Photosynthesis is the biochemical process in which energy from sunlight is converted by plants, algae, and some bacteria into sugars, which are used by the organism as food. That is, these organisms convert the energy of the sun into a different form of energy.
However, there is a least one exception: a little bacterium deep under the Pacific Ocean which manages photosynthesis without sunlight.
The bacterium's survival rests on dim light generated by the heat in hydrothermal vents close to eight thousand feet deep in the ocean. Sunlight may penetrate as deep as six hundred or so feet into the ocean, but not eight thousand.
The discovery has changed what we understand about the requirements for life.
Maybe photosynthesis is taking place in other previously unexpected places too, like under the ground. If light other than sunlight can support photosynthesis, then maybe, just maybe, it's taking place on planets that don't have a star like our sun.