A planetary system 897 light years away from us has two planets orbiting a star that rotates backwards.
It used to be assumed that a spinning star’s equator should line up with the orbital plane of its planets, because the star and planets both ultimately grow from the same spinning molecular cloud. As a consequence, the star should spin in the same direction that the planets orbit. But the K2-290 system doesn’t follow this rule.
The K2-290 system consists of three stars and has two planets orbiting the main star, K2-290 A.
Simon Albrecht at Aarhus University in Denmark and his colleagues determined that, compared with both planets’ orbits, K2-290 A’s rotational axis is tilted by approximately 124 degrees. This means that the star actually spins in roughly the opposite direction to its two orbiting planets.
By comparison, in our solar system, the sun’s spin axis is tilted only about 6 degrees compared with the planetary orbits, which means the planets orbit in roughly the same direction as the sun rotates.
The misalignment seen in K2-290 has been seen before in other planetary systems. One theory is that turbulence during star formation could cause misalignments between a star and its planets.
“Whatever nature can produce, it seems to be produced somewhere,” says Albrecht.
But K2-290 is unique in that both planets are orbiting in the same plane. This hints that something unusual happened early in the planetary system’s history, after the spinning molecular cloud had evolved to become a star surrounded by a protoplanetary disc from which the two planets eventually grew.
“The fact that [the planets] appear to be coplanar means that maybe it wasn’t a dynamically violent mechanism that caused them to migrate, maybe it was the disc,” says Chris Watson at Queen’s University Belfast, UK. “So then, you have to look at how did you end up with the star and the planet-forming disc tilted in the first place.”
Albrecht and his colleagues believe that the whole system became misaligned because of the presence of a companion star – perhaps K2-290 B – that could have exerted gravitational forces which moved the disc.
“A lot of the way we interpret the spin-orbit misalignment assumes that the actual planetary disc was aligned with the whole star in the first place,” says Watson. “You open doors to ‘hmm we don’t really know what’s going on’ if you’re able to misalign that disc in some way.”