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, placing it in the bottom 5% of all exoplanetary host stars (see current data in the Extrasolar Planets Encyclopaedia). The system contains at least three gas giant planets, widely spaced within a radius of about 3 AU.

Architecture of the HD 37124 system. Colored circles indicate the relative sizes of the 3 planets, assuming the minimum masses provided by Wright et al. 2009, the mass-radius relationships provided by Fortney et al. 2007, and modest cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line.

The architecture of the HD 37124 system is notable insofar as its three giant planets are very similar in mass, all with minimum values of about two-thirds Jupiter. In most other known multi-planet systems, including our own, one planet is considerably heavier than the rest. But in HD 37124, the most massive planet, c, is only 10% heavier than the least massive, d.

1. The innermost planet was discovered first, receiving the designation b. At 0.64 MJUP and 0.53 AU, planet b has a relatively circular orbit smaller than that of Venus, with a period of about 155 days.
   
   
2. The middle planet, d, is 0.62 MJUP, with a semimajor axis of 1.64 AU. This is larger than the orbit of Mars; the associated period is about 2 years and 4 months.
   
   
3. The outer planet, c, at 0.68 MJUP and 3.19 AU, orbits at a distance equivalent to the Asteroid Belt. Its period is longer than 6 years. This planet's orbit has the highest eccentricity of the three, but at 0.2 it is still relatively moderate. According to Gozdziewski et al. (2006), the orbits of the two outer planets approximate a mean motion resonance of 5:2, whereas 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 before planet c was identified. The evenly spaced orbits now reported (Butler et al. 2006a, Wright et al. 2009) suggest that this system is complete out to the region of the third planet. More distant planets would need semimajor axes 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 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, which is larger than other sources indicate (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 Mars. Nevertheless, as suggested by Scharf (2006), tidal stresses might produce higher temperatures on potential moons of this second planet, perhaps sustaining global volcanism and surface bodies of water.

Last update August 2009