HAT-P-12b: A Low-Density Sub-Saturn Mass Planet Transiting a Metal-Poor K Dwarf

We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V=12.8 K4 dwarf GSC 03033-00706, with a period P = 3.2130598 +- 0.0000021 d, transit epoch Tc = 2454419.19556 +- 0.00020 (BJD) and transit duration 0.0974 +- 0.0006 d. The host star has a mass of 0.73 +- 0.02 Msun, radius of 0.70 +- ^0.02_0.01 Rsun, effective temperature 4650 +- 60 K and metallicity [Fe/H] = -0.29 +- 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211 +- 0.012 MJup, and a radius of 0.959 +- ^0.029_0.021 RJup yielding a mean density of 0.295 +- 0.025 g cm^-3. Comparing these observations with recent theoretical models we find that HAT-P-12b is consistent with a ~ 1-4.5 Gyr, mildly irradiated, H/He dominated planet with a core mass Mc <~ 10 Mearth. HAT-P-12b is thus the least massive H/He dominated gas giant planet found to date. This record was previously held by Saturn.

💡 Research Summary

The paper reports the discovery and characterization of HAT‑P‑12b, a transiting exoplanet orbiting the moderately bright (V = 12.8) K4 dwarf GSC 03033‑00706. The planet was initially identified by the HAT‑Net wide‑field photometric survey, which collected over ten thousand images with the HAT‑6, HAT‑9, and HAT‑10 telescopes between 2005 and 2006. A Box‑Least‑Squares (BLS) search revealed a periodic dip with a period of 3.213 days, a depth of ~1.4 % and a duration of ~2.34 hours. Follow‑up high‑precision radial‑velocity (RV) measurements were obtained with the Keck I/HIRES spectrograph and the SOPHIE spectrograph on the OHP 1.93 m telescope. Eight RV points showed a semi‑amplitude of 35 m s⁻¹, consistent with a low‑mass gas giant of roughly 0.2 MJup.

During the RV analysis a weak positive correlation between the RV values and the spectral line bisector spans was noticed. To rule out false‑positive scenarios, the authors performed an extensive blend analysis using the BLENDER technique, testing hierarchical triple systems, background eclipsing binaries, and scenarios where the eclipsing body is a star rather than a planet. Color‑magnitude constraints, the shape of the transit light curve, and the lack of large bisector variations together excluded all blend configurations. The authors further argued that the observed bisector–RV correlation is likely caused by scattered moonlight contaminating the spectra during some observations, rather than a genuine astrophysical effect.

Stellar parameters were derived by combining SME (Spectroscopy Made Easy) analysis of the high‑resolution spectra with Yonsei‑Yale isochrones. The host star has a mass of 0.73 ± 0.02 M☉, a radius of 0.70 ± 0.02 R☉, an effective temperature of 4650 ± 60 K, and a metallicity of