  Most of the presently identified exoplanets have masses similar to that of
Jupiter and therefore are assumed to be gaseous objects. With the
ever-increasing interest in discovering lower-mass planets, several of the
so-called super-Earths (1 M_earth<M<10 M_earth), which are predicted to be
rocky, have already been found. Here we report the possible discovery of a
planet around the M-type star GJ 436 with a minimum mass of 4.7+/-0.6 M_earth
and a true mass of ~5 M_earth, which would make it the least massive planet
around a main-sequence star found to date. The planet is identified from its
perturbations on an inner Neptune-mass transiting planet (GJ 436b), by pumping
eccentricity and producing variations in the orbital inclination. Analysis of
published radial velocity measurements indeed reveals a significant signal
corresponding to an orbital period that is very close to the 2:1 mean motion
resonance with the inner planet. The near-grazing nature of the transit makes
it extremely sensitive to small changes in the inclination.
