Wednesday, May 13, 2015

Super Volcanoes on a Hot Alien Planet

55 Cancri e is a super-Earth with 2 times the diameter and 8 times the mass of Earth. It orbits extremely close to a Sun-like star, taking only 18 hours to complete an orbit. 55 Cancri e is about 25 times closer to its host star than Mercury is to the Sun. Being so close to its host star, the dayside of 55 Cancri e is superheated to an average temperature of roughly 2,400 K. At such a high temperature, the atmosphere of 55 Cancri e is puffed up and probably too hot for clouds to form.

Analysis of old and new data from observations of 55 Cancri e by NASA’s Spitzer Space Telescope showed that the thermal emission from the planet’s dayside atmosphere varied by ~300 percent from 2012 to 2013. This variation was detected at a 4σ confidence level and corresponds to temperatures varying from ~1,300 K to 3,000 K. A likely explanation for this variability is the presence of intense volcanic activity that is spewing enormous plumes of gas and dust.

These plumes can reach heights of up to a few thousand km above the planet’s surface where they cool and spread out, in the process, obscuring the underlying thermal emission from the planet. Plume heights of a few thousand km appear otherworldly when compared with volcanic plumes on Earth. However, such plume heights are quite plausible since Jupiter’s moon Io can spew volcanic plumes with heights of up to roughly 300 km to 500 km, or 0.16 to 0.27 times the radius of Io itself.

The high temperature on 55 Cancri e results in a weak planetary crust, perhaps even a molten surface. This is probably the reason behind the intense volcanic activity that is thought to be occurring on 55 Cancri e. Small exoplanets (i.e. about the size of Mercury) that are highly irradiated can have material simply erupting directly into space. However, such a mass-loss is unlikely for 55 Cancri e as it is much more massive and has a much stronger gravity. Instead, material from the volcanic plumes on 55 Cancri e will fall back onto the planet’s surface.

Demory et al. (2015), “Variability in the super-Earth 55 Cnc e”, arXiv:1505.00269 [astro-ph.EP]