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 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 Bruce Macintosh, a professor of physics at Stanford University and a member of the Kavli Institute for Particle Astrophysics and Cosmology. 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 results are published in the current issue of Science.
A clear line of sight
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.
Last year, the GPI was installed on the 8-meter Gemini South Telescope in Chile, and the team set out to look for planets orbiting young stars, identifying nearly 100 so far.
"This is exactly the kind of system we envisioned discovering when we designed GPI," said James Graham, professor at the University of California, Berkeley, and project scientist for GPI.
"51 Eri is one of the best stars for imaging young planets," said co-author Eric Nielsen, a postdoctoral researcher at Stanford and the SETI Institute. "It's one of the very youngest stars this close to the Sun. 51 Eri was born 20 million years ago, 40 million years after the dinosaurs died out."
As far as the cosmic clock is concerned, 20 million years is young, and that is exactly what made the direct detection of the planet possible. When planets coalesce, material falling into the planet releases energy and heats it up. Over the next hundred million years the planet radiates that energy away, mostly as infrared light.
Once the astronomers zeroed in on the star, they blocked its light and spotted light reflecting off 51 Eridani b, orbiting a little farther away from its parent star than Saturn does from the sun. The light from the planet is very faint - more than 3 million times fainter than its star - but GPI can see it clearly. Observations revealed that it is roughly twice the mass of Jupiter, half or less the mass of the young planets discovered to date.
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.
"Many of the exoplanets astronomers have imaged before have atmospheres that look like very cool stars," said Macintosh, who led the construction of GPI and now leads the planet-hunting survey. "This one looks like a planet."
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."
The key to the solar system?
In addition to expanding the universe of known planets, GPI will provide key clues as to how solar systems form. 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."
There are hundreds of planets a little bigger than Earth out there, Macintosh said, but there is so far no way to know if most of them are really "super-Earths" or just micro-sized gas and ice planets like Neptune, or something different altogether. Using GPI to study more young solar systems such as 51 Eridani, he said, will help astronomers understand the formation of our neighbor planets, and how common that planet-forming mechanism is throughout the universe.