A planet receiving a similar amount of sunlight as the Earth. It is watery and life-bearing. However, it is older and only slightly more than half as massive as the Earth. The land biota is impoverished, moribund, and only rescued from reversion to bacterial level by spasms of terminal volcanism, which temporarily warm and flood the planet, rejuvenating its biosphere and causing a rapid adaptive radiation of life forms. Worldbuilding began just after one of the last such episodes.
Seasonal variation is strong, both due to the planet's axial tilt and orbital eccentricity.
This world is irradiated by a similar level of sunlight as the planet Mars, but is a much more substantial body, weighing in at twice the mass of the Earth. Thus, it is still very geologically active, its carbonate-silicate cycle maintaining a 2.5 bar partial pressure of carbon dioxide in the atmosphere making the planet similar to Mars in its infancy. Most of Sucellus is covered by deep oceans, the only land masses are scattered island chains that represent the tips of numerous submerged volcanoes. No life has evolved here, but anaerobic life forms from Epona could be introduced easily. A satellite similar to the Earth's Moon is present with ice and frozen gases present in small polar caps.