Astronomy
The Formation of Planetary Systems Around Binary Stars
Quick fact
The first confirmed circumbinary planet, Kepler-16b, orbits two stars that eclipse each other from our viewpoint, and its discovery showed that planets can indeed form around binary stars, defying earlier expectations.
Why this is interesting
Most stars in the universe are not alone—they have a stellar companion. Yet we are only just beginning to learn how planets survive the chaotic gravitational tango of binary star systems.
Read the full explanation
Understanding The Formation of Planetary Systems Around Binary Stars
To understand how planets form around binary stars, start with the familiar picture: a young star is surrounded by a rotating disk of gas and dust. Inside this disk, dust grains collide and stick together, gradually building up from pebbles to boulders to planets. Now imagine two stars instead of one. The two stars are born together from the same cloud, and they are surrounded by a common disk of gas and dust—the circumbinary disk. This disk is like a giant flattened pancake encircling both stars. But the gravity of the two stars is not uniform; it changes as they orbit each other. This gravitational disturbance can stir up the disk, causing regions to become denser or thinner. Crucially, the inner part of the disk near the binary is strongly disturbed, often leaving a central cavity largely cleared of material. Planet formation must then take place further out in the more stable regions of the disk. Within that outer disk, the same process of accretion can occur, building planets that orbit around the entire binary system—these are called circumbinary planets. Alternatively, planets might form in a disk that surrounds only one of the stars, called a circumstellar disk, if the gravitational influence of the companion is not too disruptive.
A deeper explanation
The key mechanism is the gravitational influence of the binary on the disk and on growing planets. The binary stars orbit each other, and their combined gravitational field creates a complex, time-varying potential. This rapidly changing gravity acts like a stirring spoon, causing the inner disk to lose angular momentum and spiral inward, clearing a central gap. The gap's size is typically about two to three times the binary's separation. Because of this disruption, the circumbinary disk is truncated and heated, making the conditions for ice and dust different from those around a single star. Planet formation in such a disk must happen in the cooler, quieter outer regions where the disk's motion is more regular. Once planets form, they can migrate inward due to gravitational interactions with the disk, but the binary's gravity can also destabilize their orbits. For a planet to survive, its orbit must be well-separated from the binary; planetary orbits that come too close to the binary become chaotic and are ejected. Additionally, in binary systems where the stars are far apart, planet formation can proceed around each star independently, but only if the disk is not too tidally truncated. This explains why planets in binaries often end up on wide orbits or on orbits that are misaligned with the binary plane. Understanding these processes is vital because most stars are in binaries, so these gravitational effects are not an exception but a common pathway in planet formation.