K2-290A, the primary star in the triple stellar system K2-290, is tilted by 124 degrees compared with the orbits of both of its known planets, and has a wide-orbiting companion that is capable of having tilted the protoplanetary disk.
K2-290, also known as BD-19 4173 and TIC 70803960, is a triple system located 897 light-years away in the constellation of Libra.
The primary star, K2-290A, is a late-type F star with a mass of 1.2 solar masses and a radius of 1.5 solar radii.
The secondary star, K2-290B, is an M dwarf (red dwarf) with a projected orbital separation of 113 AU (astronomical units).
The tertiary star, K2-290C, is another M dwarf located farther away, with a projected separation of 2,467 AU.
In 2019, two transiting planets were discovered around the primary star in the K2-290 system.
The inner planet, K2-290Ab, has an orbital period of 9.2 days and a radius of 3 Earth radii, making it a hot sub-Neptune.
The outer planet, K2-290Ac, is a warm Jupiter with orbital period 48.4 days and a radius of 11.3 Earth radii.
In a new study, Aarhus University astronomer Maria Hjorth and colleagues analyzed high-resolution spectroscopic data obtained for the K2-290A system during transits of two planets in front of their host star.
Based on changes to the star’s spectrum during the planetary transits, they determined that the star rotates in the direction opposite to the planetary orbits.
“Systems similar to K2-290, with coplanar planetary orbits and a grossly misaligned host star, had been predicted to exist as a consequence of the tidal torque on a protoplanetary disk from a neighboring star,” they said.
“Another possible explanation for such systems is the tilting torque exerted on the inner system of planets by a massive planet on a wide and highly inclined orbit.”
“The Kepler-56 system features two planets on coplanar orbits and a misaligned star, and in that case, a wider-orbiting third planet was detected through long-term monitoring. Based on the mass and orbital distance of the third planet, it is possible or even probable that the planet was responsible for tilting the orbital plane of the inner two planets long after the planets formed.”
“Likewise, the HD 3167 multiplanet system was recently found to have a misaligned star, but there is not yet any evidence for either a wider-orbiting planet or a companion star.”
“Turbulence and disk torquing can lead to misaligned protoplanetary disks. However, retrograde orbits, as observed for K2-290A, are difficult to achieve via turbulence, and late infall of material will lead to a further reduction of any misalignment,” they noted.
“The unique aspect of K2-290 is that a companion star, K2-290B, has been detected with properties that make it a good candidate for the misalignment of the protoplanetary disk.”
“There is no evidence for a wider-orbiting massive planet. In addition, star/disk misalignment is an attractive explanation for K2-290 because it can easily produce retrograde orbits.”
The astronomers estimated that the rotational axis of K2-290A was tilted by approximately 124 degrees relative to the orbits of its two planets.
“The K2-290 system provides the clearest demonstration that stars and protoplanetary disks can become misaligned due to the gravitational torque from a neighboring star,” they concluded.
The study was published in the Proceedings of the National Academy of Sciences.
Maria Hjorth et al. 2021. A backward-spinning star with two coplanar planets. PNAS 118 (8): e2017418118; doi: 10.1073/pnas.2017418118