Fig. 18.—

Constraints on the bulk composition of GJ 436b as determined from models of its structure, evolution, and formation. Possible compositions are shown for six different model assumptions and/or imposed constraints. For each, the sum over the displayed mass fractions of H/He (green diamonds), water (blue squares), and “rock” (includes silicates plus iron; orange triangles) equals 1. All calculations take into account the planet’s mass M_p and radius R_p as one set of constraints; the two leftmost models consider uncertainties in M_p and R_p, which expands the possible H/He mass fraction by a factor of ~2, while the four on the right adopt specific M_p and R_p values from Torres et al. (2008). From left to right, the derived constraints are based on (i) the thermal and structural evolution models of Miller & Fortney (2011) that assume an ice:rock ratio of 1:1 and that impose constraints based on the age of the GJ 436 system; (ii) the formation, disk-planet evolution (including migration), and structural models of Figueira et al. (2009) that compute various formation paths consistent with M_p and R_p; (iii) the structure models of Adams et al. (2008) that consider precise values for M_p, R_p, and atmospheric temperatures and thus derive a smaller range of acceptable H/He values; (iv) the structural models of Nettelmann et al. (2010) that assume that water is mixed into the outermost H/He envelope; (v) structural models based on Nettelmann et al. (2010) that assume that a pure H/He layer resides above a water layer with a warm (1300 K) deep isothermal radiative region; (vi) a model similar to the one on the immediate left, except the isothermal radiative region is cool (700 K) and the planet is old (> 10 Gyr). Note from comparing the latter two models that atmospheric temperatures affect the derived H/He mass fraction. A color version of this figure is available in the online journal.