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Gas-Giant Exoplanets Cling to Their Parent Stars

Artist’s rendering of an exoplanet system with a gas-giant planet orbiting close to a star that is more massive than our Sun. Artwork by Lynette Cook courtesy Gemini Observatory/AURA.

In 2008, astronomers using the Gemini North telescope and W. M. Keck Observatory on Mauna Kea took the first-ever direct images of a family of planets around the star HR 8799. They found gas-giant planets at large orbital separations (about 25–70 times the Earth–Sun distance). This discovery, made after examining only a few stars, suggested such large-separation gas giants could be common. However, more recent Gemini results, from a much more extensive imaging search, show that gas-giant planets at such distances are in fact uncommon. Results from Gemini Observatory’s recently completed Planet-Finding Campaign—the deepest, most extensive direct-imaging survey to date—show that the vast outlying orbital space around many types of stars is largely devoid of gas-giant planets.

“It seems that gas-giant exoplanets are like clinging offspring,” says IfA astronomer Michael Liu, the leader of the campaign. “Most tend to shun orbital zones far from their parents. In our search, we could have found gas giants beyond orbital distances corresponding to Uranus and Neptune in our own solar system, but we didn’t find any.” The campaign was conducted at the Gemini South telescope in Chile, with funding from the National Science Foundation and NASA. The results will help scientists better understand how gas-giant planets form, as the orbital distances of planets are a key signature that astronomers use to test exoplanet formation theories.

IfA postdoctoral researcher Eric Nielsen, the first author on the paper about the campaign’s search for planets around stars more massive than the Sun, adds that the findings have implications beyond the specific stars imaged by the team. “The two largest planets in our solar system, Jupiter and Saturn, are huddled close to our Sun, within 10 times the distance between the Earth and Sun. We found that this lack of gas-giant planets in more distant orbits is typical for nearby stars over a wide range of masses.”

The campaign revealed similar tendencies around other classes of stars. Liu summed it up this way: “We’ve known for nearly 20 years that gas-giant planets exist around other stars, at least orbiting close-in. Thanks to leaps in direct imaging methods, we can now learn how far away planets can typically reside. The answer is that they usually avoid significant areas of real estate around their host stars. The early findings, like HR 8799, probably skewed our perceptions.”

A second paper by the team explores systems where dust disks around young stars show holes, which astronomers have long suspected are cleared by the gravitational force of orbiting planets. “It appears that instead of massive planets, smaller planets that we can’t detect directly could be responsible,” said Zahed Wahhaj (formerly a postdoctoral researcher at IfA and now with the European Southern Observatory), the lead author on the survey’s paper on dusty disk stars.

Finally, the team’s third new paper looks at the very youngest stars close to Earth. “A younger system should have brighter, easier to detect planets,” according to the lead author Beth Biller, a former IfA postdoctoral researcher who is now at the Royal Observatory Edinburgh. “Around other stars, NASA’s Kepler telescope has shown that planets larger than the Earth and within the orbit of Mercury are plentiful,” Biller explained. “The NICI Campaign demonstrates that gas-giant planets beyond the distance of the orbit of Neptune are rare.”

The observations for the campaign were obtained with the Gemini instrument known as NICI, the Near-Infrared Coronagraphic Imager, the first instrument for an 8–10 meter-class telescope designed specifically for finding faint companions around bright stars. Doug Toomey of Mauna Kea Infrared and the IfA’s Christ Ftaclas and Mark Chun built NICI with funding from NASA.

A new instrument, the soon-to-be-delivered Gemini Planet Imager, will likely reveal whether giant planets in orbits similar to the gas-giant planets of our own solar system are common.