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Halogenic Worlds

aka Chlorogaian Worlds

Chloroagaian 1
Image from LordOther
Doreen, a chlorogaian world, with a biosphere that both produces excess chlorine and utilises it as a life-supporting element

Halogenic worlds are a variety of modified planet, terraformed 780 million years ago by a now-extinct xenosophont civilization that Terragen researchers have dubbed the Halogenics. 356 of these worlds are currently within the boundaries of the Terragen Sphere, with many more outside it and observable via astronomy.

Despite the unusual abundance of chlorine on their surfaces, in some ways Halogenic worlds are not too dissimilar from AquaGaian planets. Their oceans consist mostly of water, but include a small portion of hydrogen chloride (HCl). Their atmospheres are primarily nitrogen and oxygen, but also a small fraction of molecular chlorine (Cl2). Other chlorine compounds, such as carbon tetrachloride (CCl4) and other organochlorides, are also usually present in the air and seas, although details vary depending on divergent evolutionary pathways. Such substances are strong greenhouse gases; as a result, Halogenic worlds are found further away from their stars than typical AquaGaian planets.

The geology of a Halogenic planet is very different from a typical AquaGaian. On an Earthlike planet, HCl and Cl2 are sequestered millions of times more readily than oxygen (O2); hence, even in the rare cases where organisms evolve to emit such molecules, they quickly react and cannot build up in the environment. On a Halogenic world, however, the geology has been engineered to avoid sequestration. Planets engineered by the Halogenics were geologically dead, and were artificially resurfaced with a coating of silica (also known as quartz) as part of the terraformation process. This is among the few compounds that do not react with HCl and Cl2.

The 356 Halogenic worlds in Terragen space have evolved in a variety of ways since their creation. Mountains have typically been worn down by erosion to a large degree, and no replacements have formed due to the dead geology; as a result tall mountains are rare or nonexistent, while shallow seas are abundant. Some have remained garden worlds, their seeded ecologies continuing to prosper and evolve, each exhibiting their own unique patterns of convergence and divergence. Many others have undergone severe mass extinctions, eliminating all macroscopic biota or even, in rare cases, all microbial life as well. Impacts or other astronomical and planetary factors have sometimes led to a restart of geological activity, leading to a dechlorination of the atmosphere and hydrosphere as chlorine gets bound up in rocks and salt. This is a common cause of mass extinction, although in some cases a few relict biota can adapt to the new conditions and in one case the biosphere evolved macrobiota anew.

Chlorine World Atmosphere
Image from Steve Bowers and John M Dollan

Halogenic Terraformation Process

Although there is naturally some uncertainty, careful geological study and comparative analysis of multiple Halogenic worlds has produced what is believed to be a mostly clear picture of the terraforming process used to create them.

First, the Halogenics selected a suitable terrestrial planet, likely using astronomical observations and perhaps flyby probes to confirm that the target world met their terraforming criteria. It appears that two factors were paramount: first, that the world was sufficiently large to retain an atmosphere over geologic timescales (although a pre-existing atmosphere was not required), and second, that the world was geologically dead, with no evidence of ongoing volcanism or tectonics, a requirement that favored small planets. Relatively few planets meet both requirements, which perhaps explains why more were not terraformed in this way. The presence of a pre-existing salt water ocean or perchlorates was apparently beneficial, but not required. Planets chosen had an insolation of around 0.1 to 0.5 times Earth, apparently accounting for the greenhouse effects of the resulting chlorocarbons.

Once a planet was chosen, specialized terraforming neumanns were dispatched to it. After multiplying into a planet-wide swarm, they proceeded to disassemble and reprocess all surface rocks, regolith, ices, and seas, as well as any excess components of the atmosphere. Undesired elements were combined into solid compounds and buried at depth. This waste layer was covered by an impermeable layer of silica. At the surface, an artificial topography was created, with ocean basins, smaller seas, mountain ranges, and other features such as would be found on an Earthlike world. Some additional minerals and other compounds were included in the surface regolith to help sustain the future surface ecosystem.

