摘要：HAT-P-11 is a mid-K dwarf that hosts one of the first Neptune-sized planets found outside the solar system. The orbit of HAT-P-11b is misaligned with the star’s spin—one of the few known cases of a misaligned planet orbiting a star less massive than the Sun. We find an additional planet in the system based on a decade of precision radial velocity (RV) measurements from Keck/High Resolution Echelle Spectrometer. HAT-P-11c is similar to Jupiter in its mass (MP sin i = 1.6 0.1MJ) and orbital period (P 9.3 0.5 = 1.0 - + year), but has a much more eccentric orbit (e=0.60±0.03). In our joint modeling of RV and stellar activity, we found an activity-induced RV signal of ∼7m s-1, consistent with other active K dwarfs, but significantly smaller than the 31m s-1 reflex motion due to HAT-P-11c. We investigated the dynamical coupling between HAT-P-11b and c as a possible explanation for HAT-P-11b’s misaligned orbit, finding that planet–planet Kozai interactions cannot tilt planet b’s orbit due to general relativistic precession; however, nodal precession operating on million year timescales is a viable mechanism to explain HAT-P-11b’s high obliquity. This leaves open the question of why HAT-P-11c may have such a tilted orbit. At a distance of 38pc, the HAT-P-11 system offers rich opportunities for further exoplanet characterization through astrometry and direct imaging.
关键词：planetary systems;planets and satellites: detection;planets and satellites: dynamical evolution and stability;stars: individual (HAT-P-11)
