HR8799: Imaging a System of Exoplanets
The HR 8799 planetary system is the most intriguing and spectacular system yet discovered by direct imaging. With four gas giant planets (5–7 MJup) orbiting at wide separations (15–70 AU) from an unusual, young A star, HR 8799 serves as a Rosetta Stone for atmospheric and planet formation physics. With direct access to the light from the planets themselves, extensive photometric and spectroscopic studies of the system have been undertaken, painting a fascinating picture of cloud formation, nonequilibrium chemistry, and potential elemental abundance measurements that can constrain formation pathways. The dynamical structure of the system is complex, with the four massive planets likely stabilized through participation in mean-motion resonances. Two belts of debris flank the planets, with a warm belt analogous to the solar system asteroid belt and a wide, cold ring comparable to the Kuiper belt. HR 8799 will continue to be studied extensively from the ground and space in the coming years, with the exciting possibility that additional planets remain to be discovered in the system.
The authors wish to acknowledge Christian Marois, whose persistence made HR 8799 the system that keeps on giving.
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