HAT-P-11: Discovery of a Second Planet and a Clue to Understanding Exoplanet Obliquities

June 2018 • 2018AJ....155..255Y

Authors • Yee, Samuel W. • Petigura, Erik A. • Fulton, Benjamin J. • Knutson, Heather A. • Batygin, Konstantin • Bakos, Gáspár Á. • Hartman, Joel D. • Hirsch, Lea A. • Howard, Andrew W. • Isaacson, Howard • Kosiarek, Molly R. • Sinukoff, Evan • Weiss, Lauren M.

Abstract • 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 ({M}_P\sin i=1.6+/- 0.1 M _J) 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 ∼7 {{m}} {{{s}}}^-1, consistent with other active K dwarfs, but significantly smaller than the 31 {{m}} {{{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 38 pc, the HAT-P-11 system offers rich opportunities for further exoplanet characterization through astrometry and direct imaging.

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Benjamin Fulton

Assistant Scientist