Two suns are better than one: planets thrive around binary stars

Planets may actually form more easily around double stars than around single stars like our Sun, according to new research from astrophysicists at the University of Lancashire.

Binary stars are common in our Galaxy, yet for a long time astronomers believed that the gravitational tug-of-war between two stars would make it harder for circumbinary planets, worlds that orbit both stars, to form. Famous fictional worlds such as Tatooine from Star Wars, with its iconic twin sunsets, were thought to be cosmic curiosities rather than something nature routinely produces.

However, new research published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) suggests the opposite may be true.

Using state-of-the-art computer simulations, the team modelled the evolution of gas discs surrounding young binary stars. The simulations show that the inner regions of such discs constitute a “forbidden zone”, where strong gravitational effects prevent planets from forming. But beyond this region, the disc becomes a fairground for giant planet formation, fragmenting under its own gravity to produce multiple young planets.

“Close to a binary star it’s simply too violent for planets to form,” said Dr Matthew Teasdale, the who led the research as part of his PhD project. “But move farther out and the disc becomes an ideal environment for planet formation.”

The team found that discs around binaries can make more planets through fragmentation than discs around single stars, and that a larger fraction of these objects end up as gas giant planets larger than Jupiter. Some planets can also be ejected from their systems, becoming free-floating planets drifting through interstellar space.

Dr Dimitris Stamatellos, who supervised the project, added: “Binary stars were once seen as hostile environments for planet formation. What we’re finding is that they can actually be extremely productive. Once you get past the danger zone, planets can form quickly and in large numbers.”

The findings suggest that circumbinary planets may be more common than previously thought and that gravitational instability, where massive discs break apart under their own gravity, could be an important pathway for forming these planets.

With over 50 circumbinary exoplanets already discovered, including several on wide orbits, the results help explain how these worlds can form and survive. They also open up new possibilities for future observations with facilities such as ALMA, the James Webb Space Telescope, and the upcoming Extremely Large Telescope (ELT).

So while planets may struggle to survive near their twin Suns, farther out these systems transform into dynamic planet-forming environments, suggesting that real-life Tatooines may be far less rare than we once imagined.