We present the detection and characterization of the transiting warm Jupiter KOI-12b, first identified with Kepler with an orbital period of 17.86 days. We combine the analysis of Kepler photometry with Doppler spectroscopy and line-profile tomograph
y of time-series spectra obtained with the SOPHIE spectrograph to establish its planetary nature and derive its properties. To derive reliable estimates for the uncertainties on the tomographic model parameters, we devised an empirical method to calculate statistically independent error bars on the time-series spectra. KOI-12b has a radius of 1.43$pm$0.13$ R_mathrm{Jup}$ and a 3$sigma$ upper mass limit of 10$M_mathrm{Jup}$. It orbits a fast-rotating star ($v$sin$i_{star}$ = 60.0$pm$0.9 km s$^{-1}$) with mass and radius of 1.45$pm$0.09 $M_mathrm{Sun}$ and 1.63$pm$0.15 $R_mathrm{Sun}$, located at 426$pm$40 pc from the Earth. Doppler tomography allowed a higher precision on the obliquity to be reached by comparison with the analysis of the Rossiter-McLaughlin radial velocity anomaly, and we found that KOI-12b lies on a prograde, slightly misaligned orbit with a low sky-projected obliquity $lambda$ = 12.6$stackrel{+3.0}{_{-2.9}}^circ$. The properties of this planetary system, with a 11.4 magnitude host-star, make of KOI-12b a precious target for future atmospheric characterization.
Transit observations of HD209458b in the UV revealed signatures of neutral magnesium escaping the planets upper atmosphere. The absorption detected in the MgI line provides unprecedented information on the physical conditions at the altitude where th
e atmospheric blow-off takes place. Here we use a 3D model of atmospheric escape to estimate the transit absorption signatures in the MgI line of their host stars. The detectability of these signatures depends on the brightness of the star and the escape rate of neutral magnesium. We identify a sample of potentially evaporating exoplanets that covers a wide range of stellar and planetary properties, and whose extended exospheres might be detected through MgI line observations with current UV facilities, allowing further steps in comparative exoplanetology.
