The planet

Eurus

Eurus orbits both stars in a binary system consisting of one K type star and a smaller and dimmer M dwarf. The planet has no moons, and has a very extreme axial tilt of around 90 degrees. This makes its seasons much more extreme than those of Earth’s, with anywhere even a small distance from the equator experiencing summers where the sun never sets and winters where it never rises. Near the equator, the sun stays close to the horizon at these times of year. 

The temperature extremes on the planet during summer and winter are made worse by the fact that the oceans are relatively shallow, and unlike Earth the planet has no single global sea. Because of this, the sea is relatively inefficient at distributing heat.

This axial tilt gives the more hospitable equatorial regions of the planet two main seasons; a windy season, when one hemisphere is experiencing summer and the other winter, and a still season, during autumn and spring. The day-night cycle is normal during the still season, and with less temperature variation winds are much lighter, although still stronger than those of Earth.

There are great variations in the sea level throughout the year, as a result of the temperature variations. When one hemisphere is experiencing winter, a large ice cap forms, and the seas retreat. However, once this ice cap begins to melt the dried-up sea beds flood and fill with water again. The relatively flat topography of most of the planet’s surface adds to this affect, since not much water needs to be lost for a significant change in the coastlines to occur.

Most of what appear to be continents are actually more similar to the sea beds of Earth, only permanently exposed. The actual continents have a much higher topography and cover a much smaller fraction of the surface. Two continental masses currently cover either pole, so the temperature variations in those places are greater than they would otherwise be, with the altitude high enough for the air to be significantly thinner there.

Apart from these differences, Eurus is very similar to Earth in a lot of ways, and as a result the life is too, at least in terms of their biochemistry.

Like Earth, Eurus is a liquid water planet, with oceans consisting of the substance and most life using it as a solvent. However, there is a significant amount of ammonia in the atmosphere, and some organisms close to the poles incorporate it in the cytoplasm of their cells to prevent them from freezing and rupturing during the harsh winters.

Plants use carbon dioxide as a carbon source, just like the plant life on Earth, and release oxygen into the atmosphere. This gives the atmosphere enough oxygen to sustain oxygen breathing animal life, although the levels are lower than those of Earth.

Properties

Physical

Mass: 1.59 Earth masses

Radius: 7615 km
Surface area: 728.8 km²
Density: 5.13 g/cm³ 
Surface gravity: 1.11 g

Atmosphere

Dry composition:

Nitrogen: 91%
Oxygen: 7.2%
Argon: 0.89%
Carbon dioxide: 0.51%

Ammonia: 0.022%

Methane: 0.013%

Hydrogen: 0.0011%

Atmospheric pressure: 2.13 bar

Motion

Rotational period: 32 hours

Axial tilt: 87^o
Orbital period: 128.4 days (96.3 local days)
Semi major axis: 72 million km

Etymology

The planet is named after Eurus, the Greek god of the east wind.