HD 37124 is a G4 star located at a distance of 33.2 parsecs (108 light years) in the constellation Taurus. Its mass and age are unexceptional, at 0.825 MSOL and 4 billion years, respectively (Vogt et al. 2005). However, as reported by Butler et al. (2006a), its metallicity is -0.442, the third-lowest value in the Catalog of Nearby Exoplanetary Host Stars. The system of HD 37124 sustains at least three gas giant planets, each approximately two-thirds the mass of Jupiter, following orbits evenly spaced within a maximum semimajor axis of 3.2 AU.

The innermost planet, at 0.64 MJUP and 0.53 AU, has a relatively circular orbit smaller than that of Venus, with a period of about 155 days. The middle planet, at 0.62 MJUP and 1.64 AU, has a larger semimajor axis than Mars and a period of about 2 years and 4 months. The outermost planet, at 0.68 MJUP and 3.19 AU, orbits at a distance equivalent to the Asteroid Belt, with a period in excess of 6 years. This planet has the highest orbital eccentricity of the three, but at 0.2 it is still relatively moderate, meaning that temperatures will remain stable for all three planets throughout their periods of revolution. According to Gozdziewski et al. (2006), the orbits of the two outer planets approximate a mean motion resonance of 5:2; Barnes & Greenberg (2006b) conclude that all three planets exhibit near-separatrix behavior

Two studies have investigated the stability of additional hypothetical orbits in this system (Barnes & Raymond 2004, Raymond et al. 2006), but both were conducted when only two planets had been identified. The evenly spaced orbits now presented by Butler et al. (2006a) suggest that this system is complete out to the region of the third planet, and that more distant planets would need semimajor axes that are larger by several AU.

As for habitability, we can speculate that the second and third planets may host satellite systems comparable to those of Jupiter and Saturn, but it is highly uncertain whether such moons could sustain Earthlike conditions. The determination of circumstellar habitable zones is notoriously imprecise. Jones et al. (2006) provide a generous estimate of 0.79-1.58 AU for the habitable zone of HD 37124, implying that the second planet orbits at its outer edge. However, they base their calculation on a stellar mass of 0.91 rather than the lower value now indicated by Butler et al. (2006a). Considering the sub-Solar mass and luminosity of HD 37124, it is doubtful that the orbital space of the second planet would permit temperatures even as warm as those on Mars. Nevertheless, as suggested by Scharf (2006), tidal stresses might produce higher temperatures on potential moons of this second planet, perhaps sustaining liquid water along with extensive volcanism.

Last update February 2007