Stevensonian Class Worlds

Planetary mass objects found in interstellar space

Stevensonian class
Image from John M Dollan and Steve Bowers
Slippery Dave, a Stevensonian class planet ten light years from Antares

Stevensonian type worlds, also known as Rogue planets or Planetary Mass Objects (PLANEMOS), are non-luminous worlds which reside in interstellar space. Such planets may form from a protoplanetary cloud by the process of core-collapse without actually orbiting any star in particular; but this is relatively rare, and most such worlds were originally part of a solar system but have been subsequently ejected by interactions with other bodies.

Deep space worlds of this kind are significantly more numerous than planets which remain within solar systems, but are more difficult to find.

The formation of a solar system from a protoplanetary cloud is a chaotic process. Dozens or even hundreds of planetoids may form, which collide with each other and gradually increase in mass. But as the millions of years continue, a process of attrition occurs, where most of these worlds are eventually lost. Many are shattered and incorporated into the growth of other worlds. Some are tossed into the central sun. And a great many indeed are tossed out into the emptiness of interstellar space.

For the most part, these are cold and lifeless worlds, whatever atmospheres that they might have once had long since frozen out onto the surface and as hard as granite. Even a helium atmosphere would collapse to liquid form at the temperature of the Cosmic Microwave Background.

But some worlds remain heated by their internal fires, forever dark wanderers that glow in the infrared like the last ember of a camp fire. Other worlds are gas giants, shrunken and quiescent, the tops of their clouds lost in perpetual blizzards, while in their depths the heat of their mass continues unabated, churning the atmosphere below like a boiling cauldron. Some of the coldest gas worlds in deep space have helium atmospheres with white hydrogen clouds; but many others are heated by internal radiation or by ongoing gravitational collapse.

Some Stevensonian worlds have large rocky or metallic cores, and these cores are quite often warm and even remain geologically active for billions of years. The decay of radioactive elements in the core of an Earth-like can produce about 0.0001 as much energy at the surface as would be received from sunlight in the habitable zone, so the amount of energy that reaches the surface is small but not negligible. A fraction of such worlds have reservoirs of liquid water beneath layers of ice, and hot vents producing localised sources of energy where chemotrophic biospheres may emerge. Such localised biospheres are usually isolated from one another, and may follow quite divergent evolutionary pathways accompanied by very rare mixing events.

It is an irony that these worlds are very common indeed, with an average solar system perhaps losing fifty or more such worlds in the first hundred million years of formation. Indeed, they form an appreciable amount of the dark mass of the universe. But they are difficult to find, utterly lost to the blackness and detectable only by the faint gravitational effects on light that the larger massed worlds have.

These worlds are prized by Hiders and Backgrounder clades, especially the worlds with internal heat sources. It is rumoured that a large number of Hider worlds of this type exist between the stars, unknown to even the Sephirotic Archailects; but it is also said that if the Hiders can find them, then so can the AI Gods.

Examples of Stevensonian Class worlds in deep space; Niaanyo, Hyxuym
Niaanyo from space
Image from Steve Bowers
Image from Steve Bowers
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Development Notes
Text by John M. Dollan and Steve Bowers
Initially published on 30 December 2009.


Letter to Nature by David Stevenson:
The Possibility of Life-Sustaining Planets in Interstellar Space