We report the detection of water absorption features in the day side spectrum of the first-known hot Jupiter , 51 Peg b , confirming the star-planet system to be a double-lined spectroscopic binary .
We used high-resolution ( R \approx 100 ~ { } 000 ) , 3.2 \mu m spectra taken with CRIRES/VLT to trace the radial-velocity shift of the water features in the planet ’ s day side atmosphere during 4 hours of its 4.23-day orbit after superior conjunction .
We detect the signature of molecular absorption by water at a significance of 5.6 \sigma at a systemic velocity of V _ { sys } = -33 \pm 2 km s ^ { -1 } , coincident with the 51 Peg host star , with a corresponding orbital velocity K _ { P } = 133 ^ { +4.3 } _ { -3.5 } km s ^ { -1 } .
This translates directly to a planet mass of M _ { p } = 0.476 ^ { +0.032 } _ { -0.031 } M _ { J } , placing it at the transition boundary between Jovian and Neptunian worlds .
We determine upper and lower limits on the orbital inclination of the system of 70 ^ { \circ } < i < 82.2 ^ { \circ } .
We also provide an updated orbital solution for 51 Peg b , using an extensive set of 639 stellar radial velocities measured between 1994 and 2013 , finding no significant evidence of an eccentric orbit .
We find no evidence of significant absorption or emission from other major carbon-bearing molecules of the planet , including methane and carbon dioxide .
The atmosphere is non-inverted in the temperature-pressure region probed by these observations .
The deepest absorption lines reach an observed relative contrast of 0.9 \times 10 ^ { -3 } with respect to the host star continuum flux at an angular separation of 3 milliarcseconds .
This work is consistent with a previous tentative report of K-band molecular absorption for 51 Peg b by Brogi et al .
( 2013 ) .