circumbinary

Circumbinary refers to objects that orbit around both members of a binary star system, rather than around a single star. The term is commonly used for circumbinary planets (also called P-type planets) and for circumbinary disks of gas and dust surrounding a binary.

A circumbinary planet follows an orbit about the system barycenter. The binary’s gravity creates a central cavity in the surrounding disk, and stable planetary orbits generally lie beyond a critical distance that depends on the binary mass ratio and eccentricity. The Holman–Wiegert criterion provides a widely used estimate for this stability limit. Orbits can precess and exhibit transit timing variations due to the moving gravitational potential of the binary.

Notable circumbinary planets include Kepler-16b, the first confirmed example, followed by multiple discoveries such as Kepler-34b, Kepler-35b, and the compact, multi-planet system Kepler-47. Circumbinary disks have been observed around young binaries with facilities like ALMA, revealing inner gaps and complex structures. Examples include GW Orionis and GG Tauri systems.

Circumbinary can also inform studies of habitability. Circumbinary habitable zones are defined by the combined luminosities of both stars and can be more dynamically complex due to varying insolation and orbital precession.

Detection methods include transit observations, which can be irregular because of the binary motion, eclipse timing variations, and, more challenging, radial velocity signals. The field continues to grow as surveys increase sensitivity to circumbinary configurations.