The red dwarf star GJ 674 is located less than 15 light years away in the constellation Ara. It harbors the second-closest exoplanetary system to our own, after Epsilon Eridani. This relatively young system conforms to current hypotheses regarding M dwarfs (see Ida & Lin 2005). It contains a single planet in the mass range of Uranus, with an orbital period of only a few days. The primary is an M2.5 star with a mass about 35% Solar and a bolometric luminosity 0.016 Solar. Its metallicity is -0.28, significantly less than Sol’s and unusually low for a planet-bearing star. Its age is estimated at 1 billion years or less, as compared to 4.6 billion for the Solar System. (All system data are from Bonfils et al. 2007.)

The only detected companion of GJ 674 (in standard nomenclature, GJ 674 b) fits the definition of a Hot Neptune. However, its minimum mass of only 11 MEARTH is significantly less than Neptune's (17.2 MEARTH), a deficit that places this object in the same class as the innermost planets of 55 Cancri and HD 69830. In semimajor axis and period its orbit is very similar to that of GJ 581 b, another M dwarf planet. For GJ 674 these values are 0.039 AU and 4.7 Earth days, well within the usual limits for Hot Jupiters and Hot Neptunes (e.g., 0.1 AU and 7 days).

Like all epistellar planets, GJ 674 b must be tidally locked, always presenting the same hemisphere to its host star. Similarly, it must have reached its present location by migrating inward from its place of formation, probably around the system's ice line.

The host star's chromosphere seems more stormy than those of other M dwarfs known to harbor exoplanets, as GJ 674 exhibits a moderate level of X-ray and magnetic activity. The discovery team reported that variations in the star's radial velocity initially suggested the presence of two planets instead of one. However, the data that they interpreted as a hypothetical second planet were shown to represent a persistent star spot in the host's upper atmosphere, analogous to a huge sunspot on our own system's primary (Bonfils et al. 2007). Thus Bonfils and colleagues describe this star as a "spotted dwarf." Its persistent stormy region covers an estimated 2.6% of the total surface area.

The radial velocity data for GJ 674 contain no residuals suggestive of any additional planetary companions. Nevertheless, the detected planet's orbital eccentricity of 0.2 is unexpectedly large for a short-period object. Epistellar planets generally have tidally circularized orbits with eccentricities approaching zero. The cause of this decentering – virtually identical to the eccentricity of Mercury – is an open question. Perturbation by an additional planet in the system remains a plausible explanation.

Last update April 2007