SAN DIEGO (DPA) - Scientists had yet to discover proof that a single planet can orbit two stars when Luke Skywalker was shown watching a twin sunset from Tatooine in the 1977 science-fiction film Star Wars.
But they later nailed it. And San Diego State University astronomers have just revealed fresh evidence that there's likely to be many of these Tatooine-like worlds far, far from Earth.
Scientists Jerry Orosz and Bill Welsh reported on Monday (April 15) in Astronomical Journal that they have found a third planet circling a pair of stars in a region of constellation Cygnus, roughly 3,400 light-years way.
Professor Welsh and Professor Orosz were part of the team that found the first two "circumbinary" planets in 2011.
"The system is more complicated than we knew," Prof Orosz said on Tuesday.
"This newly discovered planet is the largest of the three, and it is located between the two others. We didn't see it until we had more data of that area. It shows you the staggering variety in the galaxy."
This is the first time scientists have found three planets orbiting a pair of stars. The newest planet has been given the name Kepler-47d.
The evidence came from Nasa's Kepler space telescope, which spent years searching the cosmos for Earth-like planets. Astronomers were able to spot them because planets periodically pass in front of stars, causing their perceptible light to briefly dim.
"Think of a bug crawling across the headlight of a motorcycle," Prof Orosz said. "The human eye wouldn't pick it up if the person was too far away. But a super camera would. And that's what Kepler was."
The Kepler mission ended last year, after discovering thousands of planets beyond our solar system known as exoplanets.
Prof Orosz and Prof Welsh focused their attention on a system known as Kepler-47, which has two stars, making it a so-called binary system.
About half of the stars seen in the night sky are binaries.
One of the stars in Kepler-47 is far bigger than the other. The stars orbit each other every 7.45 days. The planets orbit the stars.
"There are no other systems like this that we know of," Prof Welsh said on Tuesday. "So Kepler-47 is telling us that multi-planet circumbinary systems do exist. Not that long ago, such planets were not thought to be possible because of the rapidly changing gravitational pull from the two stars."
But don't look to our own solar system for comparisons.
The new planet "is between the size of Neptune and Saturn, but much less massive", Prof Welsh said. "It is much less dense than Saturn, which as some of us learnt as kids, is less dense than water.
"The chemical composition is unknown - we don't have the tools just yet to get their make-up. But we are close - a decade or so away."
How long would it take a spacecraft to travel to Kepler-47d?
"I don't know exactly," Prof Orosz said. "But it would be tens of millions of years."