The seas and oceans were filled with water and a small portion of HCl. The atmosphere had large portions of O2 and a few dozen parts per million Cl2 added to it. A small portion of these compounds reacted with surface materials to produce chlorinated minerals and regolith, but the vast majority remained stable thanks to the silica crust. Although the planet usually lacked a magnetosphere, photolysis and loss of chlorine to space was mitigated by the high weight of the element, and other compounds were generally retained due to the planet chosen being sufficiently large.

In the final stages of establishing the geology and hydrosphere, the planets were seeded with an ecology of both microbial and macrobial organisms. In each case, although common descent is evident from the biochemical pathways involved, it appears that each world's initial set of multicellular organisms was unique. Whether all of these organisms were gengineered or neogenic, or at least some were taken from the Halogenics' original biosphere (if one ever existed), is not known. However, it is clear that each ecology was set up to be in balance with no major extinctions shortly after seeding, and to sustain planetary chemical cycles involving nitrogen, oxygen, chlorine, and so forth. Chlorine incorporated into the regolith would become part of the flora and fauna, and in turn be decomposed and return to the air and seas.

With the terraformation complete, the neumanns remained present for some millennia longer, likely monitoring the environment for emergent instabilities and attempting to adjust as needed. After a period of time, however, the neumanns entered the final phase of their mission, burrowing deep into the crust and deactivating, to be buried permanently and never again affect the surface ecosystem.

On most of the worlds, this occurred as planned. However, in some cases, errors in the neumanns' programming propagated and led to evolution in their goals, creating a mechosystem. In most such cases, this process was not very efficient, and after at most a few hundred thousand years, the unplanned cyberlife went extinct. This was what happened on Doreen, with the remains of the neumanns found on the surface or buried in relatively shallow sediments thought to have possibly inspired the now-extinct sophonts that evolved there as they developed their own technology. On Vert, however, the neumanns' programming proved especially evolvable, and the resulting mechosystem was adaptable enough to survive alongside the biosphere all the way to the present day.

Chorus Xenoflora
Image from Chris Shaeffer
The purple xenoflora and green skies of Chorus

Biology

All known life forms on Halogenic worlds exhibit the same fundamental biochemistry. This, together with some consistent aspects of cell morphology, is considered to be a strong sign of common descent. As on most AquaGaian garden worlds, life on Halogenic worlds produces chemical energy from sunlight by using it to reduce available hydrogen-bearing compounds, and the most common hydrogen donor is water, simply because it is so abundant. This form of photosynthesis releases oxygen. However, Halogenic worlds also have a significant stock of hydrochloric acid, and photosynthetic organisms also make use of that resource, and release chlorine. The splitting of water and hydrochloric acid both release hydrogen ions and high-energy electrons, which are then used to produce carbohydrates and other organic compounds. The usual carbon source is carbon dioxide. There are therefore two dominant kinds of photosynthesis on Halogenic worlds:

2HCl + CO2 ---> CH2O + Cl2

in which hydrochloric acid and carbon dioxide are consumed and organic compounds and chlorine produced, and

H2O + CO2 ---> CH2O + O2

the far more common process familiar from Terragen and similar biochemistries.

Which metabolism is used varies by clade, with many able to facultatively use both. Release of chlorine also adds to the oxygen in the atmosphere, since the chlorine reacts with water to release oxygen and produce hydrogen chloride again. The combination of the various oxygen and chlorine releasing photosynthetic pigments is typically purple or purplish-black to human eyes. Respiration on Halogenic worlds is the reverse of photosynthesis, and many organisms are capable of using either chlorine or oxygen as fuel, although there are variations between clades and species. Such fauna breathe out not only carbon dioxide and water but also hydrochloric acid.

All life on the known Halogenic worlds shares the same genetic code. The detailed chemistry differs, but the genetic molecule is a double-stranded polymer remarkably similar in structure and function to DNA.

