The angle between the rotation axis of a star and the orbital plane of its planets or disk.Tilt, in the context of stellar systems, refers to the angle between a star's rotation axis and the orbital plane of its planets or circumstellar disk. This angle, also known as obliquity, is a fundamental parameter that describes the geometry of a planetary system. A tilt of 0° means the planets orbit in the same plane as the star's equator, while a tilt of 90° indicates a polar orbit, and 180° would be a retrograde orbit. The tilt of a star's rotation axis relative to its disk can reveal important information about the system's formation history, including past gravitational interactions or planetary migration.
The measurement of stellar obliquity became possible in the late 1990s with the discovery of exoplanets and the development of the Rossiter-McLaughlin effect, which was first used to measure the tilt of a star-planet system in 2000.
The tilt is determined by measuring the star's rotation velocity and the orientation of its rotation axis, often through spectroscopic observations that detect the Doppler shift of spectral lines across the stellar disk. By comparing this with the orbital plane of the planets, determined through transit observations or direct imaging, astronomers can calculate the obliquity. A significant tilt suggests that the system has undergone dynamical processes such as planet-planet scattering, Kozai-Lidov oscillations, or interactions with a binary companion, which can tilt the orbital plane relative to the star's spin axis.
Measuring stellar tilt helps astronomers understand the formation and evolution of planetary systems, including the migration of hot Jupiters and the dynamical history of multi-planet systems.
Our own Sun has a tilt of about 7° relative to the orbital plane of the planets, which is relatively small, but some exoplanetary systems have been found with tilts exceeding 90°, meaning the planets orbit backwards relative to their star's rotation.










