A Planet With Two Suns

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.

The planet Kepler-16b "Tatooine" in orbit around its binary host stars. The orbits are drawn more or less to show scale, but please don't take a ruler to them! The stars and planet are decidedly not to scale.

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.


13 responses to “A Planet With Two Suns”

  1. A question I would like to investigate is the long-term stability of a system like this. The binary star is very close, but perturbations to “Tatooine’s” orbit seem inevitable. How long would it keep its current orbit? Would it change shape, size, etc.? The three-body problem eternally haunts us.

  2. Is this planet considered to be in the “habitable zone” for any type of life as we know it, including microbial?

    1. The planet is definitely not in the habitable zone as we usually think of it (meaning sufficient heat from the star(s) to keep water liquid, in the absence of other sources of warmth). The stars put out a lot less light and heat than our Sun, so the habitable zone is a lot closer in. A planet this large can keep warm just by its bulk, but if it’s like similar planets in our Solar System (the “ice giants” Uranus and Neptune), I’m inclined to think the environment is wrong.

  3. Funny, but a lot of reports fail to mention something important.

    “Beyond the wow factor, astronomers said the discovery — as so many discoveries of so-called exoplanets have done — had thrown a wrench into another well-received theory of how planets can and cannot form. “In other words,” said Sara Seager, a planetary expert at the Massachusetts Institute of Technology who was not part of the discovery team, “people don’t really know how to form this planet.”

    It was long thought, Dr. Seager said, that for its orbit to be stable, a planet belonging to two stars at once would have to be at least seven times as far from the stars as the stars were from each other. According to that, Kepler 16b would have to be twice as far out as it is to survive.

    “This planet broke the rule,” she said.

    Nature can do things that Platonic over-idealizations say are impossible.

    1. lol That is awesome! It isn’t the first and God willing it won’t be the last time that scientist have been wrong. Good scientists make mistakes. Better scientists learn from them. Let’s see what the brains that be right now learn from this mistake.

    2. I’ve been chatting off and on with people who actually know something about planet formation (since I don’t know very much), and I think it’s safe to say that our models are being refined all the time. Exoplanets have forced us to revise the view that was based on our Solar System alone, and it’s very probable more than one planet-formation scenario will occur in nature.

      Also, we need to beware of the assumption that because the planet is there *now* that it has always been in that orbit. As I said in my first comment, I don’t think this orbit can be long-term stable, so it’s probably it moved since its formation. We may be seeing the system at a special point in its history. (Lisa, if I were still your prof., I could give this to you as a senior project. It’s an interesting 3-body simulation that would be good practice!)

  4. […] of a wanderer, but calling them “planets” seems inappropriate, since orbiting a star (not just the Sun) is a common requirement in modern definitions for planets. So, what do we call these objects? […]

  5. […] in our galaxy, perhaps more than there are stars. This makes sense for a number of reasons: if planets form in binary star systems, the complex interplay of gravity tends to kick at least some of the planets out of the system, […]

  6. […] with access to a computer. Interesting discoveries can be made by ordinary people in this way; the famous “Tatooine” exoplanet, in orbit around two suns, was identified by a PlanetHunter user, to cite just one […]

  7. […] with access to a computer. Interesting discoveries can be made by ordinary people in this way; the famous “Tatooine” exoplanet, in orbit around two suns, was identified by a PlanetHunter user, to cite just one […]

  8. […] for example, the exoplanets known as Kepler-16b (“Tatooine”, which I wrote about on this blog), Kepler-34b, and Kepler-35b (which I covered for Ars Technica). These three planets are […]

  9. […] In fact, most stars in the same mass range as the Sun are in binaries or larger associations, so finding planets in those systems is a hopeful […]

  10. Since this blog entry was first written, a planet has been discovered orbiting Alpha Centauri b.

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