History Main / BinarySuns

[[folder:Types]]
There are three realistic possibilities for this arrangement:
* Type I: "Twin Suns" -- The suns rise and fall with each other, indicating that the suns orbit each other closely and the planet orbits their shared center of mass. This is called a circumbinary or "P-type" orbit.
* Type II: "Close Stars" -- The suns may occupy different parts of the sky, indicating that one sun orbits the other farther out than the planet, but not far enough for the stars to have their own separate habitable zones.
** Subtype II a: "Bright And Dark Seasons" -- With low axial tilt or near the equator, at one point both suns will appear close in the sky, and it becomes night when both set. About half a planet year later, usually one sun will be in the sky, and there will hardly be a real night.
** Subtype II b: "Midnight Sun" -- With high axial tilt or near the poles, the outer sun may remain [[EndlessDaytime above]]/[[AlwaysNight below]] the horizon for many years.
* Type III: "1 1/2 Suns" -- The planet has one "Sun", with night and day according to it, but another star in the same system is identifiable as a small sun that contributes some heat and sometimes leaves night more like twilight. This indicates that the planet orbits a single star in a double-star system, and the stars orbit each other at a large enough distance for one or both to have its/their own separate habitable zone(s). This is by far the most likely arrangement in reality as far as habitable planets go, and many such planets including [[https://en.wikipedia.org/wiki/Proxima_Centauri_b Proxima Centauri b,]] the closest known exoplanet to Earth, have already been discovered (although Proxima b is something of an aversion, since Proxima Centauri orbits so far from Alpha Centauri A & B that they would appear only somewhat brighter than Jupiter does from the Earth).

* Type IV: "Between Two Suns" -- The planet is positioned at the first Lagrange point in the suns' orbit around eachother (the point in between them where their gravitational pulls cancel eachother out), providing for EndlessDaytime. The reason this wouldn't work is because It's a case of UnstableEquilibrium, and thus the minor gravitational pull from the other bodies in the system (or, barring that, other nearby stars) would nudge the planet enough to eventually pull it out of "orbit".
* Type V: "Figure 8" or similar -- The planet orbits one sun, then the other, in a regular way. If the suns of Type I are too far from each other, it may also become this.[[note]]Technically a figure-8 is called a "lemniscate of Bernoulli", and one thing wrong with this scenario is that this is an order-6 curve, not the order-2 curve a real stable orbit would be.[[/note]]

Type IV and V would realistically decay into:
* Type VI: "Chaotic Orbit" -- A planet in chaotic orbit might exist, but wouldn't be able to support life. Also, the chaotic orbit would eventually throw the planet into one of the suns, or out of the system.[[note]]''Brief'' figure 8 orbits are possible -- the free-return trajectories flown on the Apollo missions were figure 8s -- but these are never stable.[[/note]]
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%%* ''Literature/{{Eldraeverse}}'': The eldrae's homeworld, Eliéra, is in a type II a system. And given that it's a nearly flat planet-sized {{Precursor|s}} artifact rather than a natural planet that explains how it has seasons.%%ZCE. Explain what "type II" means and how seasons are relevant to this.

* ''Literature/WhiteSand'': The world of Taldain is a Type IV example, suspended at the midpoint between a blue-white supergiant that creates EndlessDaytime on the "Dayside" and a second star surrounded by a nebula that filters most of its emissions into the UV spectrum, creating the "Nightside". It's noted that the only reason this setup is stable is that there's a PhysicalGod holding everything in place.

* ''VideoGame/{{Spore}}'': Some systems are Type 1. Habitable planets in such systems are typically farther away from the two suns, especially if one or both are blue stars. Since a planet in such a system would be getting illumination and consequently heating from both stars, and blue stars are the brightest and hottest stars of all, [[ShownTheirWork this makes a lot of sense]].

* ''VideoGame/{{Stellaris}}'': Many systems are Type 1 or 3 binary or trinary systems, and many of them have habitable planets (usually, but not always, orbiting the innermost star). It's even possible to have your species' homeworld in such a system.

* The ''Blog/WhatIf'' blog mentions Types I and II of this trope, calling them "circumbinary" and "[[BuffySpeak the other kind]]", while [[http://what-if.xkcd.com/150/ discussing]] [[AlienSky how rainbows would form on a planet with two suns]].

* Type III: "1 1/2 Suns" -- The planet has one "Sun", with night and day according to it, but another star in the same system is identifiable as a small sun that contributes some heat and sometimes leaves night more like twilight. This indicates that the planet orbits a single star in a double-star system, and the stars orbit each other at a large enough distance for one or both to have its/their own separate habitable zone(s). This is by far the most likely arrangement in reality as far as habitable planets go, and many such planets (including [[https://en.wikipedia.org/wiki/Proxima_Centauri_b Proxima Centauri b,]] the closest known exoplanet to Earth) have already been discovered.