The system of Gliese 436 (often abbreviated GJ 436) consists of a single Neptune-mass planet in a short-period orbit around a red dwarf star of spectral type M2.5. 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. In Solar units, its luminosity is 2.6%, its mass is 0.452, and its radius is 0.464 (Torres 2007). The star’s age is not well constrained, but estimates favor a minimum of about 3 billion years (Maness et al. 2007, Torres 2007). Brice-Olivier Demory and colleagues report a rotational period of 48 days and infer that the stellar atmosphere is stormy, with persistent starspots (Demory et al. 2007).

Among Sun-like stars (spectral types F, G, and K) that host extrasolar planets, an enhancement in heavy elements is common (Fischer & Valenti 2005). Whether the same trend applies to M dwarfs is uncertain, however, because the metal content of these stars remains a matter of debate (Bailey et al. 2009, Johnson & Apps 2009). For GJ 436 the situation is especially unsettled, as recent sources offer widely different values for its metallicity: from a low of -0.32 (Bean et al. 2006) to a high of +0.25 (Johnson & Apps 2009). Because our understanding of the system’s past evolution and present architecture hinges, in part, on the host star’s metal content, current analyses are limited by this uncertainty.

The system's known planet, GJ 436 b, is a classic Hot Neptune. It has been shown to transit across the face of its primary, permitting the planet’s mass and radius to be established with reasonable accuracy (Gillon et al. 2007, Deming et al. 2007, Torres 2007, Pont et al. 2009). Guillermo Torres provides a mass of 23.2 MEA, making the planet one-third again as heavy as Neptune (17.2 MEA). Nevertheless, values ranging from ~20 to ~25 MEA have been proposed, including margins of error. The planet’s radius is known with some precision, with findings of 4.0-4.3 Earth radius, compared to about 3.9 Earth radius for Neptune itself. The resulting mass-radius relationship implies a bulk composition similar to Neptune’s, despite the huge difference in the two planets’ orbital configurations. (Gillon et al. 2007, Deming et al. 2007).

Taking into account what we can deduce about the planet’s native environment, Pedro Figueira and colleagues argue for the following breakdown: a rock/metal core accounting for 45%-70% of the total mass; a mantle of ice contributing 15%-40%; and a hydrogen-helium atmosphere contributing 10%-20% (Figueira et al. 2009). Unlike ice as we know it on Earth, the 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 0.0287 AU (Torres 2007), 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. Paul 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 period is short even for a Hot Jupiter, at about 2.64 Earth days. Given its proximity to the host star, stellar tides will have eliminated any potential moons.

GJ 436 b has an orbital eccentricity of about 0.15, which is anomalously high for a short-period planet (Deming et al. 2007, Batygin et al. 2009). Such objects generally have circular orbits with eccentricities approaching zero. Because the timescale for circularization by stellar tides is much shorter than the age of the planetary system, current investigations have addressed the possibility that a second planet at a larger semimajor axis is perturbing the orbit of GJ 436 b. However, continuing radial velocity observations appear to rule out any gas giants within a few AU of the central star (Demory et al. 2007, Batygin et al. 2009). Super Earth to Neptune-mass planets have therefore become the preferred suspects.

Konstantin Batygin and colleagues have calculated a variety of stable configurations for a hypothetical second planet, proposing an object of 5 to 11 Earth masses whose orbital period falls between 16 and 70 days and whose orbital eccentricity lies in the range of 0.167 to 0.782 (Batygin et al. 2009). In their models, higher (lower) eccentricities characterize larger (smaller) orbits, and many of their potential planets occupy the system’s habitable zone. Although they concede that a single additional planet is the simplest solution to the problem, they also propose an alternative in which planet b has two outer companions, each an ice giant of about 22 Earth masses. In their example configuration, the periods of the two planets are 14.24 and 27.33 days, and their eccentricities lie in the range of 0.13 to 0.16, like that of the detected planet.

The shape of planet b’s orbit has still more to tell us about the system environment. A short-period planet on a circular orbit would be rotationally synchronized, so that its day and year would be the same length. However, Drake Deming and colleagues argue that the eccentricity of GJ 436 b causes its rotation to be asynchronous, resulting in a 19-day synodic period (Deming et al. 2007, Batygin et al. 2009).

Last update July 2009