  The naked-eye star 55 Cancri hosts a planetary system with five known
planets, including a hot super-Earth (55 Cnc e) extremely close to its star and
a farther out giant planet (55 Cnc b), found in milder irradiation conditions
with respect to other known hot Jupiters. This system raises important
questions on the evolution of atmospheres for close-in exoplanets, and the
dependence with planetary mass and irradiation. These questions can be
addressed by Lyman-alpha transit observations of the extended hydrogen
planetary atmospheres, complemented by contemporaneous measurements of the
stellar X-ray flux. In fact, planet `e' has been detected in transit,
suggesting the system is seen nearly edge-on. Yet, planet `b' has not been
observed in transit so far. Here, we report on Hubble Space Telescope STIS
Lyman-alpha and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous
observations cover two transits of 55 Cnc e and two inferior conjunctions of 55
Cnc b. They reveal the star as a bright Lyman-alpha target and a variable X-ray
source. While no significant signal is detected during the transits of 55 Cnc
e, we detect a surprising Lyman-alpha absorption of 7.5 +/- 1.8% (4.2 sigma) at
inferior conjunctions of 55 Cnc b. The absorption is only detected over the
range of Doppler velocities where the stellar radiation repels hydrogen atoms
towards the observer. We calculate a false-alarm probability of 4.4%, which
takes into account the a-priori unknown transit parameters. This result
suggests the possibility that 55 Cnc b has an extended upper H I atmosphere,
which undergoes partial transits when the planet grazes the stellar disc. If
confirmed, it would show that planets cooler than hot Jupiters can also have
extended atmospheres.
