets cool rapidly as they age, GPI is most sensitive to young, hot planets at large orbital distances.

51 Eri b is only 20 million years old, and orbits nearly 13 astronomical units (AU) away from the primary star in the system, 51 Eri A (one AU is the approximate distance of the Earth from the Sun). 51 Eri A is seven times brighter and about twice as massive as the Sun, giving exoplanet 51 Eri b an effective temperature between 600 and 750 Kelvin.

The 51 Eri system also contains two distant companion stars—the compact binary M-dwarf system GJ 3305AB orbiting 2000 astronomical units away from 51 Eri A and b. The four component system  is approximately 95 light-years from Earth.

GPI also observed the spectrum of 51 Eri b's atmosphere to measure the composition and try to determine whether or not the exoplanet has clouds. The team found that the planet's atmosphere is dominated by methane and that the exoplanet might have clouds. Jupiter, also, has a methane-rich, cloudy atmosphere, which means that 51 Eri b could, indeed, very closely resemble our own Jovian planet.

Future observations of 51 Eri b will reveal more about the planet's orbital characteristics and better determine the planet's mass. 51 Eri b serves as “a bridge from wider-orbit, hotter and more massive planets to Jupiter-like scales.”