The binary system of Gliese 86 (also HD 13445) is located in the direction of Eridanus at a distance of 36 light years (10.91 parsecs). It is among the nearest exoplanetary systems to the Sun. The two companion stars share a close, eccentric orbit with a semimajor axis of about 18.4 AU, an eccentricity of 0.39, and a suggested period of 70 years (Lagrange et al. 2006). Over the course of an orbit their separation varies from 22.4 AU at apastron to 14.4 AU at periastron. By comparison, Uranus orbits the Sun at an average distance of 19.2 AU.

The brighter star, Gliese 86 A, is a main-sequence orange dwarf (spectral type K1, mass 0.77 MSOL) considerably cooler and less metallic than our Sun. It harbors a single massive gas giant planet (3.9 MJUP) in a tight, circular orbit. The detected planet, Gliese 86 b, falls just outside the range of Hot Jupiters, with a semimajor axis of 0.11 AU and a period of almost 16 days. It is unusually massive for a planet so close to its star; the handful of systems with similarly large planets at comparable separations includes Tau Bootis, HIP 14810, HD 195019, and HAT-P-2. Whether any additional planets might survive in orbits exterior to Gliese 86 b remains unknown. If they do exist, they will probably be much less massive than the detected planet.

The dimmer companion star, Gliese 86 B, is a white dwarf whose current mass lies between 0.48 and 0.64 MSOL, with 0.54 MSOL as the preferred value (Lagrange et al. 2006, Desidera & Barbieri 2007). Its presence in such close proximity to the planet host star raises many questions about the system’s evolutionary history. The white dwarf was originally the brighter and more massive of the two stars, with an initial mass of about 1.2 MSOL (Desidera & Barbieri 2007) and a likely main-sequence spectral type in the range of F0-F9. It has lost at least half of its original mass in the series of explosions that marked its post-main sequence evolution into a white dwarf. Therefore, its original shared orbit with Gliese 86 A must have been even tighter, since mass loss results in a “relaxation” or widening of binary orbits.

For the Gliese 86 binary, then, Desidera & Barbieri suggest a main-sequence semimajor axis of about 13 AU, with the orbital eccentricity remaining unchanged. This value implies an original periastron separation between the two stars of only about 10 AU. The protoplanetary disks surrounding each of the two stars must have been severely truncated by this crowding, raising doubts that a gas giant planet equivalent to four Jupiters could ever have assembled in such an environment. Nevertheless, the planet exists, while the details of its formation remain mysterious.

We can assume that the immediate vicinity of the Gliese 86 binary has always been relatively warm and depleted of volatile elements. The system’s age is estimated at about 8 billion years (Desidera & Barbieri 2007), making it considerably older than our Solar System. Star B evidently evolved into a red giant a few billion years ago, with its violent contraction into a white dwarf occurring between 1 and 2 billion years ago. During this explosive epoch, both Star A and its massive planet must have intercepted numerous expanding clouds of superheated gases. Overall, Gliese 86 is one of the least congenial exoplanetary systems detected so far.

Last update July 2007