Gliese 436 b was discovered in August 2004 by R. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley, respectively, using the radial velocity method. Together with 55 Cancri e, it was the first of a new class of planets with a minimum mass different to Neptune. The planet was recorded to transit its star by an automatic process at NMSU on January 11, 2005, but this event went unheeded at the time. In 2007, Gillon led a team that observed the transit, grazing the stellar disc relative to Earth. Transit observations led to the determination of its exact mass and radius, both of which are very similar to that of Neptune, making Gliese 436 b at that time the smallest known transiting extrasolar planet. The planet is about four thousand kilometers larger in diameter than Uranus and five thousand kilometers larger than Neptune and slightly more massive. Gliese 436b orbits at a distance of four million kilometers or one-fifteenth the average distance of Mercury from the Sun.
Physical characteristics
The planet's surface temperature is estimated from measurements taken as it passes behind the star to be. This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star, which was prior to this measurement, estimated at 520 K. Whatever energy tidal effects deliver to the planet, it does not affect its temperature significantly. A greenhouse effect could raise the temperature to much higher degrees than the predicted 520–620 K. In 2019, USA Today reported that the exoplanet's burning ice continued to have scientists "flabbergasted." Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms, which would remain solid despite the high temperatures, because of the planet's gravity. 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 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. In part for this reason, it has also been hypothesized to be a possible helium planet. In June 2015, scientists reported that the atmosphere of Gliese 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail long.
Orbital characteristics
One orbit around the star takes only about two days, 15.5 hours. Gliese 436 b's orbit is likely misaligned with its star's rotation. The eccentricity of Gliese 436 b's orbit is inconsistent with models of planetary system evolution. To have maintained its eccentricity over time requires that it be accompanied by another planet. A study published in Nature found that the orbit of Gliese 436 b is nearly perpendicular to the stellar equator of Gliese 436 and suggests that the eccentricity and misalignment of the orbit could have resulted from interactions with a yet undetected companion. The inward migration caused by this interaction could have triggered the atmospheric escape that sustains its giant exosphere.
