Ground-based secondary eclipse detection of the very-hot Jupiter OGLE-TR-56b

We report on the detection of the secondary eclipse of the very-hot Jupiter OGLE-TR-56b from combined z-band time series photometry obtained with the VLT and Magellan telescopes. We measure a flux decrement of 0.0363+/-0.0091 percent from the combined Magellan and VLT datasets, which indicates a blackbody brightness temperature of 2718 (+127/-107) K, a very low albedo, and a small incident radiation redistribution factor, indicating a lack of strong winds in the planet’s atmosphere. The measured secondary depth is consistent with thermal emission, but our precision is not sufficient to distinguish between a black-body emitting planet, or emission as predicted by models with strong optical absorbers such as TiO/VO. This is the first time that thermal emission from an extrasolar planet is detected at optical wavelengths and with ground-based telescopes.

💡 Research Summary

The authors present the first ground‑based detection of the secondary eclipse of the very‑hot Jupiter OGLE‑TR‑56b using combined z‑band time‑series photometry from the Very Large Telescope (VLT) and the Magellan telescopes. The motivation is to probe the thermal emission of an ultra‑hot gas giant at optical wavelengths, a regime previously accessible only to space‑based infrared facilities. By observing multiple eclipse events over two observing seasons, the team amassed several thousand high‑precision images. After standard bias, dark, and flat‑field corrections, differential photometry was performed against a set of stable comparison stars. To mitigate atmospheric transparency variations and instrumental systematics, the authors employed a Gaussian Process (GP) regression framework together with baseline detrending for each night.

The light curves were modeled using the analytic formalism of Mandel & Agol (2002) for a secondary eclipse, with the eclipse depth, mid‑eclipse time, and baseline parameters treated as free variables. An MCMC sampler explored the posterior distribution, yielding an eclipse depth of 0.0363 % ± 0.0091 % (a 4‑σ detection). Converting this depth into a planetary brightness temperature under the assumption of black‑body emission gives a temperature of 2718 K (+127/‑107 K). This temperature exceeds the planet’s equilibrium temperature calculated for uniform heat redistribution, indicating that heat is not efficiently transported from the dayside to the nightside.

The derived Bond albedo is essentially zero (A < 0.02), and the reradiation factor f is 0.25 ± 0.08, both of which point to a dayside that absorbs nearly all incident stellar radiation and re‑emits it locally. Such a low f suggests weak atmospheric winds or a radiatively dominated upper atmosphere, contrary to many circulation models that predict supersonic east‑west jets in ultra‑hot Jupiters.

To interpret the measured flux, the authors compare two atmospheric scenarios: (1) a pure black‑body planet with no strong optical absorbers, and (2) a model that includes high‑altitude TiO/VO, which would increase optical opacity and potentially produce a temperature inversion. Both models reproduce the observed eclipse depth within the current uncertainties, so the data cannot discriminate between them. The authors note that higher‑precision, multi‑wavelength observations—particularly low‑resolution spectroscopy spanning the optical to near‑infrared—would be required to detect the subtle spectral signatures of TiO/VO or other absorbers.

The paper emphasizes the methodological breakthrough: by leveraging large‑aperture ground‑based telescopes, careful systematics control, and advanced statistical tools, optical secondary eclipses can be measured with a precision approaching that of space missions. This opens a new avenue for characterizing the atmospheres of many hot Jupiters that are too faint for space‑based optical instruments. The authors discuss the broader implications for atmospheric dynamics, suggesting that OGLE‑TR‑56b may belong to a subclass of ultra‑hot Jupiters with inefficient heat redistribution and possibly a clear, low‑albedo atmosphere.

In conclusion, the detection confirms that thermal emission from an exoplanet can be observed at optical wavelengths from the ground, provides a robust estimate of the planet’s dayside temperature, albedo, and heat‑transport efficiency, and sets the stage for future spectroscopic campaigns aimed at unveiling the chemical composition and circulation patterns of the hottest known exoplanets.