The Gamma Cephei system is notable for harboring three different kinds of objects -- a subgiant star, an M dwarf star, and a gas giant planet -- all within a radius of 20 AU. The primary, Gamma Cephei A, is a K1 star “on the first ascent of the giant branch” (Torres 2006), located at a distance of 13.8 parsecs (45 light years) in the constellation Cepheus. This orange subgiant has a mass of about 1.4 MSOL and an age of about 6.6 billion years (Neuhauser et al. 2007, Torres 2006). Its binary companion, Gamma Cephei B, is a red star of spectral class M4 with a mass of 0.40 MSOL.

The semimajor axis of the two stars' shared orbit is about 20 AU, with an eccentricity of 0.4 and a period between 67 and 68 years (Neuhauser et al. 2007). These parameters imply a periastron of 12 AU and an apastron of 28 AU. (The corresponding values for Alpha Centauri are quite similar.)

The mass of Gamma Cephei A indicates that it was formerly an F-type star like Upsilon Andromedae (mass 1.31 MSOL). It has burned through the hydrogen at its core and expanded in diameter by a factor of about 3, while lowering its effective temperature. The net result has been an increase in the star's output of heat and light, since its overall surface area has dramatically expanded.

A single planet accompanies Gamma Cephei A, traveling on a mildly eccentric orbit (e = 0.12) with a period of about 903 days (2.5 years) and a semimajor axis of 2 AU. The planet's minimum mass is 1.6 MJUP. Its decentered orbit brings it as close to its primary star as 1.83 AU at periastron and as far as 2.3 AU at apastron (Neuhauser et al. 2007). Numerical simulations have shown that no additional stable orbits exist exterior to this gas giant (Verrier & Evans 2006, Castro & Haghighipour 2006), although Verrier & Evans suggest that an asteroid belt or perhaps one or more small rocky planets might survive inside its orbit.

Given its substantial mass and favorable orbital characteristics, this planet is likely to host a family of satellites. The primordial environment in which they must have formed (see below) would ensure that these exomoons would be rocky like the Earth's moon rather than icy like the moons of Jupiter and Saturn.

The physical characteristics of Gamma Cephei b, as the giant planet is designated, raise interesting questions as to its formation history. Theoretical considerations indicate that the original protoplanetary disk around the host star would have been truncated at only 3.34 AU by perturbations from its red dwarf companion (Neuhauser et al. 2007). This limit is well within the ice line of a star of spectral class F. Thus, planet b's native environment was completely lacking in ices and other volatiles, which are considered necessary in most cases to foster the growth of planetary cores massive enough to capture large quantities of hydrogen gas (Ida & Lin 2005). If a large proportion of the primordial nebula consisted of metallic elements, a sufficiently massive core might still have assembled (Ida & Lin 2005). Yet the metallicity of Gamma Cephei is virtually identical to that of our Sun, a star of average metallic enhancement.

The formation process of Gamma Cephei b therefore remains enigmatic. As Marzari & Barbieri (2007) point out, the present configuration of a binary system may not be indicative of its primordial status, since binary and multiple systems often have violent dynamic histories.

In any case, the system of Gamma Cephei is notable simply because its survival for more than 6 billion years supports the idea that planetary systems centered on one member of a close binary pair are common throughout the Milky Way Galaxy.

Last update April 2007