Gliese 436 B - Physical Characteristics

Physical Characteristics

One orbit around the star takes only about 2 days, 15.5 hours. The planet's surface temperature is estimated from measurements taken as it passes behind the star to be 712 K (439 °C). This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star (which had been, in a Reuters article from a month prior to this measurement, estimated at 520 K). Whatever energy that tidal effects deliver to the planet does not notably affect its temperature. Its discoverers allowed for a temperature increase due to a greenhouse effect.

Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms, which remains solid because of the planet's gravity despite the high temperatures. The planet could have formed further from its current position, as a gas giant, and migrated inwards with the other gas giants. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection.

However when the radius became better known, ice alone was not enough to account for it. An outer layer of hydrogen and helium up to ten percent in mass would be needed on top of the ice to account for the observed planetary radius. This obviates the need for an ice core. Alternatively, the planet may be a super-earth.

Observations of the planet's brightness temperature with the Spitzer Space Telescope suggest a possible thermochemical disequilibrium in the atmosphere of this exoplanet. Results published in Nature suggest that Gliese 436b's dayside atmosphere is abundant in CO and deficient in methane (CH₄) by a factor of ~7,000. This result is unexpected because, based on current models at this temperature, the atmospheric carbon should prefer CH₄ over CO.