Departure from the constant-period ephemeris for the transiting exoplanet WASP-12 b

TL;DR

This study analyzes precise transit timings of exoplanet WASP-12 b over several years, revealing a departure from constant-period predictions likely due to orbital decay or precession, with implications for star-planet interaction models.

Contribution

The paper provides new transit observations and refines the ephemeris, demonstrating orbital period change and evaluating models of orbital decay versus precession for WASP-12 b.

Findings

Transit times deviate from linear ephemeris at 5 sigma confidence

Orbital period decreasing at approximately 0.0256 seconds per year

Orbital decay model is statistically favored over precession model

Abstract

Most hot Jupiters are expected to spiral in towards their host stars due to transfering of the angular momentum of the orbital motion to the stellar spin. Their orbits can also precess due to planet-star interactions. Calculations show that both effects could be detected for the very-hot exoplanet WASP-12 b using the method of precise transit timing over a timespan of the order of 10 yr. We acquired new precise light curves for 29 transits of WASP-12 b, spannning 4 observing seasons from November 2012 to February 2016. New mid-transit times, together with literature ones, were used to refine the transit ephemeris and analyse the timing residuals. We find that the transit times of WASP-12 b do not follow a linear ephemeris with a 5 sigma confidence level. They may be approximated with a quadratic ephemeris that gives a rate of change in the orbital period of -2.56 +/- 0.40 x 10^{-2} s/yr. The tidal quality parameter of the host star was found to be equal to 2.5 x 10^5 that is comparable to theoretical predictions for Sun-like stars. We also consider a model, in which the observed timing residuals are interpreted as a result of the apsidal precession. We find, however, that this model is statistically less probable than the orbital decay.