ENCYCLOPEDIA GALACTICA Edition 428193 - Earth English Definition: 'Trojan Planet' A language English1 scientific expression for a planetary mass or masses orbiting at a stable (Joseph-Louis) Lagrange2 point between two stars in binary and trinary star systems. Systems of greater complexity are not known to have Lagrange points of long term stability. Trojan planets may also be called 'Lagrange planets' especially in trinary systems.3 Lagrange points and Trojan planets are otherwise designated as L-points and T-planets respectively or by equivalent translations. For two masses - ratio greater than twenty-five (25) to one (1) - orbiting a common centre of gravity or barycentre, there are two locations where a 'negligible' mass may be located and will maintain its relative position with respect to the more massive bodies. These locations are sixty (60) degrees4 ahead (L4) and behind (L5) the smaller mass in its (near circular) orbit around the larger. In planet-star orbits, these locations may be populated with swarms of small irregular bodies named 'Trojan asteroids'. In the case of two stellar masses, there is the potential for a Trojan mass or masses of up to one (1) Neptune (1026kg) at either L4 or L5 or both.5 The larger the stellar masses, the more massive 'negligible' bodies may become. The pairing of the binary stars is critical. Stars of similar mass and spectral class6 will not create stable L-points. For a T-planet as massive as Neptune, the smaller stellar mass would need to be at least a half (0.5) Solar masses or greater - 'K2' class. Subsequently, the larger stellar mass would need to be greater than twelve (12) Solar masses - 'A4' class. Anything smaller and planet formation becomes unlikely at stable L-points. Pairing of higher massed stars is problematic. All 'B' and 'O' class stars are rare in contrast to other stars in the main sequence. These giants form from nebulae poor in common planetary elements making planet formation unlikely again. And such stars have life expectancies only in the tens of millions of years, too short for planetary formation. Hence T-planets are most often found orbiting white to bluish-white stars paired with an orange dwarf. Technically, T-planets orbit both stars at the same time. On the surface there would be two 'suns' in the sky, major and minor designated by apparent brightness. They would be separated by sixty (60) degrees on average with a variance of as much as fifteen (15) degrees over the coarse of a year. And, with a minor of sufficient brightness, a T-planet would have an average 2-to-1 day-to-night cycle. Assuming the T-planet's rotation is not captured.7 In trinary star systems more exotic T-planets may exist. There are circumstances when unstable L-points (L1 thru L3) may have loops of stability around them. If the two smaller stellar masses orbit each other with a separation less than three (3) percent that of the largest, the system is called an 'approximate binary' with L-points between the smaller pair and the larger star. But the gravity flux created by the pair quickly orbiting each other gives rise to potential energy plateaus around the otherwise unstable L-points. Provided the motion of the three stars is orderly, matter may coalesce into a planet within these loops. In such systems a T-planet at L2 would be permanently placed in between its stars and never experience night except when one or both stars are eclipsed by companion objects. References: 1Civilization Humanity: Linguistic Types 2Civilization Humanity: Members of Note 3Only objects found at certain Lagrange points are included in the 'Trojan' class. 4Civilization Humanity: Scientific Standards 5There are five (5) Lagrange points. Points L1 thru L3 are unstable in two body systems. 6Civilization Humanity: Hertzsprung-Russell Diagram 7Definition: 'Captured Rotation'