Directions of the stellar rotation (Yellow) and orbital motion (White) in the TRAPPIST-1 system (Credit: NAOJ).
During a transit of a planet across the surface of a star, it seems by spectroscopic observations that the star's radial velocity changes with time series. This phenomenon is called the Rossiter-McLaughlin effect. The analysis of the effect give a constraint on an angle between a stellar spin of the host star and an orbital axis of the planet.
Image of TRAPPIST-1 exoplanet system
Host star of TRAPPIST-1 system is a low mass M-type dwarf star with low-surface temperature and the size of the host is as small as Jupiter. The planetary system has seven Earth-like planets orbiting the host star. In this result, astronomers found that orbital axes of the planets is not significantly misaligned with the rotation axis of the host (Credit: NAOJ).