There’s an iconic scene in the original Star Wars where Mark Hamill’s Luke Skywalker watches a double sunset above Tatooine, the desert planet where he was raised. Planets with two suns are called circumbinary planets, and the fictional Tatooine is the most famous of these worlds.
Circumbinary planets also exist in reality, although they’re rare. To date, scientists have confirmed the existence of only 18 circumbinary planets amid the roughly 6,000 planets that have been tallied outside of our solar system.
A new paper from a team of astronomers led by the University of New South Wales, Sydney, Australia, reveals a method for finding the elusive two-sun planets. Using this process, the astronomers say they’ve already identified 27 potential circumbinary planet candidates.
“It’s important to be clear that these are all candidates, not confirmed planets yet,” Margo Thornton, a UNSW Sydney Ph.D. candidate who led the study, told CNET. “Follow-up observations will tell us for sure.”
Eclipse watchers
Scientists have found most of the planets that we know about using the transit method. When a planet passes, or transits, between Earth and its star, we can see the shadow.
The dip in starlight from distant eclipses lets us track exoplanets — planets outside our solar system — light-years away from Earth.
But astronomers can only use the transit method to find planets that pass between Earth and their star. If the planet’s orbit is outside our line of sight, we can’t see it.
“Our method doesn’t have that restriction,” Thornton said. “It can find planets orbiting at all kinds of angles.”
The method that the researchers used to find the new planets is called apsidal precession. While this method has been used with binary stars before, the team says this is the first time it’s been employed in a wide-reaching search for new planets.
“These are planets every previous survey was blind to,” Thornton said.
The transit method — where a planet crosses its star from our perspective, causing a mini-eclipse — is how most planets have been discovered. It’s shown here in an artist’s rendition.
Binary stars are a pair of stars orbiting each other. This new method examines the stars’ orbits, which are revealed during a stellar eclipse, when the stars align so that one passes directly between Earth and the other.
When the researchers detect a change in the eclipse schedule that can’t be explained otherwise, it suggests a third body could be influencing the binary star’s orbits. That third body could be a circumvented planet.
“We’re not just adding to the list of known circumbinary planets,” Thornton said. “We’re opening a window onto a population that simply wasn’t accessible before.”
Two-sun planets
The findings are based on data gathered by the Transiting Exoplanet Survey Satellite. NASA launched TESS in 2018 to search for new planets.
The closest planetary candidate the team found is about 650 light-years away, while the furthest is 18,000 light-years away. The researchers think this method could help astronomers find new planets in binary star systems.
The apsidal precession method, shown here in an artist’s rendition, helps astronomers detect planets that the transit method might have missed.
“There’s a strange and wonderful feeling that comes with discovery: for a brief window, we were the only people on Earth who knew these planet candidates existed,” Thornton said.
“Before the paper was published, before anyone else had seen the data, it was just us sitting with the knowledge that there were 27 possible worlds out there that no one had ever detected before,” she said.
