Gas-Giant Exoplanets Generally Stay Close To Their Parent Stars, Research Finds

Gas-giant planets are far more likely to orbit very closely to their parent star than they are to orbit at distances farther away, according to new research made possible by the Gemini Observatory’s Planet-Finding Campaign.

Image Credit: Gemini Observatory/AURA
Image Credit: Gemini Observatory/AURA

“It seems that gas-giant exoplanets are like clinging offspring,” says Michael Liu of the University of Hawaii’s Institute for Astronomy and leader of the Gemini Planet-Finding 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.”

Eric Nielsen of the University of Hawaii, notes that the new findings have important implications for the field. “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.”

There are of course e captions to this trend though — back in 2008 researchers using the Gemini North telescope and WM Keck Observatory in Hawaii “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 new research has shown though that such large distances between gas-giant planets and their parent star are atypical, and perhaps rather uncommon.

Liu words it 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.”

In related research, the team also investigated systems “where dust disks around young stars show holes, which astronomers have long suspected are cleared by the gravitational force of orbiting planets.”

“It makes sense that where you see debris cleared away that a planet would be responsible, but we did not know what types of planets might be causing this. It appears that instead of massive planets, smaller planets that we can’t detect directly could be responsible,” said Zahed Wahhaj of the European Southern Observatory.

“A younger system should have brighter, easier to detect planets,” according to the lead author Beth Biller of the Max Planck Institute for Astronomy. “Around other stars, NASA’s Kepler telescope has shown that planets larger than the Earth and within the orbit of Mercury are plentiful. The NICI Campaign demonstrates that gas-giant planets beyond the distance of the orbit of Neptune are rare.”

The two new studies are both set to be published in The Astrophysical Journal.

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