Trojan Planet

Basic Information

A Trojan Planet is a smaller planet traveling in the same orbit around a star as it's larger sister planet. (A Trojan Asteroid is the same thing, except smaller. They are pretty common. The Jovian Trojans are the specific trojan asteroids that exist on either side of Jupiter in our solar system.)

In any given planet's orbit, there are two points at which a smaller planet could exist and be stable: roughly 60^o ahead and roughly 60^o behind the larger planet. This 60-degree position roughly corresponds to the Lagrange Points of the two-body (star and planet) system. To be stable, the Trojan Planet must also be traveling at the same speed as the larger planet, and the trojans must be much smaller and less massive than the big planet in the middle. So you'll only find trojan planets in the orbit of really big planets (but a smaller planet could have trojan asteroids.)

Trojan Planets, if they exist in a given system, will be located at the Lagrange Points designated L4 and L5 in relation to the two larger bodies (typically a star and some sort of gas giant) of the system. Outside of the 5 Lagrange Point configurations, one planet being in the orbital path of another is a recipe for disaster. They'd either crash into each other, or fling one another out of orbit. (The Lagrange Point L3 is the next-most-likely spot for another body in the system, such as a Counter-Earth. Such a planet at L3 will be far less stable than the Trojan Planets at L4 and L5. So in theory you could have one at the that spot, but the slightest drift or disturbance could cause either it, or the whole system, to collapse.)

Sources

Bibliography

1. wikipedia

Game and Story Use

• Could be useful for creating an interesting space system.
  • Two trojan planets could be locations for an early warning system or military base protecting the larger planet.
  • They might be a good place to stage space travel, especially if the gas giant or other huge planet in the center is so massive as to require extreme speeds to reach escape velocity. You burn a lot of fuel getting up to full speed, so it's a good idea to swing by one of the trojans either to fuel-up or to use it's gravity well as a slingshot further out in the solar system.
  • If something (like a huge impact or explosion) disturbs the orbit of one trojan planet, it could cause a secondary greater disturbance somewhere down the road that bounces the planets around like billiards balls.
• This all sounds modern and sci-fi, but the existence of Trojan Planets was known way before you'd expect it. Joseph-Luis Lagrange understood (/invented) the math of it well enough to predict that there'd be Trojan Asteroids ahead and behind of Jupiter all the way back in 1772. So this could make an interesting bit of characterization that the players won't expect: You could have an intellectual NPC at the time of the French or American Revolution who knows enough about physics and astronomy to predict the orbital path of asteroids whose very existence won't be proven for centuries. Check out the big brains on Lagrange!
• If precursor aliens wanted to leave a precursor artifact where we couldn't reach it until we'd begun local space travel, they'd put it at the L4/L5 spots in our orbit (or just, you know, on the moon). If they wanted to hide it from our view, they'd put it at L3 where we couldn't see it until we'd really spread out and mastered space travel within our own solar system.