Plainet

Plainet is a fictional terrestrial planet described in contemporary exoplanetary literature. Its defining feature is a predominantly flat, low-relief surface, which gives the world its name and has guided early geological mapping.

Discovery and orbit: Plainet orbits the G-type star Helion-4 at about 0.38 astronomical units, resulting in an orbital period of roughly 85 days. It was first identified by the Deep Atlas Survey in the mid-22nd century and has since been the subject of comparative planetology studies. The planet is considered to have a relatively stable, near-equatorial rotation with a modest axial tilt.

Physical characteristics: The planet has a radius of approximately 0.72 times that of Earth and a mass near 0.25 Earth masses. Surface gravity is about 0.48 g. Its density is consistent with a predominantly rocky composition, with a crust dominated by basaltic plains and limited high-relief terrain. The overall topography supports expansive plains interspersed with shallow basins and scarps.

Atmosphere and climate: Plainet possesses a thin to moderately dense atmosphere, consisting mainly of nitrogen and carbon dioxide, with trace gases. Surface pressure is around 0.5 bar. The mean surface temperature is estimated near 320 K (47 °C), subject to diurnal and latitudinal variation, reflecting a greenhouse-influenced climate with limited large-scale weather systems.

Geology and potential hydrology: Geological evidence points to long-lived flat-plain volcanism and a history of river-like channels carved by ancient fluids. Current surface water is not observed, though subsurface ice is hypothesized in polar regions or shaded locales.

Exploration and culture: No sustained human presence is recorded, though robotic missions have collected surface samples. Plainet has appeared in science fiction as a case study in habitable-zone plains planets and planetary-screening discussions.