One of the best ways to learn how our solar system evolved is to look to younger star systems in the early stages of development. Now, a team of astronomers has discovered a Jupiter-like planet within a young system that could serve as a decoder ring for understanding how planets formed around our sun. The first planet detected by the Gemini Planet Imager is 100 light-years away but shares many of the characteristics of an early Jupiter.
The new planet, called 51 Eridani b, is the first exoplanet discovered by the Gemini Planet Imager, a new instrument operated by an international collaboration headed by  Professor Bruce Macintosh. It is a million times fainter than its parent star and shows the strongest methane signature ever detected on an alien planet, which should yield additional clues as to how the planet formed.
The Gemini Planet Imager (GPI) was designed specifically for discovering and analyzing faint, young planets orbiting bright stars. While NASA’s Kepler space observatory has discovered thousands of planets, it does so indirectly by detecting a loss of starlight as a planet passes in front of its star. GPI instead searches for light from the planet itself.
“To detect planets, Kepler sees their shadow,” said Macintosh. “The Gemini Planet Imager instead sees their glow, which we refer to as direct imaging.” The astronomers use adaptive optics to sharpen the image of a star, and then block out the starlight. Any remaining incoming light is then analyzed, with the brightest spots indicating a possible planet.
In addition to being the lowest-mass planet ever imaged, it’s also one of the coldest — 800 degrees Fahrenheit, whereas others are around 1,200 F — and features the strongest atmospheric methane signal on record. Previous Jupiter-like exoplanets have shown only faint traces of methane, far different from the heavy methane atmospheres of the gas giants in our solar system.
All of these characteristics, the researchers say, point to a planet that is very much what models suggest Jupiter was like in its infancy.
Of course, it’s not exactly like Jupiter — its 800 F temperature is still hot enough to melt lead — but there are signs it will evolve into a familiar shape. “In the atmospheres of the cold giant planets of our solar system, carbon is found as methane, unlike most exoplanets, where carbon has mostly been found in the form of carbon monoxide,” said Mark Marley, an astrophysicist at NASA Ames Research Center. “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.”
Astronomers believe that the gas giants in our solar system formed by building up a large core over a few million years and then pulling in a huge amount of hydrogen and other gases to form an atmosphere.
But the Jupiter-like exoplanets that have so far been discovered are much hotter than models have predicted, hinting that they could have formed much faster as material collapses quickly to make a very hot planet. This is an important difference. The core-buildup process can also form rocky planets like Earth; a fast and hot collapse might only make giant gassy planets. 51 Eridani b is young enough that it “remembers” its formation.
“51 Eri b is the first one that’s cold enough and close enough to the star that it could have indeed formed right where it is the ‘old-fashioned way,'” Macintosh said. “This planet really could have formed the same way Jupiter did — the whole solar system could be a lot like ours.” http://news.stanford.edu/pr/2015/pr-exoplanet-macintosh-eridani-081315.html
This is an artistic conception of the Jupiter-like exoplanet 51 Eridani b, with the hot layers deep in its atmosphere glowing through the clouds. Because of its young age, this cousin of our own Jupiter is still hot and carries information on the way it was formed 20 million years ago. Credit: Danielle Futselaar and Franck Marchis, SETI Institute
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