Planets without a sun, also known as rogue planets or free-floating planets, do not have a source of light like a star. They are not gravitationally bound to any star and instead wander through space on their own. However, despite lacking their own internal light source, these planets can still have some form of illumination.
There are a few ways rogue planets can receive light:
Reflection: Just like any other object in space, rogue planets can reflect light from nearby sources, such as distant stars or other celestial objects. Even though they are not orbiting a specific star, they may pass through regions where starlight is present. The reflected light from these external sources can provide some illumination to the planet's surface.
Heat: Rogue planets can emit their own faint glow in the form of infrared radiation. Planets generate heat due to residual internal heat from their formation processes, as well as from gravitational interactions with other objects. This thermal radiation can be detected and observed by specialized instruments capable of detecting infrared light.
Auroras: If a rogue planet possesses a magnetic field, it can interact with the charged particles in space and create auroras. Auroras are natural light displays caused by the interaction of the planet's magnetic field with charged particles in the planet's atmosphere. This phenomenon can produce a dim, atmospheric glow even without the presence of a nearby star.
It's worth noting that rogue planets exist in a wide range of sizes, compositions, and environments, so the amount and nature of their illumination can vary significantly depending on individual characteristics.