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.
Two involve cataclysmic variable stars (DP Leonis and NN Serpentis).
None of the four systems bear much resemblance to our own solar system, or the planetary systems of science fiction such as Tatooine.
Kepler-16 (AB)-b – a transiting circumbinary planet
In light of this, Kepler’s new discovery is probably the mission’s most exciting to date.
The Kepler telescope is continuously observing a patch of the northern sky, watching more than 100,000 stars to see whether any periodically “blink” as a result of an unseen planet (or faint star) passing between us and the brighter star in question.
In this particular case, the astronomers working with Kepler’s observational data have found a previously unidentified eclipsing binary star system.
The two component stars of the system orbit their common centre of gravity with a period of 41 days, meaning they are somewhat closer together than the orbit of Mercury around our sun.
They weigh in at around 0.7 and 0.2 times the mass of our sun, and Kepler’s observations have allowed the scientists working on the data to learn a great deal about them.
In fact, the smaller of the two stars, only just discovered, is now the smallest known “main-sequence” (i.e. normal) star for which a precise mass and radius have been calculated.
That alone makes this star system remarkable, and means it will greatly benefit our understanding of the formation and evolution of stars.
More remarkable still is the detection of a Saturn-mass planet orbiting around these two stars.
The planet, which goes by the snappy title of Kepler-16 (AB)-b, is therefore the first circumbinary planet to be found orbiting two “main-sequence” stars.
While they’re both significantly less massive than our sun, and therefore much less luminous, this is still far closer to the idea of a circumbinary planet around sun-like stars than anything announced before.
Thanks to the complexity of the system, the authors have already learned a great deal about the planet – they have even been able to accurately determine its mass, radius, and density (it turns out it is about three-quarters the diameter of our planet Jupiter, and somewhat denser than Saturn, which suggests it contains far more rocky material than that planet).
In fact, the orbit of the planet is now so well known that the scientists predict it will cease to transit the stars by 2018, after which no such transits will be observable for about 30 years.
While the current planet orbits too far from its host stars to potentially host satellites that could house life, it’s usually the case that, where there is one planet, more will be found.
It’s not beyond the bounds of possibility that there could be other planets in the Kepler-16 system that might prove more promising locations for life.
Far from being purely a staple of science fiction, therefore, it seems that there really are planets with two suns, like the famous Tatooine.
It’s quite possible that, in the coming years, scientists using Kepler will announce the first Earth-like planets in such systems.
Kepler-16 (AB)-b could well be the the first step on the way.