Rarity of gas giant planets orbiting far from their host stars

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The first of a series of papers detailing the achievements of the Gemini/NICI direct imaging planet-finding campaign has revealed a paucity of giant planets in distant (greater than about 10 astronomical units) orbits around their host stars. The team surveyed a large number of A and B stars, finding that analogues to the HR 8799 system, whose planets lie very far from the host star, are rarities. The campaign would have been able to image many planets on orbits larger than that of Neptune around the Sun if they existed but these expected planets were not observed. While it seems giant planets do exist preferentially close to parent stars, rocky planets may also follow the same trend: the inset shows an impression of the magma exoplanet UCF 1.01, which is perhaps itself similar to alpha Centauri Bb. Tiny transiting systems around M dwarfs and solar-type stars have been found by Kepler, while these new results show that truly huge planetary systems are unusual among a sample of local nearby stars more massive than the Sun. On the gas giant findings:

Eric Nielsen of the University of Hawaii, who leads a new paper about the (NICI) 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,” he points out. “We found that this lack of gas-giant planets in more distant orbits is typical for nearby stars over a wide range of masses.”

Two additional papers from the campaign will be published soon and reveal similar tendencies around other classes of stars. However, not all gas-giant exoplanets snuggle so close to home. In 2008, astronomers using the Gemini North telescope and W.M. Keck Observatory on Hawaii’s Mauna Kea took the first-ever direct images of a family of planets around the star HR 8799, finding gas-giant planets at large orbital separations (about 25-70 times the Earth-Sun distance). This discovery came after examining only a few stars, suggesting such large-separation gas giants could be common. The latest Gemini results, from a much more extensive imaging search, show that gas-giant planets at such distances are in fact uncommon.

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Liu sums up the situation 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.”

The paper is Eric Nielsen et al., “The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars”, in press at ApJ (arXiv preprint).

Images: top, wired.com (image) below, Zeit News (zeitnews.org).

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