Planetorbits

Planetorbits describe the paths that planets follow as they revolve around a more massive body, typically a star. In classical celestial mechanics, most planetary orbits are well modeled as ellipses with the gravitational source at one focus, a result of Kepler’s laws and Newtonian gravity. The details of an orbit are captured by a set of orbital elements that include size, shape, and orientation.

The main orbital elements are the semi-major axis (a), which sets the orbit’s size; eccentricity (e), which describes its shape; inclination (i), the tilt relative to a reference plane; the longitude of the ascending node (Ω) and the argument of periapsis (ω), which define the orientation of the ellipse; and the mean anomaly (M) or epoch, which specifies the position along the orbit. The orbital period (P) is related to the semi-major axis and the central mass by Kepler’s third law.

Formation and evolution play central roles in shaping planetorbits. Planets form in protoplanetary disks and interact gravitationally with disk material and with other planets, leading to migration, resonant configurations, and changes in eccentricity and inclination. Gravitational perturbations cause precession of orbit orientations, while tidal forces can circularize close-in orbits. Dynamical processes such as planet–planet scattering and disk-driven migration help explain the diverse architectures observed in planetary systems.

Observationally, orbits constrain a planet’s potential habitability and formation history. In exoplanet research, orbits are predominantly inferred from periodic starlight dimming during transits and from stellar wobbles in radial velocity measurements, with additional methods providing refinements to orbital parameters.