Gliese 876 D - Physical Characteristics

Physical Characteristics

Since Gliese 876 d has only been detected indirectly by its gravitational effects on its star, properties such as its radius, composition, and temperature are unknown. On the assumption of a Venerean distribution of temperature and a maximum albedo of 0.8, the temperature was estimated at 430–650 K.

The low mass of the planet has led to suggestions that it may be a terrestrial planet. This type of massive terrestrial planet could be formed in the inner part of the Gliese 876 system from material pushed towards the star by the inward migration of the gas giants.

Alternatively the planet could have formed further from Gliese 876, as a gas giant, and migrated inwards with the other gas giants. This would result in a composition richer in volatile substances, such as water. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection. In this model, the planet would have a pressurised ocean of water (in the form of a supercritical fluid) separated from the silicate core by a layer of ice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor and free oxygen produced by the breakdown of water by ultraviolet radiation.

Distinguishing between these two models would require more information about the planet's radius or composition. The planet does not appear to transit its star, which makes obtaining this information beyond current observational capabilities.