The Planet

 

Amthalassa

Amthalassa orbits around two K type stars in a quaternary star system. It has one, small asteroid-like moon orbiting close to its surface, and is the only body in the planetary system to possess complex, diverse multi celled life. Although the planet is too cool to host liquid water, which is frozen on the surface, it still has large oceans clearly visible from space. Unlike Earth, these oceans consist not of water but of liquid ammonia, liquid at the low temperatures present on the planet. The planet's dense atmosphere consists largely of nitrogen, with some hydrogen and methane. 

It is a rocky planet, with a crust and mantle primarily of silicates, as well as an iron core. This iron core is molten, and the planet spins fast enough to generates a magnetic field protecting life on the surface from harmful radiation. It is also tectonically active. 

With higher levels of methane than carbon dioxide, the planet's equivalent to plant life uses the former as their primary source of carbon rather than the latter. This process releases hydrogen into the atmosphere; although this hydrogen is slowly lost into space, it is replaced at a fast enough rate for levels to remain stable. 

Animal life uses the hydrogen released by plants as an energy source, with their equivalent of respiration involving its reaction with unsaturated hydrocarbons. Plants often produce unsaturated hydrocarbons as a means of storing energy (this energy storage was important in the planet's dimmer past before aeroplankton consumed the tholin layer). 

The planet is quite volcanically active, providing life with a steady source of methane.

Properties

Physical properties

Mass: 0.664 earth masses

Density: 4.88 g/cm³

Diameter: 11,592 km

Surface gravity: 0.804g

Atmosphere

Nitrogen: 96%

Hydrogen: 3.5%

Argon: 0.2%

Methane: 0.01%

There are variable levels of ammonia vapour, up to 2% (the percentages shown above are for the dry composition). 

There are also smaller amounts of carbon dioxide, carbon monoxide, nitrous oxide, oxygen, and ethane (as well as other hydrocarbons) 

Atmospheric pressure at sea level: 7.52 atmospheres

Surface temperature: Around minus 50 degrees Celsius on average, below minus 70 near the poles or at high altitudes and minus 40 or higher near the equator.  

Orbit and motion

Semi major axis: 135 million km

Orbital period: 278 earth days

Rotational period: 19 hours

Axial tilt: 30 degrees

Etymology

The name Amthalassa is a shortening of ammonia thalassa, thalassa meaning sea in Greek.