The nearby systems of GJ 777 (also known as HD 190360) and HD 217107 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. Further, both systems center on Sun-like stars of spectral class G, and both stars exhibit enhanced metallicity. In both systems, additional terrestrial-mass planets might exist between the two detected giant planets, although the outer giants would place severe constraints on these intermediate orbits.

GJ 777, located almost 52 light years away, is the only nearby exoplanetary system to contain three stars. The yellow star that hosts the two planets is accompanied by a pair of M dwarfs orbiting at about 3000 AU (Desidera & Barbieri 2006). At this wide separation, it is unlikely that the two red stars (GJ 777 Ba and Bb) play any role in the dynamic evolution of the planets orbiting GJ 777 A.

The Extrasolar Planets Encyclopaedia identifies the primary as an “evolved” subgiant of spectral class G6 IV, with an estimated age of 12 billion years. This is more than twice as old as our Sun, and if accurate, it places GJ 777 among the earliest generations of stars in the Milky Way. After so many eons, the star is burning through the last of its hydrogen. Although GJ 777 is similar in mass to our Sun, its radius has puffed up 2.35 times as it continues to evolve toward the giant phase.

The system's inner planet has a minimum mass of 0.06 MJUP, slightly heavier than Neptune, and a semimajor axis of 0.13 AU. Its orbital period of about 17 days means that this planet (c) is tidally locked, always presenting the same hemisphere to its host star. Despite the star's proximity and great age, Lecavelier des Etangs (2006) argues that planet c has not lost any mass through photoevaporation. Unfortunately, we have no evidence regarding its composition – whether it contains a substantial rocky core or whether its proportion of volatiles is more typical of a gas giant or an ice giant planet.

We do know from transit data that the Hot Neptune in orbit around the red dwarf star GJ 436 has an internal structure much like our own cold Neptune: a modest rocky core, a very deep layer of methane-ammonia-water ice, and a significant atmosphere of hydrogen and helium (Gillon et al. 2007, Deming et al. 2007). GJ 777 c may bear a family resemblance to GJ 436 b.

With a minimum mass of 1.55 MJUP and a semimajor axis of 3.99 AU, the outer planet (GJ 777 b) is probably a gas giant similar in structure to Jupiter or Saturn. However, its orbit is substantially more elliptical than theirs, with an eccentricity of 0.36. This value implies a periastron of about 3 AU and an apastron of about 5 AU.

Barnes & Greenberg (2007b) have performed a numerical simulation in which they placed an Earth-mass planet on a circular orbit around GJ 777 with a semimajor axis of about 1.5 AU. Although this hypothetical planet ultimately proved unstable, its survival for 1 million years of simulation time led the authors to conclude that some combination of parameters might result in long-term stability for a third planet in this system.

Unfortunately, even if an Earth twin somehow survived within the main sequence habitable zone of GJ 777, the star's currently evolved state, with its enlarged radius and hotter output, would have reduced this planet to a wasteland.

Last update September 2007