My imaginary planet Eden has no sun. It is not even in a galaxy, and the entire Milky Way galaxy can be seen from it as an immense starry swirl in the sky. However like the earth it has a hot core, and this heat has provided the energy for life.
When I originally dreamed up this planet (for the short story ‘The Circle of Stones’) I had no idea whether such a thing actually existed, but I have since discovered that, yes, it is a serious possibility (see: ‘Sunless but livable planets may be detectable‘). And after all, even on Earth, there are life forms that do not derive their energy from the sun.
Eden (I envisage) originally had a rocky surface entirely covered by ice. Life evolved in pools of meltwater under the ice. (Sub-glacial lakes, such as Lake Vostok in Antartica, are a phenomenon on Earth: the ice above provides insulation for the geothermal heat). One large life form that emerged was able to pump sap down into hot rocks underground in order to heat it up, and then pump it out through a network of roots or branches extending upwards and outwards through the ice, melting it for oxygen and nutrients, and so creating spaces for other life-forms to exploit.
Eventually considerable areas of the surface of Eden were completely cleared of ice and these large life-forms emerged into the newly formed atmosphere, and began to exploit the potential of this new open environment. Animal-like life forms emerged into the open with them, along with parasitic plant-like forms.
By the time humans arrived on Eden, huge areas of the surface were covered with forests of ‘trees’, which hummed constantly with the sound of sap being pumped up and down from hot rocks far below the surface. Like deep sea life-forms on earth, animals and plant-like forms made use of bioluminescence. The trees have luminous flowers known to the human settlers as ‘lanterns’, which attract the flying creatures they know as ‘bats’, ‘birds’ and ‘flutterbyes’. Streams and pools are also full of luminous life, as is the ocean which its human discoverers call Worldpool, for they have lost the word for ‘ocean’ through lack of use.