Circumbinary Planets Around Evolved Stars

The current state of evidence for planets around evolved star binaries is reviewed. The small sizes of compact evolved stars leads to sharp features when they are eclipsed by binary companions, enabling these eclipses to be used as precise clocks. In principle, these measurements are sensitive enough to detect the perturbations due to super-Earth mass planets in decade-long orbits around these binaries. Significant timing perturbations have now been measured in dozens of systems, with planetary orbits proposed in many cases, often with multiple planets required to fit the observed variations. However, the current situation is unclear, with almost all proposed orbits found to be inconsistent with new data or proving to be dynamically unstable. Variations in the internal structure of the companion stars are a probable source of noise in these measurements, although proposed mechanisms struggle to explain the magnitude of the timing variations seen in many systems. Growing evidence that evolved star binaries can possess circumbinary discs that bear a striking similarity to protoplanetary discs around young stars demonstrates the need for progress in this field, since they may be key sites of second-generation planet formation. Such progress is likely imminent, given the large number of newly discovered systems with either guaranteed timing stability or independent clocks.