Pyrothalassic Type

Lava Worlds

Image from LordOther
Chariot, a lavaworld near the luminous star Algorab
Pyrothallassic worlds have extensive seas or oceans of molten rocky crust, as opposed to Hephaestian type worlds worlds, which are distinguished by extensive volcanism without the formation of lava seas. The surface of a pyrothallassic world will typically be completely replaced in a matter of years.

Most pyrothallassic planets have thick, vapour-ridden atmospheres which help to insulate the surface and inhibit cooling; but some worlds (especially low-gravity lava worlds) have relatively thin envelopes with intermittent cloud cover.

Planets of this type may have one, or several, different sources of heat that causes the surface to become partially or fully fluid. Very young planets and those which have recently suffered a major impact may have extensive lava fields for extended periods, until the planet cools down (unless the world is subject to more impacts). Other planets are subject to extreme tidal stresses from nearby objects, and these may retain molten seas for an extended period.

Lava worlds fall into three main subtypes; the first has a relatively thin ocean of newly extruded lava which has erupted from a solid crust, which remains in place. Under the crust is generally a fluid mantle which itself overlays a solid core.

The second subtype is hot enough for the entire crust to have melted as well; this type is fluid all the way to the solid core, which remains solid because of pressure. The third type is fluid all the way from the surface to the core; this subtype of planet is generally smaller than other worlds of this type.

Pyrothallassic lavaworld
Image from Steve Bowers
An evaporating planet which is very close to its star, with extensive lava fields
A significant number of pyrothallassic worlds are heated by close proximity to the local star. In many cases these planets are gradually moving closer to their parent star, due to various momentum exchange mechanisms, or because of friction with particularly strong solar winds. Planets which started off as gas giants or neptunians, then migrated inwards while losing their atmospheric envelopes are known as chthonian worlds; some such worlds eventually become rocky, pyrothallasic remnants with molten seas.

Another mechanism which can cause a planet to become pyrothallassic is the normal progression of a star out of the main sequence, a process which often leads to severe heating of the innermost worlds in a solar system. Such planets are often engulfed by the star if and when it becomes a red giant.

Image from Steve Bowers
Kepler-10b, a pyrothallasic world with a mass 4 x Earth's and a temperature of 1800K
Hot worlds which retain a deep liquid and/or supercritical water layer are known as hycean worlds.
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Development Notes
Text by John M. Dollan
Additional material by Steve Bowers
Initially published on 20 November 2008.