In Star Wars, the original 1977 film, there is a brief scene in which Luke steps out of his underground desert house at dusk and looks across the desert to the setting of the planet Tatooine’s twin suns. The two suns are very close together in the sky, indicating (at least as well as geeks like me can guess from the movie) a close binary system.
Star Wars is science fiction (well, more like fantasy — there’s virtually no science in the films, though I love them and will hear no criticism). However, the Kepler mission has announced the discovery of a planet in orbit around a binary star, which is the first discovery of its type: the planet orbits around both stars together, as the figure on the right shows. (An earlier exoplanet was discovered orbiting around one star within a binary system, which would be similar to our Solar System itself orbiting another star.) The entire system is known as Kepler-16, and the planet carries the name Kepler-16b…although already people are referring to the planet as Tatooine.
The resemblance doesn’t go very far: the suns in Star Wars are obviously at least somewhat Sun-like, judging by the light and the fact that the movie was filmed on Earth, if you want to be really persnickety about it. Both stars in the Kepler-16 system are significantly smaller than our Sun, both in mass and size; as a result, their light is much less intense. Stars emit light according to a thermal spectrum: the hotter a star, the more white it appears and the more light it emits, while a cooler star of a similar size will be both redder and less bright.
In summary, the Kepler-16 system has these basic properties:
- Both stars are smaller, redder, and fainter than our Sun, as I mentioned. This means their total combined output of light is significantly less than our single Sun. (Actually, the smaller star’s detailed spectrum has not been determined yet, but since it is not as hot or bright as the larger one, astronomers can make some educated guesses. Here are some of the technical specs.)
- As a result, Kepler-16b “Tatooine”, even though its orbit is smaller than Earth’s, will receive less light and heat. Its estimated cloudtop temperature is around -100° F (-70° C), far from being the hot desert planet of the movie.
- The stars are in such a tight mutual orbit that they actually orbit a point in empty space known as the barycenter. Properly speaking, this is the average position of all the mass of the Kepler-16 system, so “Tatooine” orbits that point. However, it is traveling around both stars, so it will experience suns-rise and suns-set (to coin somewhat awkward terms).
- The stars orbit the barycenter in 41 days; “Tatooine” orbits every 229 days. This means the amount of light “Tatooine” receives fluctuates a lot, based on the relative positions of the two stars. From Kepler-16b’s perspective, they will eclipse each other, partially blocking light and further decreasing the amount of heat the planet gets. (More on the eclipses in a bit.)
- Kepler-16b is close in mass to Saturn, which is about 95 times the mass of Earth. This means its composition is gaseous, probably with a lot of ice as well. (“Ice” to an astronomer includes methane, ammonia, and other volatile substances.) In this sense, “Tatooine” may be more like Uranus or Neptune than Tunisia (where the Tatooine sequence of Star Wars was filmed).
As I wrote in a previous post, Kepler finds exoplanets by watching for fluctuations in light as a planet eclipses its host star. In this case, the eclipses came mostly from the stars crossing each other, relative to Earth, but there was an extra very tiny pair of eclipses. After ruling out the possibility of a third star in the system (or a brown dwarf), astronomers found the best fit to the data came from a planet described as above, with the orbit and mass determined using Kepler’s laws. It’s a very elegant example of how to perform a difficult bit of science!
This star system isn’t in a galaxy far far away — it’s actually relatively close to our Solar System, and you can even see it (barely) through a good pair of binoculars — but it’s far enough away that merely collecting enough data to understand what’s going on is a technical challenge. On the other hand, exoplanet systems are being discovered at a higher rate than ever before, and lest we risk becoming jaded, we are still learning new things every day. “Tatooine” is hardly Earthlike in any way, and the stars aren’t very much like the Sun, so any life existing in that system would necessarily have different modes of existence…but the fact that planets can orbit binary stars increases the number of possible systems where planets can be, and possibly life. Alpha Centauri, the closest star system to the Solar System, has a very Sun-like star in a binary orbit with another star (with a third, dimmer star in a much bigger orbit); it is not beyond possibility that planets could be in this system, and more observations will likely settle that question within a few years.
It’s not science fiction anymore. Maybe eventually we’ll even see if Nightfall is possible.