A backward-spinning star with two coplanar planets

TL;DR

This paper presents evidence that a star's retrograde rotation and misaligned planets can result from gravitational influence of a nearby star, challenging the assumption of initial star-disk alignment.

Contribution

It provides the first clear example of a star with retrograde rotation and coplanar planets caused by a companion star's gravitational torque.

Findings

Star K2-290 A is tilted by 124±6 degrees relative to its planets.

The system includes a wide-orbiting stellar companion capable of causing the misalignment.

Demonstrates that star-disk misalignments can originate from primordial gravitational interactions.

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 post-formation events -- with a backward-rotating star, a condition that is easier to obtain from a primordial misalignment than from post-formation perturbations. There are two previously known examples of a misaligned star in a coplanar multi-planet 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 $124\pm 6$ degrees compared to 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 neighbouring star.