circularizing

Circularizing is the process by which eccentric or non-circular orbital motion becomes circular through energy dissipation. In astrophysics, circularization most often refers to tidal circularization of orbits in binary star systems or planetary systems, driven by tidal forces within the bodies and by viscous processes in any circumstellar or circumbinary disk. In a close binary or a planet–star system, the gravitational pull raises tides on the bodies. The tides dissipate orbital energy as heat, removing eccentric energy and gradually reducing eccentricity, commonly accompanied by a spin synchronization between the rotating bodies and the orbital motion.

In many systems the dissipation is more efficient for close orbits, leading to eccentricity decreasing on a timescale that can range from millions to billions of years depending on internal structure, masses, radii, orbital separation, and the tidal quality factor Q. A sufficient decrease in eccentricity can leave the binary or planet on a nearly circular orbit and, in many cases, tidally lock the rotation. In accretion disks around compact objects, matter captured from a companion often arrives on non-circular trajectories and loses energy through shocks and viscosity; this damping causes the material to settle into a nearly circular, Keplerian disk. The radius at which the gas’s specific angular momentum corresponds to a circular Keplerian orbit is called the circularization radius.

Observationally, short-period binaries and many hot Jupiters exhibit small eccentricities, consistent with tidal circularization. However, external perturbations from additional companions or resonances can maintain or excite eccentricity, preventing complete circularization. Nevertheless, external perturbations can sustain eccentricity, preventing full circularization in some systems.