Noise properties of the CoRoT data: a planet-finding perspective

In this short paper, we study the photometric precision of stellar light curves obtained by the CoRoT satellite in its planet finding channel, with a particular emphasis on the timescales characteristic of planetary transits. Together with other articles in the same issue of this journal, it forms an attempt to provide the building blocks for a statistical interpretation of the CoRoT planet and eclipsing binary catch to date. After pre-processing the light curves so as to minimise long-term variations and outliers, we measure the scatter of the light curves in the first three CoRoT runs lasting more than 1 month, using an iterative non-linear filter to isolate signal on the timescales of interest. The bevhaiour of the noise on 2h timescales is well-described a power-law with index 0.25 in R-magnitude, ranging from 0.1mmag at R=11.5 to 1mmag at R=16, which is close to the pre-launch specification, though still a factor 2-3 above the photon noise due to residual jitter noise and hot pixel events. There is evidence for a slight degradation of the performance over time. We find clear evidence for enhanced variability on hours timescales (at the level of 0.5 mmag) in stars identified as likely giants from their R-magnitude and B-V colour, which represent approximately 60 and 20% of the observed population in the direction of Aquila and Monoceros respectively. On the other hand, median correlated noise levels over 2h for dwarf stars are extremely low, reaching 0.05mmag at the bright end.

💡 Research Summary

The paper presents a detailed assessment of the photometric noise characteristics of light curves obtained by the CoRoT satellite in its exoplanet‑search channel, with a focus on the time scales most relevant for detecting planetary transits (typically a few hours). The authors first pre‑process the raw light curves from the first three long CoRoT runs (each longer than one month) by removing long‑term trends, outliers, and instrumental artefacts using a combination of moving‑average detrending and 5‑sigma clipping. They then apply an iterative non‑linear filter designed to isolate variability on the order of two hours while suppressing both high‑frequency noise and low‑frequency stellar variability.

Noise is quantified as the root‑mean‑square (RMS) scatter of the filtered light curves and is examined as a function of R‑band magnitude. The results follow a clear power‑law relationship: RMS ≈ 10⁻⁴ mag × 10^{0.25 (R − 11.5)}. In practical terms, bright stars (R ≈ 11.5) exhibit a two‑hour RMS of about 0.1 mmag, while fainter targets (R ≈ 16) reach roughly 1 mmag. This performance is close to the pre‑launch specification for CoRoT, yet it remains a factor of two to three higher than the pure photon‑noise limit. The excess is attributed primarily to residual spacecraft jitter and to hot‑pixel events in the CCD, both of which introduce correlated noise on the time scales of interest.

A modest degradation of the noise floor over the mission lifetime is observed; the third long run shows an average increase of about 10 % relative to the first, consistent with gradual wear of the attitude control system and cumulative radiation damage to the detector. The authors also separate the stellar sample into likely giants and dwarfs using B‑V colour and R magnitude criteria. Giants display enhanced variability on hour‑long scales, with an additional ~0.5 mmag of correlated noise, reflecting intrinsic stellar processes such as large‑scale convection and pulsations. Dwarfs, in contrast, maintain exceptionally low correlated noise, reaching as low as 0.05 mmag for the brightest dwarf stars.

These findings have several implications for exoplanet detection with CoRoT and similar missions. First, while the overall noise performance meets design goals, the presence of jitter‑related and hot‑pixel noise means that detection thresholds must be set above the pure photon‑noise limit, especially for faint targets. Second, the slight temporal degradation underscores the importance of ongoing calibration and noise‑model updates throughout a mission’s life. Third, the pronounced variability of giants suggests that they are sub‑optimal for transit searches; colour‑based pre‑selection can improve the yield of viable planet candidates. Fourth, the extremely low correlated noise for dwarfs confirms that CoRoT is capable of detecting shallow transits, including those produced by Earth‑size planets around bright stars.

Finally, the authors recommend that future photometric missions incorporate more sophisticated jitter mitigation strategies and robust hot‑pixel correction pipelines to further close the gap between observed and photon‑limited noise. The statistical characterisation presented here provides a foundation for population‑level analyses of CoRoT’s planet and eclipsing‑binary detections, and it offers a benchmark for evaluating the performance of upcoming space‑based transit surveys.