Chlorocarbons are abundant in the biosphere, and participate in many biological pathways (unlike on many AquaGaian garden worlds, such as Earth, in which natural chlorocarbons are only rarely created or consumed by the biosphere). Some particularly resistant chloride polymers are used by land-dwelling life forms to protect themselves from excessive concentrations of hydrogen chloride or from pure water, either of which are harmful to their tissues. Flora usually has waxy leaves and bark composed of resistant organochloride polymers. Fauna have similar resistant coatings, and may become temporarily dormant if local conditions are particularly poor.

On most Halogenic worlds, swamps and shallow seas are common biomes, as a result of 780 million years of erosion wearing down the topography and depositing sediment into the seas, creating many expanses of flat land and shallow water.

Terragen Impressions

To Terragens accustomed to Earth-like environments, a typical chlorine world is not just extremely uncomfortable but deadly. Typical daytime temperatures are 40 to 60 degrees C despite the relatively low insolation, due to the strong greenhouse effect of atmospheric chlorocarbons. Light at the surface is dim and greenish due to the atmospheric chlorine and haze. The chlorine content of the atmosphere and water is toxic to most bionts, and the numerous chlorocarbons (which include small quantities of chemicals such as chloroform and mustard gas as well as a number of more subtle toxins such as PCBs) are deadly. Even a small quantity of chlorine world atmosphere leaking through airlocks is unpleasant. The rains and seas are also harmful, not only to bionts but also to vecs, as well as to many aspects of Terragen technology. Standard nano- and swarmtech may fail to function entirely (although specialized versions operate perfectly well) and diamondoid eventually erodes. The bodily fluids of local life forms (where present) are often strongly acidic; something that would be harmless sap or spit to the local biota may be corrosive to visitors. Despite this, Halogenic worlds have their own unique beauty to many enthusiasts of alien environments, and exhibits and virches of them can be found across Terragen space.

Chlorogaian
Image from LordOther
Tytalus, a Chlorogaian near Sol

Halogenic Worlds in the Terragen Sphere

Halogenic worlds are scattered throughout Terragen space and well beyond. In the time since their discovery most of the Halogenic worlds, especially those still hosting life, have been placed under protection by one or another of the Caretaker Gods or by transapients with similar inclinations. In the Sephirotic empires this is a fairly civilized process, and involves invitations or appointments of such guardians. Elsewhere the Caretakers have sometimes used force to displace would-be colonists in a system. Most colonists on the Periphery avoid moving into a system that contains a Halogenic world, since in some cases the Caretaker will place not only the world itself but the entire stellar system under Compact of Eden regulations.

A few Halogenic worlds have been put through varying degrees of 'restoration'. Depending on the planet and the preferences of the governing sophont(s), this may involve restoring eroded topography, recirculating buried elements, or even lazurogenics to restore extinct organisms or biospheres.

Due to stellar motion and the time elapsed since their formation, Halogenic worlds have become scattered quite widely in the galaxy, and to date most of them remain beyond Terragen space, though more are reached as time passes. The most notable Halogenic worlds are Outland and Tytalus (the first two discovered, whose similarities led to the Halogenic hypothesis), Chorus (home of the xenoprovolve Jade Chime Singers), Doreen (which hosted a now-extinct locally evolved sophont civilization), and Vert (home to the sole known surviving mechosystem).

 
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Appears in Topics
PlanetologyXenobiology
 
Development Notes
Text by ProxCenBound, Antares, Andrew P., and Stephen Inniss
Revised 17 April 2026 from a previous version by Stephen Inniss. Original 2000 version was by Anders Sandberg.
Initially published on 30 July 2000.

Purple xenoflora image by Chris Shaeffer added 2011
Tytalus image by LordOther added 2020
 
Additional Information
Worldbuilding disclaimer; because chlorine is an extremely reactive element, it combines almost immediately with the crust and hydrosphere of any natural planet, so halogenic worlds are extremely unlikely to occur in nature. In order to introduce the concept of chlorine-rich biospheres in Orion's Arm it is necessary to postulate an exceptionally efficient system of (alien) artificial environment homeostasis, and a complete redesign of the planetary surface to accommodate it.
 
 
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