Planets with two suns, like the famous desert world Tatooine from Star Wars, capture our imagination.
Yet in real life, these so-called circumbinary planets—planets that orbit two stars at once—are surprisingly rare.
Astronomers have now uncovered a compelling explanation, and it turns out that Albert Einstein’s theory of general relativity plays a central role.
Over the past two decades, astronomers have confirmed more than 6,000 exoplanets. Most orbit single stars, even though stars are just as likely to be born in pairs as alone.
In theory, hundreds of circumbinary planets should have been detected by missions such as NASA’s Kepler and TESS space telescopes. In reality, only 14 confirmed planets are known to orbit two stars, and none circle very tight pairs of stars that orbit each other in less than about seven days.
Researchers from University of California, Berkeley and the American University of Beirut believe they now know why.
Their study, published in The Astrophysical Journal Letters, shows that subtle but powerful effects predicted by Einstein’s general theory of relativity can destabilize planets around close binary stars and ultimately remove them from the system.
In most binary systems, the two stars have slightly different masses and orbit each other in elongated, oval-shaped paths.
A planet orbiting both stars feels a constantly changing gravitational pull, which causes its orbit to slowly rotate, or precess—similar to how a spinning top wobbles as it spins. This effect is well explained by Isaac Newton’s laws of gravity.
However, the stars themselves also experience orbital precession, and this is where general relativity enters the picture.
As the two stars interact and raise tides on each other, their orbit slowly shrinks over billions of years. As they draw closer together, relativistic effects become stronger, causing the stars’ orbit to precess faster and faster.
At the same time, the planet’s own precession slows down. Eventually, the precession rate of the planet and the stars can line up in a kind of resonance. When that happens, the planet’s orbit becomes increasingly stretched. It swings very far away at one point, but dives dangerously close at another.
According to lead author Mohammad Farhat, this leads to a dramatic outcome. Either the planet gets pulled so close that it is torn apart or swallowed by one of the stars, or it is flung completely out of the system. In both cases, the planet is effectively erased.
This process helps explain a long-standing mystery. Astronomers already knew that there is an “instability zone” around binary stars where planets cannot survive. What puzzled them was why even planets just outside this zone were so rare, especially around tight binaries. The new study shows that general relativity naturally clears out these systems over relatively short timescales—tens of millions of years—compared with the billions of years stars live.
The findings also explain why the few circumbinary planets we do see tend to orbit farther away from their stars, where they are harder to detect using current methods. These distant planets may survive, but they are unlikely to pass directly in front of their stars from our point of view, which is how most exoplanets are discovered.
The work highlights how Einstein’s theory, proposed in 1915 and famously used to explain the odd motion of Mercury’s orbit, still shapes our understanding of the universe today. In some cases, general relativity brings stability. In others, such as around tight binary stars, it quietly but efficiently destroys entire planetary systems.
In short, planets with two suns may not be impossible—but nature, guided by relativity, seems determined to make them exceedingly rare.
Source: UC Berkeley.
