Rogue Planet
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Basic Information


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. Alternately, a rogue planet could be frozen water world with a kilometers-thick shell of ice over a deep ocean with black smoker vents on the ocean floor that continually pump heat and new nutrients into the water, providing a home for local life.

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. For example, an impact with Theia blasted the surface of the Earth with lava 4.5 billion years ago, and created the Moon.

See Also:

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.
      • If the planet used to orbit a star, life may have evolved before it went rogue. That life might then adapt to the colder environment over the millennia thereafter. See alien biochemistry and brown dwarf for some ideas of how life might evolve in a place without much light or heat.
    • 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.
    • It might crash.
    • It might just get knocked off course by the gravity of the 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.
  • A rogue planet could theoretically be a "layover" or refueling station for voyages between stars. Especially if you have a stardrive that runs on some sort of fuel that might be in the ice on the planet.
    • Such a planet may be impractical for planned voyages unless you have some sort of hyperspace or jump drive. With a traditional rocket engine, slowing down enough to land and refuel could reasonably double your total transit time.
      • That is, unless the speed of the planet is so fast you don't have to slow down much. If that's the case, though, then the planet will only be available for such stops for a relatively short window of time, after which it will no longer be in a convenient location between the two destinations.
      • But if your stardrive lets you effectively teleport a light year or two at a time, or lets you speed-up/down without inertia, then a rogue planet in the middle of nowhere could be a useful waypoint.
    • Regardless of the impracticality of planned layovers, it could be some sort of emergency shelter. If one of the first few expeditions to pass this way had some sort of space disaster or close call, then some government space agency may have chosen to deploy a habitation module, a bin of spare parts, a silo of fuel, and an emergency beacon in case any future missions experienced mechanical failure on the route. There may also be some sort of automated digging equipment, in case you need to drill for geothermal energy to stay alive. It would make one heck of setting for a survival horror story.
  • Any creature living on such a planet would likely have weird alien biology, and a very different outlook than ours. There would be no day/night cycle, and no photosynthesis. They might even live under so much ice that they have no concept of space nor awareness that stars exist. See extremophile and hypothetical types of biochemistry for ideas on what might live in such conditions.
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