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

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

Research output: Contribution to journalArticlepeer-review

59 Scopus citations


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 J) and orbital period ( 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 , consistent with other active K dwarfs, but significantly smaller than the 31 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.

Original languageEnglish (US)
Article number255
JournalAstronomical Journal
Issue number6
StatePublished - Jun 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • planetary systems
  • planets and satellites: detection
  • planets and satellites: dynamical evolution and stability
  • stars: individual (HAT-P-11)


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