Detection of the tidal deformation of WASP-103b at 3 σ with CHEOPS

Abstract

Context. Ultra-short period planets undergo strong tidal interactions with their host star which lead to planet deformation and orbital tidal decay. Aims. WASP-103b is the exoplanet with the highest expected deformation signature in its transit light curve and one of the shortest expected spiral-in times. Measuring the tidal deformation of the planet would allow us to estimate the second degree fluid Love number and gain insight into the planet’s internal structure. Moreover, measuring the tidal decay timescale would allow us to estimate the stellar tidal quality factor, which is key to constraining stellar physics. Methods. We obtained 12 transit light curves of WASP-103b with the CHaracterising ExOplanet Satellite (CHEOPS) to estimate the tidal deformation and tidal decay of this extreme system. We modelled the high-precision CHEOPS transit light curves together with systematic instrumental noise using multi-dimensional Gaussian process regression informed by a set of instrumental parameters. To model the tidal deformation, we used a parametrisation model which allowed us to determine the second degree fluid Love number of the planet. We combined our light curves with previously observed transits of WASP-103b with the Hubble Space Telescope (HST) and Spitzer to increase the signal-to-noise of the light curve and better distinguish the minute signal expected from the planetary deformation. Results. We estimate the radial Love number of WASP-103b to be hf = 1.59−0.53+0.45. This is the first time that the tidal deformation is directly detected (at 3 σ) from the transit light curve of an exoplanet. Combining the transit times derived from CHEOPS, HST, and Spitzer light curves with the other transit times available in the literature, we find no significant orbital period variation for WASP-103b. However, the data show a hint of an orbital period increase instead of a decrease, as is expected for tidal decay. This could be either due to a visual companion star if this star is bound, the Applegate effect, or a statistical artefact. Conclusions. The estimated Love number of WASP-103b is similar to Jupiter’s. This will allow us to constrain the internal structure and composition of WASP-103b, which could provide clues on the inflation of hot Jupiters. Future observations with James Webb Space Telescope can better constrain the radial Love number of WASP-103b due to their high signal-to-noise and the smaller signature of limb darkening in the infrared. A longer time baseline is needed to constrain the tidal decay in this system.

A CHEOPS Search for Massive, Long-period Companions to the Warm Jupiter K2-139 b

“…K2-139 is fainter than all of the targets studied by Borsato et al (2021), and none of our transits has good coverage of both ingress and egress, which is needed for the most-precise transit times. Good coverage of both ingress and egress would lead to transit times with a precision of around 30-60 s, as measured with CHEOPS for WASP-103 (Barros et al 2022), which is slightly fainter than K2-139.…”

“…CHEOPS (Benz et al 2021) is a small (0.32 m diameter) telescope designed for high-precision monoband photometry of individual exoplanetary systems. CHEOPS was launched in 2019 December, and science operations commenced in 2020 April, with several studies (e.g., Lendl et al 2020;Barros et al 2022) already published reporting results from the mission. We observed four transits of K2-139 b with CHEOPS, under the Guest Observers' program AO-1-005 (PI: Smith).…”

Tidal Distortions as a Bottleneck on Constraining Exoplanet Compositions

“…We find also that for most planets, even those with shorter orbital periods, measured uncertainties are currently too large to be affected by such deviations, however we identify a sample of planets whose uncer-tainties may be as much as four times smaller than the potential change caused by shape distortions. One such planet, WASP-103 b, was recently found to show tentative tidal deformations using multiple transit observations (Barros et al 2022), where it is reported that the volumetric radius of a fit derived using an ellipsoidal planet model is 5-6% larger than the radius derived from a spherical planet model. This further strengthens the notion that for such planets susceptible to tidal deformation, any attempts to characterise their interior composition based on their density derived using spherical planet models are likely to be under-estimating their errors, and that there is a wall of accuracy which is limited by a lack of knowledge of their true shape.…”

Tidal Distortions as a Bottleneck on Constraining Exoplanet Compositions

“…We find also that for most planets, even those with shorter orbital periods, measured uncertainties are currently too large to be affected by such deviations; however, we identify a sample of planets whose uncertainties may be as much as 4× smaller than the potential change caused by shape distortions. One such planet, WASP-103 b, was recently found to show tentative tidal deformations using multiple transit observations (Barros et al 2022), where it is reported that the volumetric radius of a fit derived using an ellipsoidal planet model is 5%-6% larger than the radius derived from a spherical planet model. This further strengthens the notion that for such planets susceptible to tidal deformation, any attempts to characterize their interior composition based on their density derived using spherical planet models are likely to be under-estimating their errors and that there is a wall of accuracy, which is limited by a lack of knowledge of their true shape.…”