GJ 436, An Exoplanetary System Hosted by an M Dwarf Star

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M dwarf systems compared

The system of GJ 436 consists of a single Neptune-mass planet in a tight orbit around a red dwarf. Located at a distance of 10.2 parsecs (33 light years) in the constellation Leo, the primary star is substantially hotter and more massive than both GJ 581 and GJ 876, with a luminosity 0.024 Solar and a mass and diameter 47% Solar (Maness et al. 2007, Gillon et al. 2007, Deming et al. 2007). Nevertheless, all three stars have low metallicity, with GJ 436 at -0.32 (Bean et al. 2006). The star’s age is estimated at 3 billion years or more (Maness et al. 2007).

The system’s known planet, GJ 436 b, is a classic Hot Neptune. This planet has been shown to transit across the face of its primary, permitting its mass, radius, and orbital inclination to be established with reasonable precision (Gillon et al. 2007, Deming et al. 2007). Gillon and colleagues provide a mass of 22.6 MEA, making the planet heavier than Neptune (17.2 MEA). Its diameter is about 55,200 km (34,200 mi), a bit larger than that of Neptune at 49,750 km (30,845 mi). Thus GJ 436 b consists of 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). This is virtually identical to the inferred composition of Neptune, despite the enormous difference in the two planets’ orbital configurations. Unlike ice as we know it on Earth, the dense mantle of GJ 436 b must be extremely hot, existing in a high-pressure state that precludes sublimation despite the proximity of the star.

The planet’s semimajor axis is only 0.028 AU, a little larger than that of GJ 876 d and considerably tighter than the orbits of such classic Hot Jupiters as 51 Pegasi, Tau Bootis, and Upsilon Andromedae b. But because the primary star is an M dwarf, GJ 436 b is much cooler than these planets. Butler and colleagues compare it to Venus (Butler et al. 2004), a resemblance that may be enhanced by the presence of a hot cloud cover.

The planet’s orbital eccentricity of 0.14 is anomalously high for an epistellar planet (Deming et al. 2007), since these objects generally have tidally circularized orbits with eccentricities approaching zero. Its period is short even for a Hot Jupiter, at about 2.64 Earth days. The rotation of GJ 436 b may be tidally locked, so that it has a permanent bright side and a permanent dark side. Stellar tides will have eliminated any moons.

Index of exoplanetary topics

A study by Jones and colleagues found that stable orbits are possible outside GJ 436 b in the system’s narrow habitable zone, which they defined as centering around 0.24 AU (Jones et al. 2006). However, a similar study by Mandell and colleagues reached the opposite conclusion (Mandell et al. 2007). Mandell’s group argued for a slightly closer habitable zone, extending from about 0.10 AU to 0.19 AU. They considered stable orbits in this region unlikely, and they omitted GJ 436 in their list of “Potentially Habitable Exoplanet Systems.” Planet b’s eccentricity might be the reason for exclusion.

Nonetheless, residuals in the radial velocity data for GJ 436 hint at the presence of an additional planet in the system, traveling in an exterior orbit (Maness et al. 2007). Gravitational perturbation by such a planet is the most likely explanation for the high orbital eccentricity of GJ 436 b. Considering all constraints, Maness and colleagues suggest two hypothetical configurations for this object. In one scenario, the second planet has an orbital period between 5 and 14 years, a mass lower than 0.12 MJUP, and an orbital eccentricity above 0.5. In the other scenario, the second planet has a period of about 25 years, a mass of about 0.27 MJUP (i.e., slightly less massive than Saturn), and an eccentricity of about 0.2.

Last update August 2007