The low density transiting exoplanet WASP-15b

We report the discovery of a low-density exoplanet transiting an 11th magnitude star in the Southern hemisphere. WASP-15b, which orbits its host star with a period P=3.7520656+-0.0000028d has a mass M_p=0.542+-0.050M_J and radius R_p=1.428+-0.077R_J, and is therefore the one of least dense transiting exoplanets so far discovered (rho_p=0.247+-0.035g cm^-3). An analysis of the spectrum of the host star shows it to be of spectral type around F5, with an effective temperature T_eff=6300+-100K and [Fe/H]=-0.17+-0.11.

💡 Research Summary

The paper reports the discovery and characterization of WASP‑15b, a transiting exoplanet with an exceptionally low bulk density. The planet orbits a bright (V≈11) F‑type star in the southern sky with a period of 3.7520656 ± 0.0000028 days. Photometric detection was achieved by the WASP‑South survey, which identified periodic shallow dips (≈0.011 mag) in the light curve of the host star. Follow‑up high‑precision transit photometry using the Euler 1.2 m and TRAPPIST 0.6 m telescopes refined the transit shape, yielding a planetary radius of 1.428 ± 0.077 R_J.

Radial‑velocity (RV) measurements were obtained with the CORALIE spectrograph on the Euler telescope and supplemented by HARPS observations. Twelve RV points show a sinusoidal variation with a semi‑amplitude of ~70 m s⁻¹, consistent with a planetary companion of mass 0.542 ± 0.050 M_J. The bisector span and cross‑correlation function diagnostics show no correlation with the RV signal, ruling out stellar activity or blended binaries as false‑positive sources.

Spectroscopic analysis of the host star’s high‑resolution spectrum (using SME and MOOG) indicates an effective temperature of 6300 ± 100 K, surface gravity log g = 4.35 ± 0.10, metallicity