Research reveals new clues about planets born around two suns
In 2011, scientists made an astonishing discovery that was once thought to exist only in imagination – a circumbinary planet. Unlike planets like Earth, which orbit a single star, these extraordinary planets revolve around two stars simultaneously, known collectively as binary stars. This discovery challenged the fundamental theories of planet formation. Circumbinary planets, long imagined in stories such as Tatooine from Star Wars or Solaris by Stanislaw Lem, were once considered nearly impossible to form based on the theoretical tools available at the time. So, their revelation raised several questions.
Thriving in the chaos of two-star systems
For years, scientists speculated that the gravitational forces of binary stars would prevent planets from assembling. The whirling, unstable environment created by the stars should theoretically disturb the disc of gas and dust from which planets grow. Even more intriguingly, follow-up studies revealed that the frequency of circumbinary gas giants around close binary stars is nearly the same as that of planets orbiting single stars. The scientific community realised that more examples of circumbinary planets were needed to unravel these mysteries. However, detecting such planets is challenging. The EU-funded BEBOP project was established to further explore these elusive worlds. “First, we sought to prove that a well-known method for detecting planets around single stars – the radial velocity method – could also be used to identify circumbinary planets,” notes Amaury Triaud, project coordinator. Another objective was to discover new circumbinary planets and compare their population with planets orbiting single stars. These comparisons should help scientists understand how binary stars affect planet formation and, in turn, shed light on the processes that shaped not only circumbinary systems but also our own Solar System. To achieve their goals, researchers collected thousands of spectroscopic measurements at the observatories in Chile’s Atacama Desert and in France’s Provence region. The team also gathered photometric data – measuring changes in brightness caused by a planet passing in front of its star – from Concordia Station in Antarctica. Researchers validated their methods by successfully confirming previously known circumbinary planets and then began monitoring hundreds of nearby binary stars seeking new discoveries.
Breaking new ground in planet research
Between 2022 and 2024, BEBOP delivered a series of significant milestones. “We made history in 2022 by detecting a known circumbinary planet using the radial velocity method for the very first time. Shortly afterwards, in 2023, we discovered a brand-new circumbinary planet and named it BEBOP-1c,” highlights Triaud. The team went on to detect a circumbinary planet around a double-lined binary, and have a number of new discoveries in the process of being announced. BEBOP also advanced the field by detecting photometric transits of a circumbinary planet using ground-based telescopes, a feat previously only accomplished with spacecrafts. The team’s ability to use ground-based facilities not only reduces costs but also offers a more sustainable way to collect astronomical observations.
Advancing understanding of planet formation
Together, BEBOP identified nine new circumbinary planets, with a few more candidates still awaiting confirmation. The researchers observed that, while gas giant planets are just as common in binary systems as they are in single-star systems, very massive planets (more than three times the mass of Jupiter) are surprisingly rare in binary systems. “What I am particularly excited about is finding out that many circumbinary planets appear less dense than anticipated. If confirmed, this anomaly could deepen understanding of the atmospheric compositions of these worlds,” concludes Triaud.
Keywords
BEBOP, planets, circumbinary, binary stars, planet formation, radial velocity
