Gemini Planet Imager analyzes 300 stars

IMAGE: The Gemini Planet Imager is located at Gemini South Observatory in Cerro Pachón, Chile.

Image credit: 
Marshall Perrin



"Over the past twenty years, astronomers have discovered all of these solar systems that are really different from our own," said Bruce Macintosh, professor of physics at Stanford in the School of Humanities and Sciences. "The question that we want to understand ultimately is: Are there life-bearing, Earth-like planets out there? And one way to answer that is by understanding how other solar systems form."

Unlike other planet-hunting techniques, which rely on looking for signs of a planet - like the effect of its gravity on the parent star - rather than the planet itself, the Gemini Planet Imager takes direct images, picking the faint planet out of the glare of a star a million times brighter.

"The giant planets in our own solar system live between five and 30 times Earth's orbital distance, and for the first time we're probing a similar region around other stars," said Eric Nielsen, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology and lead author of the paper. "It's pretty exciting to be able to start to put together a census of the planets larger than Jupiter in the outer solar systems of some of our neighboring stars."

Maybe a special system


In the first half of the survey, the Gemini Planet Imager found fewer exoplanets than the researchers expected. However, the exoplanets they did see contributed to one of their strongest results: every one of the six planets orbited a large, bright star - despite the fact that planets are easier to see near faint stars. This shows conclusively that wide-orbiting giant planets are more common around high mass stars, at least 1.5 times more massive than the sun. Meanwhile, for sun-like stars, Jupiter's larger cousins are much rarer than the small planets discovered close to their star by missions like NASA's Kepler.

"Given what we and other surveys have seen so far, our solar system doesn't look like other solar systems," Macintosh said. "We don't have as many planets packed in as close to the sun as they do to their stars and we now have tentative evidence that another way in which we might be rare is having these kind of Jupiter-and-up planets."


One other result from the first half of the survey is that brown dwarfs - objects larger than planets but smaller than stars - are a very distinct population from planets. This may point to a different formation mechanism for this class of objects, suggesting that brown dwarfs are more similar to failed stars than super-size planets.

Combined with other techniques, this paper pinpoints a distance from a star at which the number of giant planets goes from increasing to decreasing - at about five to 10 astronomical units (one astronomical unit is the distance from the sun to Earth).

"The region in the middle could be where you're most likely to find planets larger than Jupiter around other stars," Nielsen added, "which is very interesting since this is where we see Jupiter and Saturn in our own solar system."

All three of the main findings support the hypothesis that giant planets likely form "bottom up" by accumulation of particles around a solid core, whereas brown dwarfs likely form "top down" as a result of huge gravitational instabilities in the disk of gas and dust from which a solar system develops.

Working their way to Earth


"Right now, we see these planets as fuzzy, red blobs. Someday, it's going to be a fuzzy blue blob. And that little, tiny, fuzzy, blue blob is going to be an Earth," Macintosh said. "Getting to Earths will take a space mission that's probably about 20 years away. But when it flies, it'll use a spectrograph like the one we built and deformable mirrors like what we have and software with lines of code that we've written."


"I helped take the first GPIES planet search images four and a half years ago," said Robert De Rosa, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology and co-author of the paper, who spent many nights observing with the Gemini Planet Imager in Chile and remotely from Stanford. "It's bittersweet to see it draw to a close."

Stanford University