Gas Giant Exoplanets Close To Parent Stars
June 28, 2013

Gas Giant Exoplanets Tend To Stick Close To Their Parent Stars

redOrbit Staff & Wire Reports - Your Universe Online

Results from the Gemini Observatory's recently completed Planet-Finding Campaign, the most extensive direct imaging survey to date, reveal the outlying orbital space around many types of stars is largely devoid of gas-giant planets, which appear to remain close to their parent stars. The findings could have a significant impact on theories of planetary formation.

"It seems that gas-giant exoplanets are like clinging offspring," said 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."

The results of the Campaign, which was conducted at the Gemini South telescope in Chile, will help scientists better understand how gas-giant planets form, as the orbital distances of planets are a critical signature that astronomers use to test exoplanet formation theories.

Eric Nielsen of the University of Hawaii, author of a paper about the Campaign's search for planets around stars more massive than the Sun, said the findings also 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 said.

"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 due to be published soon will reveal similar tendencies around other classes of stars.

But researchers point out not all gas-giant exoplanets remain so close to home. In 2008, astronomers using the Gemini North telescope and W.M. Keck Observatory on Hawaii's Mauna Kea obtained 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.

The discovery came after examining only a few stars, suggesting such large-separation gas giants could be common. But the current Gemini results involve a much more extensive imaging search, and show that gas-giant planets at such distances are in fact uncommon.

"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," Liu said.

"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 team authored a second new paper that 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 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, lead author on the survey's paper on dusty disk stars.

Finally, the third new paper from the team examines the very youngest stars close to Earth.

"A younger system should have brighter, easier to detect planets," said 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 soon-to-be-delivered Gemini Planet Imager will begin to bridge this gap, and will likely reveal for the first time how common giant planets are in orbits similar to the gas-giant planets of our own Solar System.

The observations for the Planet-Finding Campaign were obtained with the Gemini instrument known as NICI, the Near-Infrared Coronagraphic Imager, which was the first instrument for an 8-10 meter-class telescope designed specifically for finding faint companions around bright stars.