# The Apparently Decaying Orbit of WASP-12

## TL;DR

This study analyzes new transit and occultation data for WASP-12b, exploring whether its orbit is decaying or precessing, with implications for the planet's future and internal structure.

## Contribution

It provides the first detailed comparison between orbital decay and apsidal precession models for WASP-12b using new observational data.

## Key findings

- Orbital decay model is slightly favored over precession.
- Star's tidal quality parameter Q* is about 2×10^5 if decay is true.
- Planet's Love number is approximately 0.44 if precession explains the data.

## Abstract

We present new transit and occultation times for the hot Jupiter WASP-12b. The data are compatible with a constant period derivative: $\dot{P}=-29 \pm 3$ ms yr$^{-1}$ and $P/\dot{P}= 3.2$ Myr. However, it is difficult to tell whether we have observed orbital decay, or a portion of a 14-year apsidal precession cycle. If interpreted as decay, the star's tidal quality parameter $Q_\star$ is about $2\times 10^5$. If interpreted as precession, the planet's Love number is $0.44\pm 0.10$. Orbital decay appears to be the more parsimonious model: it is favored by $\Delta\chi^2=5.5$ despite having two fewer free parameters than the precession model. The decay model implies that WASP-12 was discovered within the final $\sim$0.2% of its existence, which is an unlikely coincidence but harmonizes with independent evidence that the planet is nearing disruption. Precession does not invoke any temporal coincidence, but does require some mechanism to maintain an eccentricity of $\approx$0.002 in the face of rapid tidal circularization. To distinguish unequivocally between decay and precession will probably require a few more years of monitoring. Particularly helpful will be occultation timing in 2019 and thereafter.