Cthonia by Holbenilord
System Eternia System
Orbital Position 2
Moons One
Notable Species Cthonians
Map Map of Cthonia
Class SHH

Cthonia is a class SHH terrestrial planet orbiting a K9V-class star in the Viperius Galaxy.


Cthonia is the second planet from the 'orange dwarf' star Eternia, in the western rim of the Viperius Galaxy.

Planetary CharacteristicsEdit

Cthonia is a habitable SHH-class planet, with a complex biosphere.


Approximately 0.75 AU from its star, Cthonia sits in the far edge of the habitable zone. It is capable of supporting life only because of its size, which allows it to have an atmosphere richer in carbon dioxide and of a greater pressure than Earth.


Cthonia has an orbital period of 36.50 hours, and an axial tilt of 15.1 degrees.


Although it is 3.57 times Earth's mass, it is only 0.94 times the density, so the gravity at the surface of the planet is 1.47G. This is due to its smaller iron core, though it still has a predominantly silicate crust.


Its surface is 46% water and 2% ice, anomalously low. It is thought this is due to the low number of comets striking the planet in its early life. The less extreme tectonic activity means there are fewer volcanoes and mountains, and the stronger pressure means wind erosion occurs more rapidly. The minimum surface temperature is -30 degrees C, the average is 28 degrees C, and the record high is 56 degrees C.


Cthonia's atmosphere is similiar to Earth's, at 71.5% nitrogen, 25.0% oxygen, 1.9% argon and trace other gases, including carbon dioxide, ozone, and water vapour. The atmosphere is dense, with a pressure 1.54 times Earth's, and highly stable.


Cthonia has a large moon, approximately two thirds the mass of Luna, orbiting at 350,000km. This moon was captured as it was flung from the outer solar system, and is as such of a different composition from the planet it orbits.

It also has a thin, fading ring system at 140,000km.

History Edit

Cthonia began forming around 4.1 billion years ago. The Great Bombardment did not effect Cthonia as severely as it did Earth, but during its formation Cthonia captured its moon.

Cthonia was a primordial garden world for billion of years, as life developed on its surface. Photosynthesisers began to produce oxygen, which oxygenated the surface and caused a great extinction of most families of life, unused to this new reactive chemical- the survivors began to adopt carbonate shells around their cells for protection.

The Fanisids, a family similiar to Earth's cyanobacteria but also our fungi, spread around the world around 2 billion years ago. Multicellular life forms developed around 250 million years ago. Predators and prey developed soon after.

Exantids began to grow on land only a few hundred million years ago, and this new food source was an incentive for the sea's herbivores to escape their predators. Then, the carnivores developed legs too.

Exantids have been growing more sophisticated since, and so have the animals. Now, they are divided between three groups (Tigrids, Kineks and Kivae), taking different roles in their ecosystems. Aquatic diversity has also increased, with Tagans, Anikens, and various Cthonian Plankton-organisms.