It’s one of the most famous and evocative images in cinematic history – Luke Skywalker gazing out at the twin suns of Tatooine as they set, in the original Star Wars movie, A New Hope.
Science fiction, science fact
Throughout science fiction, stories abound of planets orbiting multiple suns – what better way to make an otherwise “Earth-like” planet seem truly alien?
But do such planets actually exist?
Of these, only a handful have been discovered orbiting around close binary stars.
In fact, such “circumbinary” planets have only been confirmed in four stellar systems, none of which even vaguely resembles the two “sun-like” stars we’re familiar with from Star Wars.
So what is a “circumbinary” planet? Why are they unusual and exciting?
With such planets, the two stars in question orbit very close to one another, with the planet moving further out, orbiting both stars at once (technically, the planet and both stars actually orbit their common centre-of-gravity, the barycentre).
There are two main reasons why so few circumbinary planets are known.
Firstly, such planets are simply much harder to detect than planets orbiting single stars. In fact, the main radial velocity surveys searching for planets around other stars (responsible for the discovery of the great majority of exoplanets we know to date) specifically avoid such close binary star systems, such are the difficulties involved in finding planets within them using that technique.
Secondly, it might be that there really are fewer planets in close binary star systems than there are around single stars.
Indeed, a number of scientists have suggested it might be significantly more challenging to form planets around such close binary star systems than around single stars (or those that are a component within a wide binary star system).
Four circumbinary systems unlike our own
The four previously known circumbinary exoplanets are much more massive than Jupiter, and orbit at great distances from stars very different to our Sun.