Abyormen

Abyormen is a terrestrial planet slightly larger than Earth located in the Pleiades star cluster. It is the only planet in a peculiar binary star system, and has a unique orbit which renders it with two alternating, radically different climatic seasons, each lasting for about 65 Earth years (or 830 local years). Because of this, Abyormen's native lifeforms are divided in two opposite groups: the "hot life" remains active during the "hot" times only, undergoing massive death when the "cold season" begins; whereas the "cold life" does the opposite.

Appropriately, the planet is inhabited by two sapient species, both of which are simply known as Abyormenites. The Abyormenites of the "cold season" are humanoid in appearance, while the "hot season" ones are radially-symmetric, eyeless beings.

Astronomical data
Abyormen orbits a small red dwarf star known by the natives as Theer. Each Abyormen year (its rotation period around Theer) is very short, lasting no more than about 28 Earth days. Theer, in turn, is in a highly elliptical orbit around a massive blue star, called Arren by the natives and Alcyone by Humans.

Physical aspects
Abyormen is a bit larger than Earth and more geologically active. Even during the "cold" season there are active volcanoes and geyser areas. The oceans of Abyormen are not as big and rich in life as those of Earth, though there is evidence that they might have been in the past. The planet is orbited by an unspecified number of small moons, but none of them is large enough to produce a significant tide.

Most of Abyormen's terrain is volcanic. There is also an ice cap on the southern hemisphere, situated in such a place where Theer never shows up in the sky, and Alcyone rarely does so. This "dark spot" of Abyormen is the only place on the planet where the temperatures remain perpetually cold.

Abyormen's surface pressure and gravity are slightly greater than Earth's, but still tolerable by human standards.

Ecology
Every 65 Earth years, the planet and its red sun are brought close enough to the blue giant for a drastic climatic change to begin: the "cold" creatures die, leaving spores either on soil or (more commonly) in the bodies of the "hot" creatures which are starting to emerge, born from spores their ancestors had left inside the bodies of the "cold" creatures. This complex symbiotic cycle has been going for over 10 million years.

During the "hot" season, not only are the temperatures extremely high, but the atmosphere composition also changes, with thermophilic bacteria producing large quantities of nitrogen oxides. There is also a great increase in brightness, due to the proximity to the hot blue sun. The brightness is such, in fact, that sight becomes impossible, and the radially-symmetric "hot season" Abyormenites see by means of high-frequency sound waves. After 65 Earth years, this hot period ends and the "cold" one begins. The temperatures go down, the oceans return to their "normal" level and photosynthetic organisms restore oxygen to the atmosphere. The "hot" creatures then die, the dormant spores inside their bodies giving rise to a new generation of "cold" creatures.

History
It is believed that Abyormen was formed billions of years ago, near its sun Theer. Life soon started to evolve, including many lineages of large, six-legged animals. It was only in a relatively recent past, approximately 10 million Earth years ago, that the planet and its sun entered the Pleiades cluster and were captured by the blue giant Alcyone. This resulted in the mass extinction of virtually all life and the beginning of the alternating seasonal cycle which has been going on since then. All extant lifeforms on the planet, therefore, including the two sapient species, evolved during the last 10 million years, meaning that Abyormenite lifeforms are among the most adaptable lineages known in the galaxy.