Fiction:Rana system

The Rana Bioring was an inhabited planetary ring system discovered by humanity in the 23rd century, in the Delta Eridani system (formerly known as Rana), 29 light years from Sol. While it was not uncommon to find microbial spores in planetary rings, this marked the only known ring system to sport a complex ecology with various domains of life which had migrated from elsewhere, adapted to the vacuum of space. It was one of the largest known self-contained ecosystems and is under protection of the Milky Way Cooperative.

Environment
Rana is a Jupiter-mass gas giant that orbits about 6 AU from Delta Eridani, an orange subgiant that will soon join the red giant branch. The discovery of life in this system was unexpected to humanity, found by smugglers who had chanced upon the rings in an attempt to hide from authorities.

Rana formed in a much earlier time, behind the 'snowline' where icy bodies can coalesce. The ring system formed an estimated 60 million years ago from a moon strayed too close to Rana's Roche limit, and so was pulled apart by the planet's gravity well. It is a relatively young ring system that is still over a 1000 metres thick in places, and has only recently (in geological terms) settled into a flat disc of divided rings. The material of the inner ring has an orbital velocity of 32 km/s. It almost entirely composed of water ice (99.2%), the rest being 0.7% carbon dioxide and nitrogen ices (N2), a 0.1% rocky material. The ice is laden with tholins, which are organic polymers composed of CO2, methane (CH4), nitrogen and so-on. There is a thin atmosphere of water vapour, molecular oxygen and hydroxide liberated from the action of ultraviolet radiation received from the host star. Between gaps in the rings there are shepherd moons which clear the material in their orbits. Some of the moons sculpt waves and even mountains of ice particles that cast long shadows. There are also more significant satellites; Rana C orbits elliptically because it is in a 2:1 orbital resonance between Rana D. It constantly ejects plumes of water vapour that feed the rings, and might be a source for at least one domain of life in the ring system (see below).

System
Rana's most significant neighbour is Rana D, with a diameter of 9834.34 km. It has an atmosphere rich in CO2 and CH4, similar to Archean Earth. Thought to former Titan-like world, the host star's evolution has warmed the atmosphere to the point that most of moon's surface is now covered by a deep ocean of liquid water except for the subequatorial extremes.

In the next few million years, Delta Eridani will have entered the red giant phase, where it is predicted to last a few hundred million years. It is unknown what effect this phase will have have on the Rana ecology. The increase in heat received from the star will cause even more ice on moons like Rana C to sublimate and feed the ring system. But the increased radiation pressure may also decrease the overall lifespan of the rings, by blowing away the atmosphere and smaller ice grains. It is possible life will migrate to the outer star system or beyond during this process.

Ecology
The Rana ring system is host to a rich variety of flora and fauna, many belonging to completely unrelated domains of life.

Biochemistry
There are two known forms biochemistry native to the rings (although with various subgroups): CHON and silane-based life. The former primarily use the basis of carbon-oxygen bonds and use water as a solvent. The latter consist of silicon-silicon polymers and use ethane and methane as a solvent.

Most of life present in the bioring are chemotrophs. They are slow metabolisers, using the violence of ultraviolet radiation to help them liberate useful compounds from tholins. CHON organisms make use of different metabolic processes. Methanogens react carbon dioxide with molecular hydrogen to form methane and water. The sulphate reducers react hydrogen sulphide with carbon dioxide, creating formaldehyde, water and sulphur byproducts. There are also some rare organisms that capture molecular oxygen to metabolise ammonia and carbohydrates. Rarer still is the so-called ammono life which are based on carbon-nitrogen bonds (another type of CHON) and use ammonia as a solvent. These organisms compete for resources with the nitrifying metabolisers.

Not all interactions between the various domains are competitive. Some silane-based animals have evolved symbiotic relationships with methanogenic microbes, exchanging methane for hydrogen and carbon dioxide. The compounds are contained in special membranes that allow methane in and keep oxygen and water out.

Diversification
The scientific consensus was that complex life would have migrated to the rings rather than originating there, due to the very young estimated age of the rings. It would have made use of microbes already adapted for survival. The CHON domains arrived from plumes ejected by Rana C and had already evolved complexity in its subterranean oceans. Silane polymers are destroyed by water, so life based on it could not share this common origin. It possibly migrated from Rana D while it was cold enough to have retained a hydrosphere of ethane and methane, the world is now too hot and poisonous for its existence. However, it is possible that the silane-based life originated elsewhere further back in time and had been transported to Rana D before it transformed.

It was reasoned that the diversification of complex life must have been a geologically recent affair and probably happened within a few million years, akin to the Cambrian Explosion on Earth. Once organisms adapted to survival in the vacuum, the explosion was brought about by the competition and cooperation of organisms for scarce resources.

Researchers identified unique lineages of life in the various rings, distantly related to each other (making an analogy to plate tectonics causing continent drift). As the rings stabilised, the material started to differentiate, and moons and moonlets opened up gaps in the rings. Life found it much harder (though not impossible) to migrate between gaps in the ring, largely stranding them in their rings where they specialised on the local chemical abundances.

Trivia

 * Sources of the planetary images is Space engine 0.980, created by Vladimir Romanyuk for non-commercial worldbuilding purposes.