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.

Long-period Jovian Tilts the Orbits of Two sub-Neptunes Relative to Stellar Spin Axis in Kepler-129

“…In this case, the inner planets may be misaligned with the stellar spin when they formed. Hjorth et al (2021) found two coplanar transiting planets' orbits are retrograde with respect to the host star (K2-290A) in a triple system, indicating the protoplanetary disk was misaligned due to perturbations from the neighboring star. Spalding & Batygin (2014) and Spalding & Millholland (2020) argue that the quadrupolar gravitational potential of a tilted, rapidly rotating host star would torque the orbits of close-in planets.…”

Subaru Telescope —History, active/adaptive optics, instruments, and scientific achievements—

“…157) The further detection of a backward-spinning star with two coplanar planets is also reported. 158) These findings add constraints on the formation scenario of planetary systems.…”

Planet Hunters TESS III: two transiting planets around the bright G dwarf HD 152843

“…The RM effect helps to shed light onto the dynamical history of the system, as mechanisms such as planet-disk interactions help to preserve the initial spinorbit alignment, while planet-planet interactions promote misalignment (e.g., Chatterjee et al 2008;Deeg et al 2009;Storch et al 2017). The number of multi-planet systems with measured obliquities remains small (e.g., Hjorth et al 2021;Dalal et al 2019). We estimate the RM effect to be 3.71 +0.89 −0.74 m s −1 and 9.56 +2.65 −2.7 m s −1 for HD 152843 b and c, respectively (Winn 2010).…”