Relativity Explains Lack of Planets Around Tight Binary Stars

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Astronomers have long puzzled over the scarcity of planets orbiting binary star systems, despite the fact that most stars are born in pairs and most stars have planets. A new study suggests that the missing planets are being destroyed or ejected from these systems due to the effects of general relativity as the binary stars slowly spiral inward over time.

Why it matters

This research helps explain a longstanding mystery in exoplanet science and demonstrates how Einstein's theory of general relativity can significantly impact planetary systems, even in relatively common stellar configurations. It suggests that many more planets may exist around binary stars, but are simply too difficult for current telescopes to detect.

The details

The study focused on tight binary star systems, where the two stars orbit each other very closely, often completing an orbit in less than seven days. As these binary pairs slowly spiral inward over millions to billions of years due to tidal forces and interactions with surrounding material, the relativistic precession of the stars' orbits speeds up, while the precession of any orbiting planets slows down. At a critical point, the two precession rates lock together, dramatically stretching the planet's orbit into a long, narrow ellipse. This causes the planet to dive dangerously close to the stars, leading to either the planet's tidal disruption, engulfment by one of the stars, or its eventual ejection from the system.

• Over millions to billions of years, the binary stars spiral inward due to tidal forces and interactions with surrounding material.

What they’re saying

What’s next

The researchers suggest that future telescopes and observational techniques may be able to detect more planets orbiting binary star systems, especially those at wider distances where they are less likely to be destroyed by the effects of general relativity.