A team of Canadian, American and British astronomers used the Keck and Gemini North telescopes on Mauna Kea in Hawai'i to take infrared images of the young, massive star HR 8799. Using advanced instrumentation and image processing, they have discovered three faint objects close to that star. Comparison of images from different years show that the three are all moving with and orbiting around the star, proving that they are associated with it rather than just being unrelated background objects coincidentally-aligned in the image.
HR 8799 is located about 130 light years from Earth in the constellation of Pegasus. The star is faintly visible to the naked eye, but only to those who live well away from bright city lights.
The planets are young enough that they are still glowing from heat released when they formed, estimated to be sixty million years ago. Analysis of the brightness of the objects at multiple wavelengths show that these objects are about six, nine and eleven times the mass of Jupiter. As in our solar system, these giant planets orbit in the outer regions - roughly 25, 40 and 70 times the Earth-sun separation. The furthest planet orbits just inside a disk of dusty debris, similar to that produced by the comets of the Kuiper Belt of our solar system (just beyond the orbit of Neptune at 30 times the Earth-Sun distance). In some ways, this planetary system seems to be a scaled-up version of our solar system orbiting a larger and brighter star.
"For the first time, we have detected and confirmed by direct imaging observations actual planets orbiting a star other than the Sun," said Christian Marois, a Research Associate at the NRC Herzberg Institute of Astrophysics and lead author of an article published in Science. "We have known for a decade through indirect techniques that the Sun was not the only star having planets in orbit around it, but we finally have an actual image of an entire system. This is a milestone in the search and characterization of planetary systems around stars."
More than two hundred exoplanets have been detected in the past decade, but almost all are known through indirect studies of their gravitational tug on their parent star. The indirect technique only measures the mass and orbit of the planet. "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 Dr. Bruce Macintosh, a scientist at the Lawrence Livermore National Laboratory who has been co-leading the attempts to image an extrasolar planet for almost a decade.
Up to now, only a few brown dwarfs (object of mass in between stars and planets, also known as failed stars) and some ambiguous objects near or above the brown dwarfs/planets boundary have been detected by direct imaging. All of these are orbiting at large distances from their star and are not believed to have formed the same way the planets of our solar system did.
The Keck and Gemini observations were possible due to the high angular resolution achievable with adaptive optics systems that significantly reduces the blur caused by Earth's atmosphere. "Adaptive optics allows the large ground-based telescopes to match or exceed the performance of the Hubble Space Telescope," said Dr. Macintosh. In addition, the detection of all three planets were made possible by an advanced observation strategy (called Angular Differential Imaging) that helps to separate the light of the host star to reveal the faint planets.
The parent star HR 8799 has about 1.5 times the mass of the Sun and is 5 times more luminous but is significantly younger. Infrared observations by satellites have shown evidence for a massive disk of cold dust orbiting the star. According to Ben Zuckerman, (a UCLA professor of physics and astronomy and a co-author on the paper), who has been studying dust disks orbiting nearby stars for decades, "HR 8799's dust disk stands out as one of the most massive in orbit around any star within 300 light years of Earth."
Analyis of available data indicates that the star is about 60 million years old. "No one technique can yield a precise age for the star and its planets, so our age estimate is based on three independent lines of evidence that involve the spectrum and luminosity of HR 8799 and its motion in our Milky Way galaxy," Zuckerman noted. Planets do not have any internal source of energy; they are thus slowly cooling down and becoming less luminous with time. Co-author Travis Barman, an astronomer at Lowell Observatory, commented that "knowledge of the age of HR8799 is critical for linking the observed luminosities of the planets with their masses. The younger/older are the planets, the smaller/larger would be their masses." The derived luminosities show that the three companions are massive planets. "Detailed comparison with theoretical model atmospheres confirms that all three planets possess complex atmospheres with dusty clouds partially trapping and re-radiating the escaping heat," Barman remarked.
More information (Keck Observatory)
More information (Gemini Observatory)