The Gemini Planet Imager — a special planet-hunting instrument mounted on the Gemini South Telescope in Chile — has captured an image of its very first exoplanet, or a planet outside our Solar System. The details are published this week in the journal Science, marking one of the few times an exoplanet has been directly imaged from the surface of Earth. Dubbed 51 Eri B, the space rock is a large gas giant about twice the mass of Jupiter, with lots of water vapor and methane swirling around in its atmosphere. Learning more about this gassy planet could provide researchers with clues about how planets form throughout the Universe.
Imaging an exoplanet directly like this is incredibly difficult; a planet's host star is usually so luminous that it often washes out all nearby objects. That's why most exoplanet imagers, such as NASA's Kepler spacecraft, use indirect methods for locating planets outside our Solar System. These include measuring things like a star's periodic dimming — when a star temporarily dims as a planet passes in front of it.
One of the few times an exoplanet has been directly imaged from the surface of Earth
Yet indirect imaging provides very limited information about an exoplanet, only revealing things like size, mass, and location. Direct imaging allows scientists to get a much more comprehensive view. "We actually see the planet as a distinct dot and measure its properties, like its atmospheric spectrum, or its temperature," study author Bruce Macintosh, a professor of physics at Stanford University, tells The Verge. "It doesn’t let us count dozens of planets like Kepler, but we can study smaller planets in great detail."
The Gemini Planet Imager was specifically designed to image large gas giants like 51 Eri B straight on. It works by using a combination of corrective optics and specialized mirrors. The optics help to filter out the various lights in Earth's atmosphere that might interfere with the data. The researchers then focus the light from a far away star onto a mirror with a hole in it. The star's light goes through the hole, allowing the researchers to see any large planets that might be nearby. "The planet is a little off to the side of the star. It's just off to the side so it doesn’t fall down that hole, and its light bounces off the mirror," says Macintosh.
The Gemini Observatory is located in the mountains of La Serena, Chile.
Despite all this special hardware, the exoplanets still have to be big and hot so that they give off enough infrared light to be seen. Macintosh says 51 Eri B is the perfect example of the kind of gas giants the research team is looking for. The exoplanet orbits a star that's relatively close — only 100 light-years from Earth — and fairly young — just 20 million years old; our Solar System dates back to 4.5 billion years ago.
This star's youth means Gemini captured images of the exoplanet in the first stages of its formation. Additionally, its size and chemical properties make it the most Jupiter-like planet that's ever been discovered elsewhere in the galaxy.
"51 Eri B, with a mass only slightly larger than Jupiter’s, gives humanity a view of our own Jupiter when it was just 20 million years old," says Geoff Marcy, an astronomer at the University of California-Berkeley, who was not involved in the study. "What did Jupiter look like when it was being born? For the first time, we have direct pictures and spectra to tell us."
Macintosh says studying this early formation of a gas giant can give us details about the types of conditions that are needed for different planets to form. "As we study that more, we can try to say this is how you form giant planets, and in turn, this is how you could form an Earth-like planet," says Macintosh. 51 Eri B could help tell us if our planet is rare or more common than we thought.
An image of the exoplanet 51 Eridani b was taken by the Gemini Planet Imager in the near-infrared on Dec. 18 2014. (J. Rameau and C. Marois)