In an unprecedented discovery, a team of Canadian, U.S. and British astronomers have used telescopes atop the summit of a dormant Hawaiian volcano to capture images of three giant planets orbiting a star known as HR 8799. Comparisons of images obtained in different years show that the three planets, each roughly ten times the mass of Jupiter, are all moving with and orbiting around the star, proving that they are associated with it and are part of a solar system.
The research team, led by astronomer Dr. Christian Marois of the National Research Council Canada/Herzberg Institute of Astrophysics, used advanced instrumentation and image-processing techniques to detect the three faint planets against the bright glare of their host star. The images were captured by the Gemini North and Keck telescopes at the Mauna Kea Observatories in Hawaii.
"It's amazing to have a picture showing not one but three planets," said Dr. Bruce Macintosh, a scientist at the Lawrence Livermore National Laboratory and a project collaborator. "The discovery of the HR 8799 system is a crucial step on the road to the ultimate imaging of another Earth."
The findings will be published online on November 13, 2008 in Science, an international weekly science journal.
The primary star, barely visible to the naked eye, lies 130 light years from Earth in the constellation of Pegasus. Its mass is about 1.5 times that of the Sun and its age is about 60 million years, significantly below that of the Sun. Infrared observations by satellites have shown evidence of a massive disk of cold dust orbiting the star.
Ben Zuckerman, a UCLA professor of physics and astronomy and a co-author of the paper, has been studying dust disks orbiting nearby stars for decades. A similar dust disk exists in our solar system, produced by dust from the comets of the Kuiper Belt located just beyond the orbit of Neptune.
"HR 8799's dust disk stands out as one of the most massive in orbit around any star within 300 light years of Earth," says Zuckerman.
Unlike stars which sustain fusion of hydrogen in their cores, planets do not have any internal source of energy to maintain their temperature. As a result, they are slowly cooling down and becoming less luminous with time. Study co-author Travis Barman, an astronomer at Lowell Observatory in Arizona, notes that "knowledge of the age of HR 8799 is critical for linking the observed luminosities of the planets with their masses," since a planet of given mass would have different luminosities at different ages. This means that at a given age, more massive planets are more luminous than less massive ones. For the HR 8799 system, the observed luminosities imply that the three planets have masses between five to thirteen times that of Jupiter, given the age of 60 million years mentioned above.
"Detailed comparison with theoretical models confirms that all three planets possess complex atmospheres with dusty clouds partially trapping and re-radiating the escaping heat," says Barman.
In some ways, the HR 8799 planetary system seems to be a scaled-up version of our solar system, with more massive planets in orbit around a larger and brighter star. These giant planets orbit relatively far away from their star, at roughly 25, 40 and 70 times the distance between the Earth and the Sun.
More than two hundred planets in other solar systems have been detected in the past decade through indirect studies of their gravitational tug on their parent star. This indirect technique only measures the mass and orbit of the planet. Direct imaging offers many more possibilities of study, including the ability to detect planets at wider separations from their stars, as was in the case of this study.
"By making an image that shows the planet directly, we can study its properties in detail – measure its temperature and composition and try to understand its atmospheric structure," said Bruce Macintosh.
Dr. David Lafrenière, a postdoctoral fellow at the University of Toronto and member of the discovery team, says the pioneering work of Dr. Marois and colleagues in exoplanet imaging over the past couple of years has finally paid off.
"This is truly an amazing discovery!" he says. "Surely astronomers from all around the world will be very busy over the next few years to figure out the most intricate details of this fascinating system."
Christian Marois – NRC Herzberg Institute of Astrophysics, Victoria, BC
Bruce Macintosh – Lawrence Livermore National Laboratory, Livermore, CA, USA
Travis Barman – Lowell Observatory, Flagstaff, AZ, USA
Benjamin Zuckerman – Astronomy Department, University of California, Los Angeles, CA, USA
Jennifer Patience – School of Physics, University of Exeter, Exeter, UK
Inseok Song – University of Georgia, Athens, GA, USA
David Lafrenière – Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON
René Doyon – Département de Physique and Observatoire du Mont Mégantic, Université de Montréal, Montréal, QC
Artwork of the system is available at the following URL: www.gemini.edu/threeplanetart
Image caption for the picture of the planets:
Picture of the HR 8799 planetary system captured with the Keck telescope at near-infrared wavelengths. The three planets are seen as the small red dots at approximately 1 o'clock, 5 o'clock and 10 o'clock, in the clockwise direction. The orbital separation of the planets are 24, 38 and 68 times the Earth-Sun distance, and their masses are 5-13 times that of Jupiter. The noise at the centre of the image is left over light from the bright central star and the advanced image processing applied.
Image caption for the artwork image:
Artist's conception of the multiple planet system HR 8799, initially discovered by Gemini North adaptive optics images and confirmed with W.M. Keck Observatory imaging. Credit: "Gemini Observatory Artwork by Lynette Cook"
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