Double Planet

Basic Information

A Double Planet is actually two planets, a planetary form of Binary System.

Actually, it's hard to say where exactly the cut-off between a Double Planet and a Planet-Moon system is. Our Moon is an exceptionally large moon for a planet of our size, but by some definitions it's a second half of the Earth-Moon Double Planet system. The various definitions have to do with ratios of gravity or mass between the objects, or between them and the star they orbit. Earth-Moon may qualify as a Double Planet not because the Moon is so big (some of the moons of Jupiter are bigger), but because it's so big relative to the size of the planet it orbits. If the Earth was much bigger or the Moon much smaller, we'd clearly not be a Double Planet.

Important things to know about Double Planets is that the gravity of the two planets has an effect on each-other. This results in large, regular tides. It also stabilizes the magnetic poles of the planets, and slows their rotation. As a result, one or both planets may be better suited to sustain life as we know it.

How a Double Planet might form:

• Giant Impact Theory
• Capture Theory
• Lunar Fission Hypothesis
• Co-formation Hypothesis

See also:

• Orbit of the Moon
• Movement of the Earth
• Weird Moon

Sources

Bibliography

1. Wikipedia

Game and Story Use

• One of the locations in your game could be a Double Planet.
  • Likely place for aliens to evolve, at least ones that are more like us than like extremophiles or energy beings.
    • Very different life-forms could evolve on each of the two planets.
      • There'd be some interesting culture shock when the inhabitants of one planet developed the ability to travel to the other.
    • Also possible the same exotic lifeform might exist on both worlds. Say, a plant-like organism that has really resilient spores. It evolves on one world, and gets blasted off by volcanic activity or an asteroid impact. The spores land on the other world, and "colonize" it. From there, evolution diverges, but may run somewhat parallel.
  • Probably a little easier to Terraform than some other planets might be.
• Are there any ramifications or implications of our planet being a Double Planet? What might it mean?
  • In some great space Republic, each planet gets one vote or representative. Does Earth get one or two? Does Luna (or a Lunar Colony) get one? Do planets have rights that are different from moons?
  • Is our moon our sovereign territory, or a neutral planet?
    • What happens if aliens colonize our moon?
      • Then we're buggered. If they can get here without us spotting them a long way off, they've got the capability to take us out without blinking.
        • Not necessarily. Let's say a generation ship were headed in our direction, and we noticed it even as much as 10 years before it got here. In those 10 years, we'd clearly take plenty of interesting steps towards preparing ourselves for the arrival of alien life. No doubt we'd largely be assuming that the aliens intended to eventually come to Earth, so we wouldn't exactly rush to colonize the majority of the moon before they got here (and more than perhaps a few automated observation posts would probably be an overly ambitious goal for 10 years given all the other things we'd be motivated to do in that time). Given alien biochemistry and/or sufficiently advanced technology, it's not impossible that our moon might be a serviceable place for the aliens to set up shop and scrape by. There's billions of us, and maybe only thousands of them. They've got better tech, but not so much better that worldwide victory is automatic. Just because they could nuke (or hypervelocity impact) us into oblivion doesn't matter if they need to preserve our ecosystem (or, less likely, planned to live on our moon in the first place). We also have nukes, after all. If reinforcements are decades away, they might decide it's better not to poke the bear, at least at first.
        • It's also possible the aliens colonizing our moon could be a von Neumann probe, an automated or remote-controlled exploratory device, a single craft with minimal crew, and/or a system for embryo space colonization. A variety of scenarios could exist where aliens might choose diplomacy, stealth, or even just try to ignore us.