The tilt of a planet's orbit relative to a reference plane, such as the sky plane.Orbital inclination is the angle between a celestial body's orbital plane and a reference plane, typically the ecliptic (the plane of Earth's orbit around the Sun) for solar system objects, or the sky plane for exoplanets. It determines how 'tilted' an orbit is: an inclination near 0° means the orbit lies nearly in the reference plane, while 90° indicates a polar orbit, and values above 90° signify retrograde motion, where the body orbits opposite to the reference direction. This parameter is fundamental for describing planetary systems, binary stars, and artificial satellites.
The concept dates back to Kepler's laws and was formalized in celestial mechanics by Laplace and others in the 18th century. For exoplanets, the first measurements of orbital inclination came in the 1990s with the discovery of hot Jupiters via the transit method.
The inclination is measured as the angle between the orbital plane and the reference plane, defined at the ascending node—the point where the body crosses the reference plane moving upward. For exoplanets, inclination is often inferred from transit observations (where the planet passes in front of its star) or from radial velocity measurements combined with astrometry. A high inclination can reveal that a planet orbits in a tilted or even retrograde path, which often signals past gravitational interactions, such as planet-planet scattering or the influence of a companion star.
Orbital inclination is critical for planning satellite launches (e.g., geostationary orbits require near-zero inclination) and for interpreting exoplanet data, including determining a planet's true mass and the likelihood of transits.
Some exoplanets have retrograde orbits with inclinations near 180°, meaning they circle their star in the opposite direction to the star's rotation—a sign of violent past interactions.
