Hephaestian type worlds

Hot planets with extensive vulcanism

Hephaestean type
Image from Steve Bowers
Hephaestus, a thermohephaestian planet with a thick atmosphere, in the Tau Ceti system

These planets have very active vulcanism, with large numbers of volcanoes and a geology that changes on a yearly basis. Some hephaestian worlds have plate tectonics with highly active subduction zones, where volcanoes cluster near the plate edges; others have drip-and-plume volcanism, or shield volcanoes which form above localised hotspots beneath the largely static crust.

The atmospheres of these planets vary greatly according to the world's size and mass, from having thick, Cytherean-like atmospheres to almost non-existent ones, where the feeble gravity loses any elements almost as soon as they are released from the surface. Composition of these atmospheres is dominated by chemical species originating from volcanic erruptions and outgassing events. Depending on the makeup of the planet various sulfur oxides, carbon oxides, hydrocarbons, halocarbons, water, and both molecular and atomic oxygen and sulfide have been recorded as major components of Hephaestian atmospheres. The chaotic hostile environment of Hephaestian worlds often proves a challenge to survive in.

These worlds are generally heated by proximity to a star due to direct radiation (subtype ThermoHephaestian) or by tidal flexing as a moon of a gas giant (subtype EuHephaestian). Some worlds are heated significantly by tidal effects from their planetary neighbors. As is often the case there are a number of rare outliers worlds that don't neatly fall into a classification. This include Hephaestian that are heated by an unusually large number of radioactive isotopes and those that sustain prolonged bombardment by impactors.

Early in their life cycle many naturally-occurring terrestrial type worlds start as pyrothallassic worlds with lava oceans, then pass through a Hephaestian phase. These hot worlds cool down on the timescale of tens of thousands to millions of years; some go on to develop various kinds of plate tectonics, while others develop stagnant-lid crust features which are much more stable. Many pass through several stages of this kind over their history, dependent on internal and external factors.

Examples: Io (EuHephaestian), Ixion, Hephaestus (ThermoHephaestian).

Ixion (Alpha Centauri Bb)
Image from Steve Bowers
Ixion, a thermohephaestian planet with a thin atmosphere, in the Alpha Centauri system
Related Articles
Appears in Topics
Development Notes
Text by Mark Ryherd
Initially published on 11 May 2013.