Counter-Earth: Difference between revisions

[[The Other Wiki]] [http://en.[wikipedia.org/wiki/Fictional_planets_of_the_Solar_System:Fictional planets of the Solar System#Counter-Earth |has a list]].

* Believe it or not, there actually is a way that this trope could be justified. The counter-Earth "point" (actually a circular orbit) is L3, one of the five [http://en.[wikipedia.org/wiki/Lagrangian_point:Lagrangian point|Lagrange Points]] (actually closed orbits in a three-body system). Unfortunately L3 is not a stable orbit: if the body is even a millimetre out of position, or if it is perturbed in the slightest (say by the gravitation of Venus, Mars, Jupiter) it will move into a chaotic trajectory and eventually leave the vicinity of the L3 point in an unpredictable fashion. This would take on the order of only tens to hundreds of thousand years (less for large perturbations), which is a geologically short time.

** There are five Lagrange Points. L1 and L2 lie directly in front of and directly opposite the Sun from Earth's perspective, at the points where the sun's gravity and Earth's add or cancel just the right amount to produce a circular orbit with the same period as Earth's year. A counter-earth would be at Earth's L3 point. However, L1, L2 and L3 are unstable; if an object was even one millimeter out of alignment (easily happens due to there being other planets around, perturbing orbits and mischief like that) the forces would be out of balance and the object would slowly drift away from the Lagrange point. L4 and L5 are 60 degrees ahead of and behind the Earth, and are also called the [http://en.[wikipedia.org/wiki/Trojan_points:Trojan points#L4_and_L5L4 and L5|Trojan Points]] after the Trojan asteroids, which lie around Jupiter's L4 and L5 points (any time a light object orbits a heavy one, there will be Lagrange points). The L4 and L5 points are stable (a small perturbation would put the object into a dynamically-stable, quasi-periodic [http://en.[wikipedia.org/wiki/Lissajous_orbit:Lissajous orbit|Lissajous orbit]] that would keep it near the Lagrange point), but any object there would be easy to see: an object in the L4 point would be a perpetual morning star, rising four hours before the Sun every day and preceding the Sun on its annual circuit of the ecliptic, and an object in L5 would be a perpetual evening star setting four hours after the Sun every night, and trailing the Sun around the ecliptic.

** There is another possibility, called a [http[wikipedia://en.wikipedia.org/wiki/Horseshoe_orbitHorseshoe orbit|horseshoe orbit.]] A horseshoe orbit does not require vast differences in mass, and an object in one with Earth could temporarily be opposite the sun. However, it would still be detectable.

*** [http[wikipedia://en.wikipedia.org/wiki/Theia_Theia (planet)#Theia |A hypothetical body in that formed in the L4/L5 spot]] may have created the Moon.