TESS Revisits WASP-12: Updated Orbital Decay Rate and Constraints on Atmospheric Variability

Abstract

After observing WASP-12 in the second year of the primary mission, the Transiting Exoplanet Survey Satellite (TESS) revisited the system in late 2021 during its extended mission. In this paper, we incorporate the new TESS photometry into a reanalysis of the transits, secondary eclipses, and phase curve. We also present a new K s -band occultation observation of WASP-12b obtained with the Palomar/WIRC instrument. The latest TESS photometry span three consecutive months, quadrupling the total length of the TESS WASP-12 light curve and extending the overall time baseline by almost two years. Based on the full set of available transit and occultation timings, we find that the orbital period is shrinking at a rate of −29.81 ± 0.94 ms yr −1 . The additional data also increase the measurement precision of the transit depth, orbital parameters, and phase-curve amplitudes. We obtain a secondary eclipse depth of 466 ± 35 ppm, a 2σ upper limit on the nightside brightness of 70 ppm, and a marginal 6 • .2 ± 2 • .8 eastward offset between the dayside hotspot and the substellar point. The voluminous TESS dataset allows us to assess the level of atmospheric variability on timescales of days, months, and years. We do not detect any statistically significant modulations in the secondary eclipse depth or day-night brightness contrast. Likewise, our measured K s -band occultation depth of 2810±390 ppm is consistent with most ∼2.2 µm observations in the literature.

Measuring the orbit shrinkage rate of hot Jupiters due to tides

“…From our result of Q , the estimated spiral in-fall timescale is around 5 × 10 7 yr, which is longer than originally expected. For comparison, WASP-12b, the only planet with confirmed tidal decay so far has a corresponding Q = (1.50 ± 0.11) × 10 5 (Wong et al 2022), which is two orders of magnitude lower than the minimum value we found for WASP-18. WASP-12 is also an F-type star, with an effective temperature of 6150 K, with a mass similar to WASP-18.…”

“…Its orbit is shrinking at a rate of 29.81 ± 0.94 ms yr −1 , leading to a decay timescale of 3.16 ± 0.10 Myr. The tidal quality factor of Q = (1.50 ± 0.11) × 10 5 is thought to be on the lower side of the predicted values (Wong et al 2022).…”

“…WASP-12b, a hot Jupiter orbiting a late F-type star, was the first target confirmed as having a decaying orbit caused by tidal interactions (Patra et al 2017;Maciejewski et al 2018;Turner et al 2021b;Wong et al 2022). It orbits its star with a period of 1.09 days, and has a mass of 1.5 M J and a radius of 1.90 R J .…”