Two "b"s in the Beehive: The Discovery of the First Hot Jupiters in an Open Cluster

We report the discovery of two giant planets orbiting stars in Praesepe (also known as the Beehive Cluster). These are the first known hot Jupiters in an open cluster and the only planets known to orbit Sun-like, main-sequence stars in a cluster. The planets are detected from Doppler-shifted radial velocities; line bisector spans and activity indices show no correlation with orbital phase, confirming the variations are caused by planetary companions. Pr0201b orbits a V = 10.52 late F dwarf with a period of 4.4264 ± 0.0070 days and has a minimum mass of 0.540 ± 0.039 M _Jup, and Pr0211b orbits a V = 12.06 late G dwarf with a period of 2.1451 ± 0.0012 days and has a minimum mass of 1.844 ± 0.064 M _Jup. The detection of two planets among 53 single members surveyed establishes a lower limit of 3.8^+5.0 _{- 2.4}% on the hot Jupiter frequency in this metal-rich open cluster. Given the precisely known age of the cluster, this discovery also demonstrates that, in at least two cases, giant planet migration occurred within 600 Myr after formation. As we endeavor to learn more about the frequency and formation history of planets, environments with well-determined properties—such as open clusters like Praesepe—may provide essential clues to this end.