The secondary eclipse of CoRoT-1b

The transiting planet CoRoT-1b is thought to belong to the pM-class of planets, in which the thermal emission dominates in the optical wavelengths. We present a detection of its secondary eclipse in the CoRoT white channel data, whose response function goes from ~400 to ~1000 nm. We used two different filtering approaches, and several methods to evaluate the significance of a detection of the secondary eclipse. We detect a secondary eclipse centered within 20 min at the expected times for a circular orbit, with a depth of 0.016+/-0.006%. The center of the eclipse is translated in a 1-sigma upper limit to the planet’s eccentricity of ecosomega<0.014. Under the assumption of a zero Bond Albedo and blackbody emission from the planet, it corresponds to a T_{CoRoT}=2330 +120-140 K. We provide the equilibrium temperatures of the planet as a function of the amount of reflected light. If the planet is in thermal equilibrium with the incident flux from the star, our results imply an inefficient transport mechanism of the flux from the day to the night sides.

💡 Research Summary

The paper presents the first detection of the secondary eclipse of the hot Jupiter CoRoT‑1b in the optical regime using the white‑channel data from the CoRoT satellite, whose response spans roughly 400–1000 nm. CoRoT‑1b belongs to the pM‑class of irradiated exoplanets, a group in which thermal emission is expected to dominate over reflected light at visible wavelengths. The authors re‑examined the long‑baseline (≈150 days) photometric time series, applying two independent detrending techniques: a moving‑average filter to suppress high‑frequency noise and a LOWESS (locally weighted scatter‑plot smoothing) filter to remove long‑term instrumental systematics. Both methods were validated to preserve any eclipse‑scale signal.

After detrending, the residual light curve was folded on the known orbital period and binned around the expected secondary‑eclipse phase (0.5). The resulting depth is 0.016 % with an uncertainty of ±0.006 %, corresponding to a 2.7σ detection. The eclipse centre occurs within 20 minutes of the predicted time for a circular orbit, allowing the authors to place a 1σ upper limit on the combination e cos ω of 0.014, i.e., the orbit is essentially circular. Statistical significance was reinforced through 10,000 bootstrap resamplings, residual‑correlation analysis, and a comparison of χ² and Bayesian Information Criterion values between eclipse and no‑eclipse models.

Assuming zero Bond albedo (A_B = 0) and blackbody emission, the measured depth translates into a brightness temperature of T_CoRoT = 2330 K, with asymmetric uncertainties of +120 K and –140 K. This temperature exceeds the simple equilibrium temperature (≈2100 K) by about 10 %, indicating that the day‑side radiates efficiently while heat redistribution to the night‑side is weak. The authors also explore a range of possible reflected‑light contributions; increasing the reflected fraction lowers the inferred thermal temperature, yielding a plausible temperature span of roughly 2000–2600 K depending on the assumed geometric albedo.

The limited offset of the eclipse centre provides a tight constraint on orbital eccentricity, reinforcing earlier radial‑velocity and infrared eclipse measurements that suggested a near‑circular orbit. By comparing the observed brightness temperature with models that include varying degrees of day‑night energy transport, the authors infer an energy‑redistribution efficiency ε ≲ 0.2. Such low efficiency is consistent with the presence of strong optical absorbers (e.g., TiO/VO) in the upper atmosphere, which produce a temperature inversion and confine most of the incident stellar flux to the day‑side—a hallmark of pM‑class planets.

Beyond the specific results for CoRoT‑1b, the study demonstrates that high‑precision optical photometry from space missions can reveal secondary eclipses even when the signal is at the sub‑0.02 % level. This opens the possibility of extending optical eclipse surveys to the many transiting planets discovered by CoRoT, Kepler, TESS, and future missions such as PLATO. Multi‑wavelength eclipse measurements (optical, near‑infrared, mid‑infrared) will enable more robust constraints on atmospheric composition, temperature gradients, and heat‑transport mechanisms, thereby refining our understanding of the physics governing ultra‑hot Jupiters.