Frey

Frey is the fourth planet in the Vana system in the lower Cygnus arm of the Milky Way, and the homeworld of the freyan race. It is one of the few chlorogaian worlds, and the only one known to have generated sentient life. The Vana system possesses twelve planets, the last two being distant Neptunian giants. Its star is a large and luminous white giant.
Its atmosphere contains a high fraction of free chlorine, and its seas are rich with hydrochloric acid; it has roughly 69% ocean cover. It is a highly geologically active world, with many volcanoes and hot springs, and rapidly-shifting tectonic plates. Otherwise it is geologically a fairly conventional terrestrial world with a solid inner and liquid outer iron core, a silicate and titanium-rich mantle, and a crust composed heavily of silicates and rich in titanium, chromium, and calcium, a result of its parent star's high metallicity. It is because of the metallicity that Frey has a high proportion of chlorine, resulting in surface deposits of chloride rocks, atmospheric chlorine, and an acidic hydrosphere. It has an atmosphere of 74.1% nitrogen, 20.4% oxygen, 5% chlorine, and 1% other trace gases. The inability to form an ozone layer is offset by the atmosphere's density, the hazy effect of the chlorine and other greenhouse gases, and geomagnetic effects to protect the planet from the worst of Vana's stellar radiation.
Frey developed complex life around two billion years ago, evolving from sea-dwelling microbial life. Its life developed to make use of hydrochloric acid as well as water to absorb hydrogen; the combination of this life cycle with chlorine-spewing volcanoes replenishes the free chlorine in the atmosphere, while the low-settling chlorine normally reacts with water to form hydrochloric acid. Its surface developed moss, lichens, and woody flora around its many marshy areas. Its seas evolved pseudo-molluscs as the dominant form of life; some evolved for land dwelling, crawling or shambling in the marshlands as early as 350 million years ago. The ancestors to the freyans evolved as hunter-gatherers, reaching anatomical modernity around a quarter of a million years ago and developing agriculture by 8,000 BCE. They great wealth of resources on their world, readily revealed by the acidification of the surface rocks, led to early metallurgy and development of complex civilization. Industrialism arose in the 1200s CE.