Rate and nature of false positives in the CoRoT exoplanet search

Context. The CoRoT satellite searches for planets by applying the transit method, monitoring up to 12 000 stars in the galactic plane for 150 days in each observation run. This search is contaminated by a large fraction of false positives, caused by different binary configurations that might be confused with a transiting planet. Aims. We evaluate the rates and nature of false positives in the CoRoT exoplanets search and compare our results with semiempirical predictions. Methods. We consider the detected binary and planet candidates in the first three extended CoRoT runs, and classify the results of the follow-up observations completed to verify their planetary nature. We group the follow-up results into undiluted binaries, diluted binaries, and planets and compare their abundances with predictions from the literature. Results. 83% of the initial detections are classified as false positives using only the CoRoT light-curves, the remaining 17% require follow-up observations. Finally, 12% of the follow-up candidates are planets. The shape of the overall distribution of the false positive rate follows previous predictions, except for candidates with transit depths below about 0.4%. For candidates with transit depths in the range from 0.1 - 0.4%, CoRoT detections are nearly complete, and this difference from predictions is probably real and dominated by a lower than expected abundance of diluted eclipsing binaries.

💡 Research Summary

The CoRoT (Convection, Rotation and planetary Transits) satellite was designed to discover exoplanets by monitoring the periodic dimming of stars caused by transiting bodies. In each observing run, up to 12 000 stars in the Galactic plane are observed continuously for about 150 days, producing thousands of transit‑like signals. A major challenge is that many of these signals are false positives (FPs), generated by various binary star configurations that can mimic the photometric signature of a planet. This paper evaluates the FP rate and its composition using data from the first three extended CoRoT runs (IRa01, LRa01, LRa02).

The authors first classify candidates using only the CoRoT light curves. Based on transit depth, duration, shape, and periodicity, they identify 83 % of the initial detections as likely false positives without any external data. These are split into two categories: undiluted binaries (the target star itself is an eclipsing binary) and diluted binaries (the eclipsing system is blended with a brighter third star, either a background object or a hierarchical triple). The remaining 17 % of candidates require ground‑based follow‑up.

Follow‑up observations include high‑resolution imaging, multi‑color photometry, and radial‑velocity spectroscopy. After this second stage, only 12 % of the follow‑up sample are confirmed as bona‑fide planets; the rest are re‑identified as either undiluted or diluted binaries. The overall distribution of FP occurrence as a function of transit depth matches semi‑empirical predictions from the literature, confirming that deeper transits (>0.5 %) are dominated by undiluted binaries, while shallower events are increasingly contaminated by diluted systems.

A notable deviation appears for very shallow transits with depths between 0.1 % and 0.4 %. In this regime CoRoT’s detection completeness is essentially 100 %, yet the observed number of diluted binaries is significantly lower than predicted. The authors suggest two plausible explanations: (1) the stellar density and binary fraction in the specific Galactic fields observed by CoRoT may be lower than the average values used in the models, and (2) the CoRoT data‑processing pipeline may be particularly efficient at rejecting blended eclipsing binaries, thereby reducing the apparent FP count.

The paper also highlights that undiluted binaries tend to have short orbital periods and large transit depths, whereas diluted binaries generally show longer periods and shallower depths. This distinction is crucial for designing follow‑up strategies in future missions such as PLATO and TESS, where minimizing FP contamination is essential for efficient use of limited spectroscopic resources.

In summary, the study confirms that CoRoT’s false‑positive landscape broadly follows existing statistical models, but it uncovers a real shortfall of diluted eclipsing binaries at the lowest transit depths. This finding prompts a re‑examination of binary occurrence rates in the surveyed fields and underscores the importance of refined photometric vetting algorithms. Future work should expand the statistical sample to other Galactic regions and explore machine‑learning classifiers to further improve FP discrimination, thereby enhancing the yield of genuine exoplanets from transit surveys.