An international research team has discovered a Jupiter-like planet 100 light-years away that could help astronomers understand how planets formed in our solar system. Called 51 Eridani b, it is the first planet detected by the Gemini Planet Imager (GPI), a new instrument that started running last year on the Gemini South telescope in Chile. Details about the new exoplanet, the lightest planet ever imaged, are published today by the journal Science.
“This discovery is of paramount importance in human exploration of the range of planets in our universe,” said Rebecca Oppenheimer, an author on the paper and curator and chair of the American Museum of Natural History’s Department of Astrophysics. “It marks a point where indirect detection of planets and directly seeing and analyzing their atmospheres are joining forces. We are no longer simply seeing the ‘tip of the iceberg’ of the broad diversity of planets in the Universe. Perhaps solar systems similar to ours are not uncommon.”
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. 51 Eridani b shares many of the characteristics of an early Jupiter and shows the strongest methane signature ever detected on an alien planet, which should yield additional clues as to how it formed.
GPI was designed specifically for discovering and analyzing faint, young planets orbiting bright stars through what is called “direct imaging.” The instrument uses adaptive optics to sharpen the image of a star and blocks out the starlight with a special device called a coronagraph, designed and built by a team led by Oppenheimer at the Museum. Any remaining incoming light is then analyzed, and the brightest spots indicate a possible planet.
“This is exactly the kind of planet we envisioned discovering when we designed GPI,” said James Graham, a professor at the University of
51 Eridani b is the first distant world GPI has detected out of the almost 100 stars surveyed since the machine started searching in January 2014. The light from the planet is very faint—more than three million times fainter than its star—but GPI can see it clearly. Observations revealed that it is roughly twice the mass of Jupiter. Other directly imaged planets are five times the mass of Jupiter or more.
In addition to being the lowest-mass planet ever imaged, it's also the coldest—700 degrees Fahrenheit, whereas others are around 1,200 Fahrenheit —and features the strongest atmospheric methane signal on record. These characteristics, the researchers say, point to a planet very similar to what it’s believed Jupiter looked like in its infancy.
Astronomers think 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 gasses to form an atmosphere. But the Jupiter-like exoplanets that have been discovered so far 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 the Earth; a fast and hot collapse might only make giant gassy planets. 51 Eridiani b is young enough that it “remembers” its formation.
There are hundreds of planets a little bigger than Earth out there, the researchers say, but there is so far no way to know if they are really "super-Earths" or just micro-sized gas and ice planets like Neptune or something altogether different. Using GPI to study more young solar systems such as 51 Eridani will help astronomers understand the formation of our neighbor planets, and how common that planet-forming mechanism is throughout the universe.