WASP-39b: a highly inflated Saturn-mass planet orbiting a late G-type star (1102.1375v2)

Abstract: We present the discovery of WASP-39b, a highly inflated transiting Saturn-mass planet orbiting a late G-type dwarf star with a period of $4.055259 \pm 0.000008$\,d, Transit Epoch T${0}=2455342.9688\pm0.0002$\,(HJD), of duration $0.1168 \pm 0.0008$\,d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of $\mpl=0.28\pm0.03\,\mj$ and a radius of $\rpl=1.27\pm0.04\,\rj$, resulting in a mean density of $0.14 \pm 0.02\,\rhoj$. The stellar parameters are mass $\mstar = 0.93 \pm 0.03\,\msun$, radius $\rstar = 0.895\pm 0.23\,\rsun$, and age $9^{+3}{-4}$\,Gyr. Only WASP-17b and WASP-31b have lower densities than WASP-39b, although they are slightly more massive and highly irradiated planets. From our spectral analysis, the metallicity of WASP-39 is measured to be \feh\,$= -0.12\pm0.1$\,dex, and we find the planet to have an equilibrium temperature of $1116^{+33}_{-32}$\,K\,. Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.

WASP-39b: A Saturn-Mass Exoplanet with Anomalously Low Density

The discovery of WASP-39b, a highly inflated Saturn-mass exoplanet orbiting a late G-type star, presents intriguing values for its mass and density, significantly advancing the understanding of planetary formation and characteristics in this mass regime. The paper describes the thorough photometric and spectroscopic analysis that led to the characterization of WASP-39b, leveraging both the SuperWASP survey and additional follow-up observations.

Key Findings

WASP-39b orbits its host star with a period of approximately 4.055 days. The planet's mass and radius are measured at 0.28 ± 0.03 M_☉ and 1.27 ± 0.04 R_☉, giving it a notably low mean density of 0.14 ± 0.02 g cm^-3. This ranks WASP-39b as the third least dense planet identified by ground-based transit surveys, with lower densities only observed in WASP-17b and WASP-31b. These metrics are critical as they deviate from models predicting planetary mass without cores, indicating inflation well beyond model anticipations.

Methodology

A multi-pronged observational approach was employed to arrive at these findings. Photometric data from SuperWASP, alongside high-precision follow-up photometry from the Faulkes Telescope North and the Euler Swiss telescope, solidified the identification of transits. Radial velocity data from SOPHIE and CORALIE spectrographs established the planet's mass and confirmed its planet-like nature, excluding stellar activity or blended binaries as possible explanations for the observed data.

Implications and Discussion

The low density of WASP-39b suggests a significantly inflated radius, a phenomenon seen in only a handful of Saturn-mass planets. Current models, such as those by Fortney and Baraffe, cannot adequately explain the inflation without invoking additional physical phenomena. The implications of such findings force a reevaluation of traditional models of planetary formation and evolution, particularly for sub-Jupiter mass exoplanets.

Further, the host star’s sub-solar metallicity ([Fe/H] = -0.12 ± 0.10) corroborates the observed trend that low-density planets often orbit stars with lesser metallicity. This correlation aligns with predictions from the core-accretion model of planet formation, which posits that metal-rich environments lead to increased core masses.

Future Perspectives

The ongoing discovery and characterization of Saturn-mass exoplanets like WASP-39b facilitate a deeper understanding of planetary diversity and structure. WASP-39b's anomalous radius could serve as a case paper for investigating potential interior heating mechanisms, such as Ohmic heating, and the role of atmospheric conditions in radius inflation. The paper provides a platform for theorists to develop more complex thermal evolution models to account for such anomalies.

Future studies should focus on improving the theoretical frameworks that account for planetary cooling history, interior compositions, and other physical parameters. Improved atmospheric characterization, potentially through next-generation space telescopes, may unravel the atmospheric dynamics contributing to planetary inflation and provide a more coherent understanding of the relationship between host star properties and planetary characteristics.