A Real Twin Sunset - Can A Planet Have Two Suns?

A fictional travel poster made by NASA about Kepler-16b, sometimes known as Tatooine. This image portrays the planet as rocky, although it is likely a gas giant.

One of the most iconic scenes in Star Wars is the famous twin sunset over Tatooine, a desert planet that orbits not one, but two stars. Science fiction often takes creative liberties with its fantastic planets, but what about this one? Can a planet really have two suns?

Twin Stars

As it turns out, it’s actually normal for more than one star to exist in a solar system. When two stars orbit around each other, we call it a binary star system. We estimate that around 85% of all stars exist with at least one partner. If you look at a random star in the night sky, odds are you’re actually looking at two or more stars that are too close together to distinguish them. Sometimes you’ll even find three stars in one system, a trinary star.

Some binary stars can even be resolved with a telescope. For example, the star Mizar, a star in the handle of the Big Dipper, is actually a visible binary with its partner Alcor, which can be faintly seen with the naked eye, but through a telescope you can actually see that Mizar itself is in fact two stars, Mizar A and Mizar B, right next to each other. Alpha Centauri, the closest star system to Earth, is a trinary system, with Proxima Centauri being the star closest of the three to us.

In a binary star system, the two stars will orbit around the center of mass of the two bodies, known as the barycenter. If one of the stars is significantly more massive than the other, then the barycenter will be closer to that star, but if both are of roughly equal mass, then the barycenter will be dead in the center of the two, making it look like the two stars orbit around an invisible object in between them. The two stars making up a binary system can be similar or completely different. A giant and dwarf star or a young and old star could find themselves paired up. In some cases, a dead white dwarf star paired with a still living red giant star can siphon away material from its companion until it becomes massive enough to trigger a supernova.

An artist's rendering of a binary system where one member is leeching material from the other. This may eventually result in a supernova. Image Credit: Merikanto (CC BY 4.0)

A planet in a binary system can have a P-type or S-type orbit, depending on if it orbits one or both stars. Image Credit: Philip D. Hall (CC BY 4.0)

A Real Tatooine

But what about planets? Can binary stars host a planetary system? Once again, the answer is yes. A planet in a binary system could orbit further out from the stars, circling the barycenter of the two suns, known as a P-Type orbit, or it could orbit around one of the stars the same way the Earth circles the Sun, an S-Type orbit. A more complex orbit weaving back and forth between the two stars would likely lead to the planet being ejected from the system. As Tatooine experiences a twin sunset, with both suns setting simultaneously, it likely orbits the barycenter of both stars in a P-Type orbit.

You might have planets in a binary system, but the question of if those planets can be habitable is a little trickier. If the planet only orbits one of the stars, then it could exist within that star’s habitable zone and support life. But if it instead orbits both stars, things get harder. Unlike what Star Wars suggests, double the suns does not mean double the heat, resulting in a planet-wide desert. As the planet orbits both stars, one star might be warmer than the other. This would lead to wild temperature shifts from hot to cold based on which star is currently closer. Despite this, life could still be possible.

The first exoplanet discovered in a binary system was Kepler-16b. It was discovered by the Kepler Space Telescope in 2011 using the transit method. The planet is roughly the mass of Saturn and consists of half gas and half rock by mass. We have affectionately nicknamed this planet Tatooine. It does orbit at the outer edge of its system’s habitable zone, but being a gas giant with a surface temperature of over a hundred below zero means this is no life-supporting desert planet.

As common as binary stars are, and with prospects for life, humans may someday be able to witness an actual twin sunset. And even if we don’t, odds are that some species out there does. This is one case of science fiction mirroring science fact.