The Apparently Decaying Orbit of WASP-12b

Abstract

We present new transit and occultation times for the hot Jupiter WASP-12b. The data are compatible with a constant period derivative:Ṗ = −29 ± 3 ms yr −1and P/Ṗ = 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 ⋆ is about 2 × 10 5 . If interpreted as precession, the planet's Love number is 0.44 ± 0.10. Orbital decay appears to be the more parsimonious model: it is favored by ∆χ 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 ∼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 it does require some mechanism to maintain an eccentricity of ≈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.

Do Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?

“…To evaluate the dominant radius inflation mechanism for these planets, we follow the prescription for tidal heating given by Miller et al (2009) andDobbs-Dixon et al (2004), and assume the synchronous rotation of the planet and tidal quality factors Q p =10 4 and Q * =10 6 , within an order of magnitude of observed and model constraints (Gallet et al 2017;Patra et al 2017). We find that if the planets are actively circularizing, tidal evolution driven by the star can dominate planetary heating by an order of magnitude over irradiative mechanisms.…”

“…Following the reasoning of Villaver et al (2014), the eccentricity evolution of a planetary orbit will be dominated by planetary tides driving orbit circularization on the main sequence, and stellar tides driving tidal inspiral on the red giant branch. For example, assuming Q p =Q * ∼10 6 , and using the equilibrium tide formulations of Patra et al (2017) derived from Goldreich & Soter (1966), the timescale for orbit circularization for K2-97b is ∼5 Gyr, while the tidal inspiral timescale is 2 Gyr. This suggests that orbital decay is driven more rapidly than eccentricity evolution as the stellar radius increases, producing a population of transient planets displaying moderate eccentricities at close-in orbits around evolved stars.…”

Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

“…constraints from binary stars and analyses of other planetary systems (see the discussion in Patra et al 2017), as well as the theoretical expectation that dissipation on the RGB is weaker because of the small core mass and radius (e.g., Gallet et al 2017).…”

“…This is expressed as (e.g., Patra et al 2017) where Q * ¢ is a modified tidal dissipation factor that includes the Love number, M * and * r the stellar mass and mean density, and G the gravitational constant.…”

Roche-lobe Overflow in Eccentric Planet–Star Systems

“…This suggests that many planets are near Roche lobe overflow (RLOF) (e.g., Figure 1 in Jackson et al 2017). For example, Li et al (2010) suggested that WASP-12 b is in the process of RLOF (see also Patra et al 2017). The presence of many gas giants in similar orbits suggests that RLOF may be common among exoplanets.…”