The nearby systems of HD 217107 and GJ 777 are similar insofar as each contains one planet in an epistellar or star-grazing orbit as well as a second planet of larger mass in a much wider and more eccentric orbit. In each system the orbit of the epistellar planet has been circularized by tides caused by the host star, superseding any previous relationship that might have existed with the outer planet. Further, both systems center on Sun-like stars of spectral class G, and both stars exhibit enhanced metallicity. In both systems, additional lower-mass planets might exist between the two detected giant planets, although the outer giants would severely constrain these intermediary orbits.

Located at a distance of almost 64 light years, HD 217107 is considered a mature star, likely to be older than Sol. The Extrasolar Planets Encyclopaedia lists its age as about 7 billion years and identifies its spectral class as G8 IV, meaning that, like GJ 777, it is beginning the subgiant phase of its evolution.

Its inner planet ("b") resembles a classic Hot Jupiter, with a minimum mass of 1.4 MJUP and a semimajor axis of 0.07 AU. However, as Barnes & Greenberg (2006b) point out, its orbital eccentricity of 0.13 is unusually high for an orbit smaller than 0.1 AU. Among the eight Solar planets this eccentricity is surpassed only by Mercury (e = 0.2). One possible cause would be perturbation by an as-yet undetected planet in an external orbit. The other known companion of HD 217107 is evidently not responsible, since its semimajor axis of 5.15 AU insulates each planet from the other’s influence.

With a minimum mass of 2.46 MJUP and a large orbital eccentricity of 0.53, this outer planet ("c") would exercise strong constraints on the formation and survival of any additional planets in the system. Nevertheless, a recent study of the orbital dynamics of HD 217107 finds a high likelihood that a third planet orbits between 0.1 AU and 4 AU (Barnes & Greenberg 2007b).

Last update February 2008