The 0.5M$_J$ transiting exoplanet WASP-13b

We report the discovery of WASP-13b, a low-mass $ M_p = 0.46 ^{+ 0.06}{- 0.05} M_J$ transiting exoplanet with an orbital period of $4.35298 \pm 0.00004$ days. The transit has a depth of 9 mmag, and although our follow-up photometry does not allow us to constrain the impact parameter well ($0 < b < 0.46$), with radius in the range $R_p \sim 1.06 - 1.21 R_J$ the location of WASP-13b in the mass-radius plane is nevertheless consistent with H/He-dominated, irradiated, low core mass and core-free theoretical models. The G1V host star is similar to the Sun in mass (M${*} = 1.03^{+0.11}_ {- 0.09} M_{\odot}$) and metallicity ([M/H]=$0.0\pm0.2$), but is possibly older ($8.5^{+ 5.5}_{- 4.9}$ Gyr).

💡 Research Summary

The paper reports the discovery and characterization of WASP‑13b, a low‑mass transiting exoplanet identified by the Wide Angle Search for Planets (WASP) survey. Initial photometric data collected between 2006 and 2008 revealed a periodic 9 mmag dip occurring every 4.35298 ± 0.00004 days. Follow‑up observations were carried out with the SOPHIE spectrograph on the 1.93 m telescope at OHP and with the Liverpool Telescope (2 m) to obtain high‑precision radial velocities and additional transit light curves.

Radial‑velocity measurements (eight data points) show a semi‑amplitude K ≈ 0.13 km s⁻¹ with a scatter of ~20 m s⁻¹, confirming a planetary companion on a nearly circular orbit (eccentricity consistent with zero). The derived planetary mass is Mₚ = 0.46 M_Jup, with asymmetric uncertainties of +0.06/‑0.05 M_Jup. The transit depth, duration (~2.7 h), and shape were modeled using the Mandel & Agol (2002) analytic formalism. Because the impact parameter b is only loosely constrained (0 < b < 0.46) due to modest time resolution and atmospheric conditions during the follow‑up, the planetary radius is inferred to lie between 1.06 and 1.21 R_Jup.

Spectroscopic analysis of the host star yields stellar parameters typical of a G1 V dwarf: effective temperature T_eff ≈ 5850 K, surface gravity log g ≈ 4.44, metallicity