A peculiarity is emerging in our galaxy as astronomers discover exoplanets: planets orbiting pairs of stars, similar to Tatooine from Star Wars, appear far fewer than expected compared to those accompanying solitary suns, even taking into account observational difficulties.
This situation has intrigued researchers for several years, as it seems to challenge established astronomical models. How can we explain why binary systems host so few planetary companions?
An artist's illustration of an Earth-like planet orbiting binary stars.
Credit: NASA/JPL-Caltech
Star duos, where two suns orbit each other, are however common in the Milky Way, and estimates suggested that at least 10% of them should host exoplanets. Nevertheless, among the more than 6,000 confirmed planets to date, only 14 have been identified around such stellar pairs, which constitutes a real puzzle for the scientific community.
A team from the University of California, Berkeley, and the American University of Beirut proposes an explanation linked to Albert Einstein's theory of general relativity, adding to the inherent observational difficulties of such systems (see chapter below).
Within these binary systems, the two stars follow elliptical trajectories, exposing any accompanying planet to particularly intricate gravitational forces. This configuration generates a phenomenon called precession, where the orientation of the planetary orbit gradually shifts over time, affecting its stability.
Simultaneously, the orbits of the stars themselves are subject to precession induced by general relativity. When these precession rates synchronize, resonance occurs, stretching the planet's trajectory considerably. According to Mohammad Farhat, this interaction can destabilize the orbit, leading either to the destruction of the object if it gets too close to the stars, or to its outright ejection from the system. This mechanism would thus provide a reason for the low observation rate of planets in such environments.
Modeling indicates that these perturbations are even more frequent in tight binaries, where stars complete an orbit in a week or less. Yet, these systems are precisely those targeted by missions like NASA's Kepler and TESS, which detect planets by observing the micro-eclipses caused by their passage in front of their star. Consequently, the observed rarity could stem from these dynamic instabilities rather than a real absence of planet formation, thus introducing a bias in our current data.
It is conceivable that hundreds of worlds analogous to Tatooine exist in the Milky Way, but their identification remains arduous with the techniques used today. Future research will need to integrate these relativistic effects to refine their surveys, potentially opening the door to new discoveries in the field of exoplanets.
Methods for detecting exoplanets
The hunt for exoplanets relies mainly on indirect techniques, as these worlds are too distant to be observed directly. The transit method is the most used: it involves measuring the periodic dimming of a star's brightness when a planet passes in front of it, like a shadow.
This approach has been widely exploited by space telescopes like Kepler and TESS, which continuously monitor thousands of stars. However, it works optimally for planets whose orbits are aligned with our line of sight, but can prove less effective in binary systems where light signals are disrupted by the presence of two stars.
Other techniques exist, such as the radial velocity method, which captures the oscillations of the star induced by the gravitational pull of a planet. It is more suited to massive planets or those close to their star. In double systems, multiple gravitational interactions complicate the analysis, which probably also contributes to the low number of confirmations.
These technical limits imply that our inventory of exoplanets remains partial, particularly for unusual stellar architectures. The development of new instruments and the improvement of algorithms could help overcome these obstacles, possibly revealing in the future a larger population of planets evolving around double suns.