一颗有两颗共面行星的后旋恒星
A backward-spinning star with two coplanar planets
摘要：
人们普遍认为恒星和它的原行星盘最初是排成一行的，恒星赤道平行于盘平面。当观测结果显示恒星自转和行星轨道运动之间存在偏差时，通常的解释是，最初的对齐被行星形成后发生的引力扰动所扰乱。大多数先前已知的错位都涉及孤立的热彗星，行星与行星之间的散射或来自更宽轨道行星的长期影响是主要的解释。在理论上，恒星/盘的错位可能是恒星形成过程中的湍流或宽轨道伴星的引力扭矩造成的，但这种情况的确切例子是未知的。一个理想的例子是将一个多行星的共面系统（排除行星间的散射或其他破坏性的形成后事件）与一个反向旋转的恒星结合起来，这种情况更容易从原始的未对准中获得，而不是从形成后的扰动中获得。在共面多行星系统中有两个已知的偏离恒星的例子，但在这两种情况下都没有发现合适的伴星，也没有已知的恒星旋转是逆行的。在这里，我们发现恒星K2-290 A与它的两颗已知行星的轨道相比倾斜了$124\pm 6$度，并且有一个宽轨道的恒星伴星，能够倾斜原行星盘。该系统提供了最清晰的证据，表明恒星和原行星盘可以由于邻近恒星的引力扭矩而变得严重错位。
英文摘要：
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.
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