Spin-Orbit Alignment of KOI-2138.01

When a star rotates fast enough, centrifugal forces will cause the star to assume a detectably oblate spheroidal shape. This means the star has a larger radius at its equator than at its poles. A consequence of this is that the equatorial regions will have a lower surface gravity, and thus lower temperature and brightness; while the polar regions will have a higher surface gravity, and thus higher temperature and brightness. This darkening around the star's equator is known as gravity darkening.

KOI-2138.01 is a transiting super-Earth planet candidate in orbit around a rapidly rotating host star that has 2.29 times the radius and 2.34 times the mass of the Sun. The star's rapid rotation spins it into an oblate spheroid with a gravity darkened equator. Observing the way KOI-2138.01 transits its host star and how it passes in front of the gravity darkened equator allows the planet's orbital inclination to be determined. KOI-2138.01 is found to have a relatively flat orbit with a projected spin-orbit alignment of only 1 ± 13 degrees. The close proximity of KOI-2138.01 from its host star also gives it an estimated dayside temperature of up to ~1300 K.

Reference:
Barnes et al. (2016), "Probable Spin-Orbit Aligned Super-Earth Planet Candidate KOI-2138.01", arXiv:1512.03855 [astro-ph.EP]