Life on Tatooine: Habitable planets in binary star systems
Those familiar with Star Wars may remember the beautiful sight of the twin stars that are the home of the planet Tatooine rising above the desert planet’s horizon, like two glowing balls of fire bringing light to the scorched land. Prose aside, the two stars — known as a binary star system — are an incredible sight, and putting a world in a binary star system is certainly a tempting idea for any worldbuilder merging space travel with a potentially habitable world. But is it realistic for a planet in a binary star system to be habitable? This blog post is dedicated to figuring that out.
In my last blog post, I talked about which stars are the best for supporting life. I settled on Sun-like stars — G-class stars, specifically — as the best choices, because of their relatively long lives and near-perfect luminosities. You would think, therefore, that having a binary system of two G-class stars would be ideal — and, indeed, the two stars that Tatooine orbits seem similar to those. The nearest star system to us, the Alpha Centauri system, happens to be a binary star system (although it contains a third star, Proxima Centauri, which is actually closer to us, and may or may not be part of the system) with two Sun-like stars. What’s not to like about this?
The problem with picking such a mundane setup is that it makes one assumption: namely, that the planet must orbit both stars. This might stem from a misunderstanding I’ve seen in the past, which is that stars in a binary system must be quite close to one another. In fact, this is typically not the case. For example, Alpha Centauri A and Alpha Centauri B are typically separated by a distance roughly 23 times that between the Earth and the Sun (the latter distance is called 1 AU, which stands for Astronomical Unit). Many other systems can have separations many times that.
This means that a planet doesn’t have to orbit both stars (such an orbit is called a circumbinary orbit, and such a planet is called a circumbinary planet). Planets that do do this can receive variable amounts of radiation during the course of a year — and, in fact, their “years” may be many times longer than on Earth year. Circumbinary planets also may form in unstable orbits, in which case they may be ejected from the system. If they form too close, they’ll be quickly tossed out.
A better scenario is one in which a planet orbits just one of the stars. If the two stars are separated by a large distance — say, 100 AU — then life on a planet orbiting just one of the stars can be similar to life on Earth. The right configuration can make things quite comfortable — and still provide some awesome views of the sky at different times.
