Physicist offers a new framework for understanding how planets form around binary stars.
In one of the most enduring images in science fiction, Luke Skywalker stands at the edge of the desert on his home planet, Tatooine, gazing toward the horizon as two suns sink into the sand. The scene, from the opening minutes of Star Wars, has come to symbolize longing and possibility—the sense that entire worlds might exist beyond the limits of what we know.
Fifty years after the iconic film first reached theaters, astronomers are still searching for real-world counterparts to that vision: planets that orbit not one star, but two. Such planets do exist, but they are far rarer than scientists once expected. Jihad Touma, professor of physics at the American University of Beirut and founding director of AUB's new School of Computing and Data Sciences (SCDS), says the discrepancy undermines expectations about how planetary systems work. “Binary stars are extremely common," he explains. “We would expect planets to appear around them in roughly the same proportion as around single stars. Instead, the observations consistently show that they do not."
Now, Touma and his former student Mohammad Farhat (BS Physics '16, MA Physics '19), currently a post-doctoral fellow at the University of California Berkeley, offer an explanation. In new research, they argue that an effect long considered marginal in planetary science plays a decisive role: the slow, subtle effects of Einstein's theory of general relativity. As tightly bound stars evolve over billions of years, Touma explains, relativistic perturbations destabilize the regions where planets might form or survive, quietly clearing out many potential worlds before astronomers ever have a chance to detect them.
The finding offers a new framework for understanding planetary systems and positions AUB at the forefront of a long-standing astrophysical puzzle.
Understanding “the Force
The shortage of planets around tight binary stars—systems in which two stars orbit each other in just a few days—proved especially troubling because, as Touma puts it, “The problem didn't go away with better data. It kept showing up, and the existing explanations weren't capturing what was happening."
These systems had been observed by the same telescopes and using the same techniques that successfully identified thousands of planets around single stars, and simple selection and observational biases were ruled out as an explanation.
The absence pointed to something more fundamental. Touma began to suspect that researchers were overlooking a slow-acting process—one that unfolds over immense timescales and subtly reshapes entire systems. The clue came from a different area of his work, focused on the behavior of binary black holes. “I realized that I already had the machinery to think about this," he says. “The physics is operating on very different scales, but the underlying dynamics are remarkably similar."
Those dynamics look like this: As binary stars evolve, their orbits do not remain fixed. Over hundreds of millions to billions of years, tidal interactions slowly draw the stars closer together. Once the stars become tightly bound, effects predicted by general relativity begin to shape their motion.
“These systems are no longer governed purely by Newtonian gravity," Touma explains. “Small relativistic effects build up over time, creating repeating gravitational disturbances that disrupt regions people once thought were safe for planets."
Over time, those disturbances act as long-term destabilizers. Planets that form farther from the binary on stable orbits can gradually be driven into chaotic orbits, entering chaotic orbits that eject them from the system or send them crashing into one of the stars. Because the process unfolds over immense timescales, it is invisible in individual systems but becomes apparent when astronomers examine large populations.
“What we're proposing is something natural," Touma says. “It's built into how these binaries form and evolve. When you follow that evolution carefully over billions of years, the outcome is no longer surprising."
Rethinking Long-Standing Models
On the whole, the findings make clear that long-term, relativistic effects cannot be treated as minor details in studies of planetary systems. Instead, they must be integrated into how astronomers think about where planets can form, how long they can survive, and why some of the universe's most common star systems may be far less hospitable to planets than once believed. In other words, the research calls for a reassessment of long-standing models of stellar and planetary evolution.
That work emerged from years of collaboration with students and colleagues at the American University of Beirut, Touma says. “It's generational research," he explains. “Over many years, graduate and undergraduate students have helped develop and test the underlying models. They put us in a position to appreciate things that would otherwise have been very difficult to see."
As former students carry the ideas into new institutions and projects, the research continues to evolve, extending far beyond the original question that first drew attention to planets with two suns. In the process, it is helping astronomers build a more complete picture of how rare—and how fragile—such worlds can be.