Eridani9

Eridani9 is a fictional exoplanet described within worldbuilding and speculative fiction centered on the Eridani star system. In the in-universe catalog, it is the ninth planet orbiting the primary star, Eridani A, and is characterized as a terrestrial world with substantial surface water.

Orbit and physical characteristics are described as follows: Eridani9 orbits at roughly 0.95 astronomical units, completing a year in about 336 days. Its orbit is nearly circular, with an eccentricity around 0.04. The planet has a radius of about 1.08 Earth radii and a mass near 1.25 Earth masses, yielding a rocky composition and a density similar to Earth. These attributes support a geologically active surface with tectonic features and volcanic activity.

Atmosphere and climate are depicted as nitrogen-dominated with a surface pressure near 1.0 bar, including trace water vapor and carbon dioxide. The modeled greenhouse effect produces a temperate average global temperature in the mid-teens Celsius, with regional variations due to latitude and albedo. About half of the surface is described as being covered by oceans, with continents and shallow seas creating diverse climates and weather patterns.

Discovery and naming are fictional: Eridani9 appears in datasets and narratives created for the Eridani system, designated as the ninth planet in the sequence. In-universe missions and probes are described as having studied the world remotely, with no confirmed in-situ exploration in canonical sources. The planet serves as a case study for discussions of habitability, planetary geology, and ocean-bearing terrestrial worlds in the Eridani context.