Ground-based Secondary Eclipse Research Articles

Storms or systematics? The changing secondary eclipse depth of WASP-12b

WASP-12b is one of the most well-studied transiting exoplanets, as its highly-inflated radius and its 1.1 day orbit around a G0-type star make it an excellent target for atmospheric categorisation through observation during its secondary eclipse. We present two new secondary eclipse observations of WASP-12b, acquired a year apart with the Wide Field Camera on the Isaac Newton Telescope (INT) and the IO:O instrument on the Liverpool Telescope (LT). These observations were conducted in the $i^\prime$-band, a window expected to be dominated by TiO features if present in appreciable quantities in the upper atmosphere. We measured eclipse depths that disagree with each other by $\sim$3$\sigma$ (0.97 $\pm$ 0.14 mmag on the INT and 0.44 $\pm$ 0.21 mmag on the LT), a result that is mirrored in previous $z^\prime$-band secondary eclipse measurements for WASP-12b. We explore explanations for these disagreements, including systematic errors and variable thermal emission in the dayside atmosphere of WASP-12b caused by temperature changes of a few hundred Kelvin: a possibility we cannot rule out from our analysis. Full-phase curves observed with TESS and CHEOPS have the potential to detect similar atmospheric variability for WASP-12b and other optimal targets, and a strategic, multi-telescope approach to future ground-based secondary eclipse observations is required to discriminate between explanations involving storms and systematics.

The GROUSE project

In recent years, day-side emission from about a dozen hot Jupiters has been detected through ground-based secondary eclipse observations in the near-infrared. These near-infrared observations are vital for determining the energy budgets of hot Jupiters, since they probe the planet's spectral energy distribution near its peak. The aim of this work is to measure the Ks-band secondary eclipse depth of WASP-33b, the first planet discovered to transit an A-type star. This planet receives the highest level of irradiation of all transiting planets discovered to date. Furthermore, its host-star shows pulsations and is classified as a low-amplitude delta-Scuti. As part of our GROUnd-based Secondary Eclipse (GROUSE) project we have obtained observations of two separate secondary eclipses of WASP-33b in the Ks-band using the LIRIS instrument on the William Herschel Telescope (WHT). The telescope was significantly defocused to avoid saturation of the detector for this bright star (K~7.5). To increase the stability and the cadence of the observations, they were performed in staring mode. We collected a total of 5100 and 6900 frames for the first and the second night respectively, both with an average cadence of 3.3 seconds. On the second night the eclipse is detected at the 12-sigma level, with a measured eclipse depth of 0.244+0.027-0.020 %. This eclipse depth corresponds to a brightness temperature of 3270+115-160 K. The measured brightness temperature on the second night is consistent with the expected equilibrium temperature for a planet with a very low albedo and a rapid re-radiation of the absorbed stellar light. For the other night the short out-of-eclipse baseline prevents good corrections for the stellar pulsations and systematic effects, which makes this dataset unreliable for eclipse depth measurements. This demonstrates the need of getting a sufficient out-of-eclipse baseline.

ON THE ORBIT OF EXOPLANET WASP-12b

We observed two secondary eclipses of the exoplanet WASP-12b using the Infrared Array Camera on the Spitzer Space Telescope. The close proximity of WASP-12b to its G-type star results in extreme tidal forces capable of inducing apsidal precession with a period as short as a few decades. This precession would be measurable if the orbit had a significant eccentricity, leading to an estimate of the tidal Love number and an assessment of the degree of central concentration in the planetary interior. An initial ground-based secondary eclipse phase reported by Lopez-Morales et al. (0.510 +/- 0.002) implied eccentricity at the 4.5 sigma level. The spectroscopic orbit of Hebb et al. has eccentricity 0.049 +/- 0.015, a 3 sigma result, implying an eclipse phase of 0.509 +/- 0.007. However, there is a well documented tendency of spectroscopic data to overestimate small eccentricities. Our eclipse phases are 0.5010 +/- 0.0006 (3.6 and 5.8 microns) and 0.5006 +/- 0.0007 (4.5 and 8.0 microns). An unlikely orbital precession scenario invoking an alignment of the orbit during the Spitzer observations could have explained this apparent discrepancy, but the final eclipse phase of Lopez-Morales et al. (0.510 -0.006 / +0.007) is consistent with a circular orbit at better than 2 sigma. An orbit fit to all the available transit, eclipse, and radial-velocity data indicates precession at <1 sigma; a non-precessing solution fits better. We also comment on analysis and reporting for Spitzer exoplanet data in light of recent re-analyses.

The GROUnd-based Secondary Eclipse project - GROUSE

AbstractSecondary eclipse observations of exoplanets at near-infrared wavelengths are important to constrain the energy budgets of hot-Jupiters, since they probe the radiation from the planet's atmosphere at the peak of the spectral energy distribution. Since this wavelength range is accesible from the ground, we have started the GROUnd-based Secondary Eclipse (GROUSE) project. As part of the GROUSE project, we target a sample of hot-Jupiters at near-infrared and optical wavelengths. Planets include TrES-3b, HAT-P-1, WASP-18b and WASP-33b.

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.