As planets form in the swirling gas and dust around the young stars, there seems to be a sweet spot where most of the large, Jupiter-like gas giants congregate centered around the orbit where Jupiter sits today in our own solar system.
The location of this sweet spot is between 3 and 10 times the distance of Earth (3-10 astronomical units, AU). Jupiter is 5.2 AU from our Sun
This is just one of the conclusions of an unprecedented analysis of 300 stars captured by the Gemini Planet Imager, or a GPI, a sensitive infrared detector mounted on the 8-meter Gemini South telescope in Chile.
The GPI Exoplanet Survey, or GPIES, is one of two large projects that searches for exoplanets directly, by blocking stars' light and photographing the planets themselves, instead of looking for telltale wobbles in the star ̵
The analysis of the first 300 of more than 500 stars surveyed by GPIES, published June 12 in the The Astronomical Journal "is a milestone," said Eugene Chiang, and UC Berkeley professor of astronomy and member of the collaboration's theory group. "We now have excellent statistics on how often the planets occur, their mass distribution and how far they are from their stars."
The study complements earlier exoplanet surveys by counting planets between 10 and 100 AU, and the range in which the Kepler Space Telescope transit survey and radial velocity observations are unlikely to detect planets. It was led by Eric Nielsen, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University, and involved more than 100 researchers at 40 institutions worldwide, including the University of California, Berkeley
One new planet, one new brown dwarf
Since the GPIES survey began five years ago, the team has captured six planets and three brown dwarfs orbiting these 300 stars. The team estimates that about 9 percent of the massive stars have gas giants between 5 and 13 Jupiter masses beyond a distance of 10 AU, and less than 1 percent have brown dwarfs between 10 and 100 AU
The new data set provides important insight
"As you go out of the central star, giant planets become more frequent, around 3 to 10 AU, the occurrence rate peaks," Chiang said. "We know the peaks because the Kepler and radial velocity surveys find a rise in the rate going from hot Jupiters near the star to Jupiters at a few AUs from the star." GPI has filled the other end, going from 10 to 100 AU, and finding that the occurrence rate drops, the giant planets are more often found at 10 than 100. If you combine everything, there is a sweet spot for a giant planet occurrence around 3 to 10 AU. "
" With future observatories , especially the Thirty-Meter Telescope and ambitious space-based missions, we will begin imaging the planets residing in the sweet spot for sun-like stars, "said team member Paul Kalas, and UC Berkeley, associate professor of astronomy
Exoplanet survey found only one previously unknown planet – 51 Eridani b, nearly three times the mass of Jupiter – and one previously unknown brown dwarf – HR 2562 B, weighing in about 26 Jupiter masses. None of the giant planets imaged were around sun-like stars. Instead, giant gas planets were discovered only around more massive stars, at least 50 percent larger than our sun, or 1.5 solar masses.
"Given what we and other surveys have seen so far, our solar system does not look like other solar systems, "said Bruce Macintosh, chief investigator for GPI and professor of physics at Stanford. "We do not have as many planets packed in as close to the sun as they do their stars, and we now have some tentative evidence that another way in which we might be rare is having these kind of Jupiter-and-up planets."
"The fact that giant planets are more common around stars is more massive than sun-like stars," Chiang said. means that "the stars you can see in the night sky with your eye are more likely to have Jupiter-mass planets around them than the fainter stars that you need a telescope to see," Kalas said. "That's kinda cool."
The analysis also shows that gas giant planets and brown dwarfs, while seemingly on a continuum of increasing mass, may be two distinct populations formed in different ways. The gas giants, up to about 13 times the mass of Jupiter, appear to have formed by the accumulation of gas and dust on smaller objects – from the bottom up. Brown dwarfs, between 13 and 80 Jupiter masses, formed by stars, by gravitational collapse – from the top down – within the same cloud of gas and dust that gave birth to the stars.
"I think this is the clearest We have these two groups of objects, planets and brown dwarfs, form differently, "Chiang said. "The Gemini Planet Imager can sharply image planets around distant stars, thanks to the extreme adaptive optics, which quickly detects turbulence in the world."
and reduces blurring by adjusting the shape of a flexible mirror. The instrument detects the heat of bodies still glowing from their own internal energy, such as exoplanets that are large, between 2 and 13 times the mass of Jupiter, and young, less than 100 million years old, compared to our sun's age of 4.6 billion years. Even though it blocks most of the light from the central star, the glare still limits GPI to seeing only planets and brown dwarfs away from the stars they orbit, between about 10 and 100 AU
The team plans to analyze data on
Chiang noted that the success of GPIES shows that direct imaging will become increasingly important in the study of exoplanets, especially for understanding their formation.
"Direct imaging is the best way to get at young planets," he said. "When young planets are shaping, their young stars are too active, too jittery, for radial velocity or transit methods to work easily, but with direct imaging, seeing is believing"
Other UC Berkeley team members are postdoctoral fellows Ian Czekala , Gaspard Duchêne, Thomas Esposito, Megan Ansdell and Rebecca Jensen-Clem, professor of astronomy James Graham and undergraduates Jonathan Lin, Meiji Nguyen and Yilun Ma. Other team members include Nielsen, former Berkeley undergraduate, Franck Marchis, and former assistant researcher, and Marshall Perrin, Mike Fitzgerald, Jason Wang, Eve Lee and Lea Hirsch, former graduate students
. Foundation (AST-1518332), National Aeronautics and Space Administration (NNX15AC89G) and Nexus for Exoplanet System Science (NExSS) and Research Coordination Network sponsored by NASA's Science Mission Directorate (NNX15AD95G)