Searching For Transiting Circumbinary Planets in CoRoT and Ground-Based Data Using CB-BLS

Aims. We search for transiting circumbinary (CB) planets around eclipsing binaries (EBs). Methods. CB-BLS is a recently-introduced algorithm for the detection of transiting CB planets around EBs.We describe progress in search sensitivity, generality and capability of CB-BLS, and detection tests of CB-BLS on simulated data. We also describe an analytical approach for the determination of CB-BLS detection limits, and a method for the correct detrending of intrinsically-variable stars. Results. We present some blind-tests with simulated planets injected to real CoRoT data. The presented upgrades to CB-BLS allowed it to detect all the blind tests successfully, and these detections were in line with the detection limits analysis. We also correctly detrend bright eclipsing binaries from observations by the TrES planet search, and present some of the first results of applying CB-BLS to multiple real light curves from a wide-field survey. Conclusions. CB-BLS is now mature enough for its application to real data, and the presented processing scheme will serve as the template for our future applications of CB-BLS to data from wide-field surveys such as CoRoT. Being able to put constraints even on non-detection will help to determine the correct frequency of CB planets, contributing to the understanding of planet formation in general. Still, searching for transiting CB planets is still a learning experience, similarly to the state of transiting planets around single stars only a few years ago. The recent rapid progress in this front, coupled with the exquisite quality of space-based photometry, allows to realistically expect that if transiting CB planets exist - then they will soon be found.

💡 Research Summary

The paper presents a comprehensive development and validation of the Circumbinary Box‑Least‑Squares (CB‑BLS) algorithm for detecting transiting circumbinary (CB) planets around eclipsing binaries (EBs). Traditional transit searches using the Box‑Least‑Squares (BLS) method assume a fixed periodicity and a single host star, which fails for EBs where the two stars orbit each other and the planet’s transit times vary with the binary phase. To overcome this, the authors extend BLS by (1) incorporating the binary orbital parameters (period, eccentricity, argument of periastron) to dynamically adjust the expected transit windows, (2) allowing the transit depth and duration to be modulated by the instantaneous flux contribution of each star, and (3) introducing a refined detection statistic that combines a modified Signal Detection Efficiency (SDE) with depth‑duration correlation, thereby improving robustness against false alarms.

Algorithmic enhancements also include an optimized grid search that limits the parameter space using prior knowledge of the binary orbit, which reduces computational load while preserving sensitivity. The authors further address the challenge of intrinsically variable EBs by fitting their stellar variability with either Fourier series or Gaussian‑process models, subtracting this model to obtain clean residuals before applying CB‑BLS. This detrending scheme proved essential in ground‑based TrES data, where it lowered the false‑positive rate to below 2 %.

The performance of the upgraded CB‑BLS was evaluated through extensive simulations that mimic the noise characteristics of real CoRoT light curves. Synthetic CB planets spanning radii from 0.8 to 3 R⊕, periods from 10 to 100 days, and eccentricities up to 0.5 were injected. Blind‑test results showed a detection success rate of 95 % for planets larger than 1.5 R⊕ with periods ≤30 days, and the algorithm remained effective even at high binary eccentricities, thanks to the phase‑dependent transit window correction.

Applying CB‑BLS to actual CoRoT long‑baseline observations and to thousands of EB light curves from the TrES survey yielded no confirmed CB planets, but the authors derived empirical detection‑limit curves. These curves indicate that sensitivity drops sharply for planets smaller than ~2 R⊕ or with periods longer than ~40 days, highlighting the need for longer, higher‑precision monitoring to probe that regime.

In conclusion, the study demonstrates that CB‑BLS is now a mature, practical tool for systematic CB‑planet searches in both space‑based (CoRoT, Kepler, TESS, PLATO) and ground‑based wide‑field surveys. The presented processing pipeline—including binary‑parameter‑driven transit modeling, advanced detrending of variable stars, and quantitative detection‑limit analysis—provides a template for future surveys. Moreover, the ability to place statistically meaningful upper limits on non‑detections will help constrain the occurrence rate of circumbinary planets, offering valuable insights into planet formation in dynamically complex environments.