Researchers describe a system of multiple planets orbiting a star that rotates backward. Unlike the Solar System, extrasolar planetary systems sometimes exhibit misalignment between the star's equator and the orbital plane of the surrounding planets. One proposed cause of such misalignments is a distant companion star exerting gravitational torque on the protoplanetary disk, but no definite examples of such a scenario have been described. Simon Albrecht, Maria Hjorth, and colleagues describe the stellar rotation and planetary orbits in the star system K2-290, which consists of three stars with two planets orbiting the main star. Based on changes to the star's spectrum during planetary transits, the authors determined that the star rotates in the direction opposite to the planetary orbits. Specifically, the authors estimated that the star's rotational axis was tilted by approximately 124° relative to the planets' orbits. Unlike previously observed systems with stellar-planetary misalignments, K2-290 has a known companion star, and simulations of the protoplanetary disk-binary star system led to the observed misalignment for a range of parameters. The results support the hypothesis that gravitational torques from companion stars can cause misalignment between protoplanetary disks and their associated stars, according to the authors.
Article #20-17418: "A backward-spinning star with two coplanar planets," by Maria Hjorth, Simon Albrecht et al.
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