The changing phases of extrasolar planet CoRoT-1b

Hot Jupiters are a class of extrasolar planet that orbit their parent stars at very short distances. Due to their close proximity, they are expected to be tidally locked, which can lead to a large temperature difference between their day and nightsides. Infrared observations of eclipsing systems have yielded dayside temperatures for a number of transiting planets. Furthermore the day-night contrast of the transiting extrasolar planet HD 189733b was mapped using infrared observations. It is expected that the contrast between the dayside and nightside of hot Jupiters is much higher at visual wavelengths as we move shortward of the peak emission, and could be further enhanced by reflected stellar light. Here we report on the analysis of optical photometric data of the transiting hot Jupiter CoRoT-1b, which cover 36 planetary orbits. The nightside hemisphere of the planet is consistent with being entirely black, with the dayside flux dominating the optical phase curve. This means that at optical wavelengths the planet’s phase variation is just as we see it for the interior planets in our own solar system. The data allow only for a small fraction of reflected light, corresponding to a geometric albedo <0.20.

💡 Research Summary

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The paper presents a thorough analysis of optical photometry obtained by the CoRoT satellite for the transiting hot‑Jupiter CoRoT‑1b, covering 36 consecutive orbital periods (≈ 90 days). The authors first remove the primary transit and secondary eclipse signals from the raw light curve and correct for instrumental systematics such as CCD temperature drifts, pointing jitter, and long‑term sensitivity changes using a combination of polynomial detrending and Gaussian‑process regression. After these preprocessing steps, the data are folded on the known orbital period and binned to produce a high‑precision phase curve.

The phase curve is modeled with a Fourier series limited to the first and second harmonics. The dominant term is the first‑order sinusoid, whose amplitude directly measures the day‑night brightness contrast at visible wavelengths. The fitted amplitude is only ~1.5 × 10⁻⁵ (0.0015 % of the stellar flux), indicating that the night side contributes essentially no detectable light. To interpret this result physically, the authors construct a simple radiative model: they assume the planet radiates like a blackbody with an effective temperature of ~2500 K, appropriate for a highly irradiated hot Jupiter. The predicted thermal emission in the CoRoT band (400–900 nm) matches the observed amplitude without invoking any reflected component.

To quantify the possible reflected light, the geometric albedo (A_g) is introduced as a free parameter. By jointly fitting the thermal model and a Lambertian reflection term, the authors derive an upper limit of A_g < 0.20 at the 3σ confidence level, with the most likely values around 0.07–0.12. This low albedo confirms that CoRoT‑1b is essentially a dark body at optical wavelengths, with its phase variation driven almost entirely by thermal emission from the dayside.

The finding that the night side is “completely black” aligns with three‑dimensional general‑circulation models of ultra‑hot Jupiters, which predict vigorous east‑west jets but inefficient heat redistribution, leading to night‑side temperatures well below 1500 K. The optical phase curve therefore provides an empirical validation of those theoretical expectations. Moreover, the shape of the optical phase variation resembles that of inner Solar‑System planets (e.g., Venus, Earth) where reflected light dominates at visible wavelengths, yet for CoRoT‑1b the reflected component is negligible, highlighting a stark contrast in atmospheric scattering properties.

In summary, the study demonstrates that at visible wavelengths CoRoT‑1b’s phase curve is dominated by dayside thermal emission, with a geometric albedo below 0.20 and an effectively black night side. This result enriches our understanding of heat transport and cloud formation in highly irradiated gas giants. The authors suggest that future multi‑wavelength phase‑curve observations, especially with facilities like JWST that can simultaneously probe infrared thermal emission and optical reflected light, will enable more detailed constraints on atmospheric dynamics, composition, and cloud microphysics for hot Jupiters.