Hot Jupiters are gas-giant planets orbiting with periods of 3−9 days around Sun-like stars. They are believed to form in a disk of gas and condensed matter at or beyond 2.7 astronomical units (au—the Sun−Earth distance) from their parent star^{1, 2}. At such distances, there exists a sufficient amount of solid material to produce a core capable of capturing enough gas to form a giant planet. Subsequently, they migrate inward to their present close orbits³. Here I report the detection of an unusual hot Jupiter orbiting the primary star of a triple stellar system, HD 188753. The planet has an orbital period of 3.35 days and a minimum mass of 1.14 times that of Jupiter. The primary star's mass is 1.06 times that of the Sun, 1.06 M. The secondary star, itself a binary stellar system, orbits the primary at an average distance of 12.3 au with an eccentricity of 0.50. The mass of the secondary pair is 1.63 M. Such a close and massive secondary would have truncated a disk around the primary to a radius of only 1.3 au (ref. 4) and might have heated it up to temperatures high enough to prohibit giant-planet formation^{5, 6}, leaving the origin of this planet unclear.

1. California Institute of Technology, Division of Geological & Planetary Sciences MC 150-21, Pasadena, California 91125, USA

Correspondence to: Maciej Konacki¹ Correspondence and requests for materials should be addressed to M.K. (Email: maciej@gps.caltech.edu).

Received 15 April 2005; Accepted 23 May 2005

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