In their quest to understand how our solar system evolved, scientists have been exploring the younger star systems in the early stages of development and they have struck a goldmine when they discovered a Jupiter-like planet that could serve as a decoder ring for understanding how planets formed around our sun.
Called 51 Eridani b, the first exoplanet was discovered by the Gemini Planet Imager, a new global facility headed by Bruce Macintosh at Stanford University, which is a million times fainter than its parent star and shows the strongest methane signature ever detected on an alien planet.
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. The core-buildup process can also form rocky planets like Earth; a fast and hot collapse might only make giant gassy planets. The exoplanet 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.
Highlighting the use of GPI to study more young solar systems such as 51 Eridani, he said, it will help astronomers understand the formation of our neighbor planets, and how common that planet-forming mechanism is throughout the universe.
Unlike NASA’s Kepler space observatory which detects thousands of planets from the loss of starlight as a planet passes in front of its star, the Gemini Planet Imager (GPI) was designed to search 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.”
Precisely envisioned for GPI, the new deisgn is key to find such exoplanets, 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.”
Once the astronomers zeroed in on 51 Eridani b, they blocked its light and spotted light reflecting off the star, 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, he said.
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.”