Revisiting the Orbital Dynamics of the Hot Jupiter WASP-12b with New Transit Times

TL;DR

This study re-evaluates the orbital evolution of the hot Jupiter WASP-12b by compiling 391 transit timings from space (TESS) and ground-based surveys (ETD, ExoClock) plus seven new light curves. Three timing models are tested—linear, orbital decay, and apsidal precession—with the orbital-decay model yielding the best statistical support, a decay rate of about $-30.31$ ms yr$^{-1}$, and an inferred stellar tidal quality factor near $1.61 imes 10^{5}$. A planetary Love number of $k_p \,\approx\,0.66$ suggests a Jupiter-like interior, though the nonzero eccentricity allows apsidal precession as an additional contributing effect. The results imply rapid tidal evolution for WASP-12b and emphasize the need for continued high-precision monitoring to distinguish between decay and precession as the dominant mechanism behind the observed timing variations.

Abstract

In this study, we examine the transit timing variations (TTVs) of the extensively studied hot Jupiter WASP-12b using a comprehensive dataset of 391 transit light curves. The dataset includes 7 new photometric observations obtained with the 1.3 m Devasthal Fast Optical Telescope, the 0.61 m VASISTHA telescope, and the 0.3 m AG Optical IDK telescope, along with 119 light curves from the Transiting Exoplanet Survey Satellite (TESS), 97 from the Exoplanet Transit Database (ETD), 34 from the ExoClock Project, and 134 from previously published sources. To ensure homogeneity and precision, we modeled all 391 light curves and determined their mid-transit times. A detailed transit timing analysis revealed a significant orbital decay rate of $-30.31 \, \mathrm{ms \, yr^{-1}}$, corresponding to a stellar tidal quality factor of $Q'_\star = 1.61 \times 10^{5}$, thereby confirming that the orbit of WASP-12b is indeed decaying rapidly. Furthermore, the computation of model selection metrics ($χ^2_r$, BIC, AIC) favors orbital decay as the most likely explanation. However, the presence of an eccentricity above the threshold value allows apsidal precession to remain a viable alternative. We also derived a planetary Love number of $k_p = 0.66 \pm 0.28$, consistent with Jupiter's value, suggesting a similar internal density distribution. Therefore, while orbital decay is strongly supported, apsidal precession cannot be ruled out as another contributing effect, highlighting the necessity of continued high-precision monitoring to resolve the system's orbital evolution.

Figures (15)

• Figure 1: Light curve of WASP-12b observed by the 1.3 m Devasthal Fast Optical Telescope at Devasthal, Nainital of Aryabhatta Research Institute of Observational Sciences (ARIES). Here, the error bars corresponding to the normalized flux are shown in light blue.
• Figure 2: Light curves of WASP-12b observed by the 0.61 m telescope installed at IERCOO campus of ICSP, Kolkata. Here, the error bars corresponding to the normalized flux are shown in purple.
• Figure 3: Light curves of WASP-12b observed by the 0.3 m AG Optical IDK telescope at the Utah Desert Remote Observatory in Beryl, Utah. Here, the error bars corresponding to the normalized flux are shown in light green.
• Figure 4: O-C diagram for analysing 391 mid-transit times of WASP-12b. The blue filled square show the data from 2024PSJ.....5..163A, the red filled squares are from 2011AJ....141..179C, the green filled square is from 2017AJ....153...78C, the black filled circle is from 2024MNRAS.534..800A, the brown filled asterisks are from 2009ApJ...693.1920H, the magenta filled circles are from 2024AA...686A..84L, the yellow filled upward triangles are from 2011MNRAS.411.1204M, the green filled circles are from 2013AA...551A.108M, the orange filled right triangles are from the quality 1 and quality 2 data of ETD, the blue filled circles are from Exoclock, the red filled circle is from 2016AA...588L...6M, the blue filled asterisk is from 2020ApJ...888L...5Y, the yellow filled squares are from TESS data, the purple filled squares are from the new ground-based data for 7 nights. The dashed black line, red and blue curves represent the linear, orbital decay and apsidal precession models. The lines are drawn for 100 randomly chosen sets of parameters from the Markov chains of posteriors of the orbital decay (brown) and apsidal precession (cyan) models. The models are extrapolated for the next $\sim 10$ years to illustrate the broad spectrum of possible solutions.
• Figure 5: The normalized relative flux of WASP-12b as a function of the time (the offset from mid-transit time and in TDB-based BJD) of individual transit observed by TESS between epochs (3965 - 4546) : the points are the data of raw flux, solid red lines are best-fit models for model flux, and E is the calculated epoch number.