High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets1. Such images were possible because of the wide orbits (>25 astronomical units, where 1 AU is the Earth- Sun distance) and youth (<100 Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. An important area of contention in the exoplanetcommunity is whether outer planets (>10 AU)more massive than Jupiter formby way of one-step gravitational instabilities2 or, rather, through a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium3. Here we report the presence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts4, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (>10 AU) separations.
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