A rogue planet is an exoplanet or planetar (a celestial body of planetary mass), that rather than orbiting a star instead floats in the void of space. Technically, they orbit the center of the galaxy but they do so from a far greater distance than the mental image that statement may conjure. It's called "rogue" because it moves to the beat of it's own drum, instead of marching in place with stars and other planets.
For a long time, conventional wisdom held that such rogue planets would be very rare, but lately scientists have begun to suspect that they are actually quite common. There may in fact be more rogue planets than there are stars in our galaxy. The creation of star systems can actually expel planets more easily than we'd imagined. For example, when a young Jovian Planet cuts through the stellar disk and ends up in close orbit of its star (becoming a hot jupiter in the process, the gravitational forces of its passage may fling smaller planets about at great velocities. Other rogue planets might be planemo smaller than a brown dwarf that formed in otherwise empty space just lacked the mass to become stars.
Likewise, science had long expected such rogue planets to be cold and lifeless. However, recent papers have suggested that it's possible for the internal heat of a planet to keep the surface from being frozen. In order for this to happen, the planet would either need a dense atmosphere, a companion moon, or both. A thick atmosphere would help keep heat in, and a moon could actually provide additional heat via tidal forces.
Since they don't emit bright light, a rogue planet might be overlooked by spacecraft or astronomers. Since they are free floating they could cross the path of a ship, or even a solar system, and cause a lot of trouble. Remember, most of the universe is moving at very high speed relative to one another.
Game and Story Use
- A rogue planet enters our solar system.
- It could be on a collision course that will have disastrous results. It's the end of the world as we know it! Even a near miss could cause massive devastation, possibly knocking the Earth out of orbit.
- It could be home to an alien species.
- It could have the ruins of a long-dead civilization.
- A spacecraft "flying blind" due to malfunctioning sensors, poor navigational charts, and/or the requirements of a particular FTL drive might find itself running afoul of a rogue planet.
- If the PCs are on a spaceship that gets damaged (in combat, or due to some other hazard) they might have to put down on a nearby rogue planet for repairs.
- Or they may set down there for privacy or stealth reasons.
- There may be space pirates or criminals using the rogue planet for any of the reasons mentioned above. They probably won't like the PCs snooping around.