@article{03309c38baa7449da8ff358703b24eae,
title = "A backward-spinning star with two coplanar planets",
abstract = "It is widely assumed that a star and its protoplanetary disk are initially aligned, with the stellar equator parallel to the disk plane. When observations reveal a misalignment between stellar rotation and the orbital motion of a planet, the usual interpretation is that the initial alignment was upset by gravitational perturbations that took place after planet formation. Most of the previously known misalignments involve isolated hot Jupiters, for which planet–planet scattering or secular effects from a wider-orbiting planet are the leading explanations. In theory, star/disk misalignments can result from turbulence during star formation or the gravitational torque of a wide-orbiting companion star, but no definite examples of this scenario are known. An ideal example would combine a coplanar system of multiple planets—ruling out planet–planet scattering or other disruptive postformation events—with a backward-rotating star, a condition that is easier to obtain from a primordial misalignment than from postformation perturbations. There are two previously known examples of a misaligned star in a coplanar multiplanet system, but in neither case has a suitable companion star been identified, nor is the stellar rotation known to be retrograde. Here, we show that the star K2-290 A is tilted by 124◦ ± 6◦ compared with the orbits of both of its known planets and has a wide-orbiting stellar companion that is capable of having tilted the protoplanetary disk. The system provides the clearest demonstration that stars and protoplanetary disks can become grossly misaligned due to the gravitational torque from a neighboring star.",
keywords = "Binary host, Exoplanets, Obliquity, Primordial inclination, System formation",
author = "Maria Hjorth and Simon Albrecht and Teruyuki Hirano and Winn, {Joshua N.} and Dawson, {Rebekah I.} and Zanazzi, {J. J.} and Emil Knudstrup and Bun'ei Sato",
note = "Funding Information: ACKNOWLEDGMENTS. M.H., S.A., and E.K. acknowledge support from the Danish Council for Independent Research through DFF (Danmarks Frie Forskningsfond) Sapere Aude Starting Grant 4181-00487B and the Stellar Astrophysics Center, which is funded by Danish National Research Foundation Grant DNRF106. This work was supported by JSPS (Japan Society for the Promotion of Science) KAKENHI Grants16K17660 and 19K14783. Work by J.N.W. was supported by the Heising-Simons Foundation and NASA Award 80 NSSC18K1009. R.I.D. is supported in part by NASA XRP (Exo-planets Research Program) Grant NNX16AB50 G. We thank Akito Tajitsu and Sanghee Lee for assisting with the Subaru observations. We acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The data analyzed in this paper were obtained with the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundaci{\'o}n Galileo Galilei of the Istituto Nazionale di Astrofisica at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias as part of Program A39TAC 2; the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan as part of Program S19A122; and the Very Large Telescope with data collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO DDT (Director{\textquoteright}s Discretionary Time) Program 2103.C-5041(A). Funding Information: This paper also includes data collected by the K2 mission, which was funded by the NASA Science Mission directorate. This research made use of Lightkurve, a Python package for Kepler and TESS (Transiting Exoplanet Survey Satellite) data analysis (Lightkurve Collaboration). Funding Information: M.H., S.A., and E.K. acknowledge support from the Danish Council for Independent Research through DFF (Danmarks Frie Forskningsfond) Sapere Aude Starting Grant 4181-00487B and the Stellar Astrophysics Center, which is funded by Danish National Research Foundation Grant DNRF106. This work was supported by JSPS (Japan Society for the Promotion of Science) KAKENHI Grants16K17660 and 19K14783. Work by J.N.W. was supported by the Heising-Simons Foundation and NASA Award 80 NSSC18K1009. R.I.D. is supported in part by NASA XRP (Exoplanets Research Program) Grant NNX16AB50 G. We thank Akito Tajitsu and Sanghee Lee for assisting with the Subaru observations. We acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The data analyzed in this paper were obtained with the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundaci{\'o}n Galileo Galilei of the Istituto Nazionale di Astrofisica at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias as part of Program A39TAC 2; the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan as part of Program S19A122; and the Very Large Telescope with data collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO DDT (Director{\textquoteright}s Discretionary Time) Program 2103.C-5041(A). Publisher Copyright: {\textcopyright} 2021 National Academy of Sciences. All rights reserved.",
year = "2021",
month = feb,
day = "23",
doi = "10.1073/pnas.2017418118",
language = "English (US)",
volume = "118",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
publisher = "National Academy of Sciences",
number = "8",
}