If you could stand on the surface of Kepler-16b, you'd have two shadows. At sunset, you would see an orange star about the size of the sun and next to it a much fainter red star. As the stars slipped toward the horizon, they would change places in the sky, like partners in a square dance.
You would not need to be Luke Skywalker visiting his home planet of Tatooine in the movie "Star Wars" to watch the twin sunset. The only science fiction in this story is how to make the 200 light-year journey to Kepler-16, a binary star system jointly sharing the Saturn-sized planet, Kepler-16b.
Unlike Tatooine, however, Kepler-16b is likely a gaseous world, with no solid "surface" to walk on. The density of the world is higher than Saturn, similar to that of water (but there are no indications that suggest any water is present in Kepler-16b's atmosphere).
The finding, reported by scientists on NASA's Kepler planet-hunting space telescope team, adds a new page into Mother Nature's recipe book for extrasolar planets.
"It's the first one that circles two stars, so it's a fundamentally different kind of planet," lead researcher Laurance Doyle, with the SETI Institute in Mountain View, Calif., told Discovery News.
From a distance about as far as Venus orbits the sun, Kepler-16b circles both its parent stars in 221 days. The stars, which on average have about 21 million miles between them, fly around each other about every 41 days.
The whole system is perfectly aligned to Kepler's viewing spot, with the bodies crossing paths so that tiny amounts of their radiating starlight regularly, repeatedly and predictably vanish and reappear as the stars and the planet fly past one another.
The telescope points at a fixed position in space, looking for changes in light streaming from about 155,000 target stars in the constellations Cygnus and Lyra.
Because the Kepler-16 system is so perfectly aligned, scientists believe the planet formed alongside its parent stars from a common disk of gas and dust. The plane in which the two stars orbit is aligned within one-third of a degree of the orbit of the planet.
"It shows the gears of celestial mechanics turning and interlocking so vividly, you almost want to reach out and touch it," exoplanet scientist Marc Kuchner, with NASA's Goddard Space Flight Center in Greenbelt, Md., told Discovery News.
"Kepler himself would have been dizzy with excitement," he added, referring to the telescope's namesake, Johannes Kepler, the 16th century scientist who deduced the laws of planetary motion.
Both the Kepler-16 stars are smaller than the sun, so their planet lives largely outside the so-called habitable zone where liquid water could exist on its surface (if it had a "surface," which it doesn't). Water is believed to be an essential ingredient for life as we know it.
The planet is estimated to experience temperatures that plummet from -100 to -150 degrees Fahrenheit (-70 to -100 degrees Celsius).
Timing also played a role in the discovery, notes Doyle.
Computer models show that in early 2018, the planetary transits across the larger star will disappear from Kepler' view until around 2042. The passages across the smaller star's face, already slipping from view, will vanish in May 2014, and won't be back for 35 years.
The research is published in this week's Science.