Fictional planets are often accused of being unlikely, or plainly unrealistic, in terms of astrophysics. In the case of Arrakis (Dune), it was the single biome to create troubles. For Star Wars’ Tatooine, instead, the culprits were its two suns. How correct are these statements? Since Kepler and its planet hunting mission, the body of knowledge regarding exoplanets and planetary systems in general has significantly evolved. And after having discovered planets that, according to what we previously knew, should not even exist, we have been forced to reconsider a few assumptions.
First assumption: you can find planetary systems around single stars only. A binary system, making orbits unstable, prevents the formation of the protoplanetary disk and planetoids. Considering the fact that the majority of the stars out there are indeed in multiple systems, this fact has been used as a proof that planets were rare or sort of. Kepler’s mission proved it wrong. Yes, planets around single stars still seem to be more abundant, but that’s probably due to an observational bias: telescopes have an easier time with single stars because the binary system interactions mostly hide planet signatures, making them harder to spot. It hasn’t only been planets orbiting one of the two stars. Whenever stars are close enough (i.e., within an interstellar distance of 1 AU, the distance between Sun and Earth) planets will orbit around both of them, and if you walk on the planet’s surface – provided it’s rocky and not a gas giant – you will admire nice double sunsets. As on Taooine.
Have we spotted them yet? Kepler 16B fits this description. There are more, for sure. It’s just that the main technique used so far, the radial velocity method, is not well-suited for finding planets in multiple systems, especially when stars are that near. And since space contains all wonders we can dream of, and more, the Keck I telescope in Hawaii has located something weirder, namely a planet orbiting three stars, in a system called HD 188753. The primary star is Sun-like, for spectral type and mass, while the other two, smaller, are tightly bound and separated from the main one by a Sun-Saturn distance. The planet itself is what you can call a hot Jupiter, gas giants that are close to their stars and therefore hotter than Jupiter and Saturn. A view from an hypothetical solid moon orbiting this planet would witness three sunsets in close sequence.
The main headache for astrophysicists here is how these planets could actually form in these complex gravitational conditions. The main theories for gas giant formation, i.e., gravitational instability and core accretion, don’t explain HD 188753. Moreover, planets do migrate over time. Hot Jupiters located less than 1 AU from their stars have most likely not formed in their current orbit, but have moved there after their birth, raising the question how they managed to get (and remain) into a new, stable orbit.
Interested in playing with planetary orbits? Good. There is actually a game designed for it, Super Planet Crash. Developed in a series of different flavours and platforms by Systemic Console at the UC Santa Cruz, “it is used around the world by research groups and classrooms to analyze radial velocity datasets and derive the orbital properties of putative planetary systems. It includes a large array of tools for error estimation, orbital stability, plotting and animation.” (Quoting from website).
I have illustrated rules and constraints of Super Planet Crash on my blog. The game has already been used as an educational tool in many universities – MIT and Columbia, just to mention two of them – and it can get as complex as necessary by adding layers and databases. Warmly recommended to sci-fi writers. A caveat: don’t try and cram together too many planets and stars in your stories – they won’t last twenty pages.