While it is theoretically possible for planets to exist without a star or galaxy to call home, such planets are extremely rare and difficult to detect. These planets are known as rogue planets or free-floating planets.
Rogue planets are believed to form in a similar manner to regular planets, through the process of gravitational collapse within a protoplanetary disk. However, instead of becoming gravitationally bound to a star, these planets are ejected from their home systems due to gravitational interactions with other large objects or due to disruptions during the early stages of planetary formation.
In terms of supporting life, it is challenging for rogue planets to provide a stable environment for life as we know it. Without a nearby star, these planets lack a consistent source of heat and light, making them extremely cold and dark. However, there are some potential scenarios where life could exist on rogue planets.
One possibility is if a rogue planet retains enough internal heat from its formation to sustain liquid water beneath its icy surface. Such subsurface oceans, if they exist, could potentially harbor life forms adapted to extreme cold and darkness, similar to certain deep-sea organisms on Earth.
Another possibility is that a rogue planet could capture a nearby star, forming a binary system. In this case, the planet would receive heat and light from the captured star, allowing for more favorable conditions for life.
While these scenarios are purely speculative, they demonstrate that life on rogue planets is not entirely impossible. However, due to the challenges in detecting and studying these distant and elusive objects, we have limited knowledge about their prevalence and habitability.