Context :

Aims : We report the discovery of CoRoT-8b , a dense small Saturn-class exoplanet that orbits a K1 dwarf in 6.2 days , and we derive its orbital parameters , mass , and radius .

Methods : We analyzed two complementary data sets : the photometric transit curve of CoRoT-8b as measured by CoRoT and the radial velocity curve of CoRoT-8 as measured by the HARPS spectrometer Both data sets are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/vol/page . .

Results : We find that CoRoT-8b is on a circular orbit with a semi-major axis of 0.063 \pm 0.001 AU .
It has a radius of 0.57 \pm 0.02 \mathrm { R } _ { \mathrm { J } } , a mass of 0.22 \pm 0.03 \mathrm { M } _ { \mathrm { J } } , and therefore a mean density of 1.6 \pm 0.1 g cm ^ { -3 } .

Conclusions : With 67 % of the size of Saturn and 72 % of its mass , CoRoT-8b has a density comparable to that of Neptune ( 1.76 g cm ^ { -3 } ) .
We estimate its content in heavy elements to be 47–63 \mathrm { M } _ { \oplus } , and the mass of its hydrogen-helium envelope to be 7–23 \mathrm { M } _ { \oplus } .
At 0.063 AU , the thermal loss of hydrogen of CoRoT-8b should be no more than \sim 0.1 % over an assumed integrated lifetime of 3 Ga .