# Searching For Rapid Orbital Decay of WASP-18b

## TL;DR

This study investigates the orbital decay of exoplanet WASP-18b to measure the star's tidal dissipation efficiency, finding no rapid decay and suggesting the star's Q' is higher than previously inferred, indicating less tidal dissipation.

## Contribution

The paper provides the first nine-year observational analysis of WASP-18b's orbital decay, constraining the star's tidal quality factor Q' to be greater than 10^6, challenging prior expectations.

## Key findings

- No evidence of rapid orbital decay over nine years
- Q' for the host star is at least 10^6 at 95% confidence
- Supports the idea that F-stars are less tidally dissipative than solar-type stars

## Abstract

The WASP-18 system, with its massive and extremely close-in planet, WASP-18b (M$_{p}$ = 10.3M$_{J}$, a = 0.02 AU, P = 22.6 hours), is one of the best known exoplanet laboratories to directly measure Q', the modified tidal quality factor and proxy for efficiency of tidal dissipation, of the host star. Previous analysis predicted a rapid orbital decay of the planet toward its host star that should be measurable on the time scale of a few years, if the star is as dissipative as is inferred from the circularization of close-in solar-type binary stars. We have compiled published transit and secondary eclipse timing (as observed by WASP, TRAPPIST, and Spitzer) with more recent unpublished light curves (as observed by TRAPPIST and HST) with coverage spanning nine years. We find no signature of a rapid decay. We conclude that the absence of rapid orbital decay most likely derives from Q' being larger than was inferred from solar-type stars, and find that Q' $\geq $1$\times10^{6}$, at 95\,\% confidence; this supports previous work suggesting that F-stars, with their convective cores and thin convective envelopes, are significantly less tidally dissipative than solar-type stars, with radiative cores and large convective envelopes.