  In the favoured core-accretion model of formation of planetary systems, solid
planetesimals accumulate to build up planetary cores, which then accrete
nebular gas if they are sufficiently massive. Around M-dwarf stars (the most
common stars in our Galaxy), this model favours the formation of Earth-mass to
Neptune-mass planets with orbital radii of 1 to 10 astronomical units (AU),
which is consistent with the small number of gas giant planets known to orbit
M-dwarf host stars. More than 170 extrasolar planets have been discovered with
a wide range of masses and orbital periods, but planets of Neptune's mass or
less have not hitherto been detected at separations of more than 0.15 AU from
normal stars. Here we report the discovery of a 5.5 (+5.5/-2.7) M_earth
planetary companion at a separation of 2.6 (+1.5/-0.6) AU from a 0.22
(+0.21/-0.11) M_solar M-dwarf star. (We propose to name it OGLE-2005-BLG-390Lb,
indicating a planetary mass companion to the lens star of the microlensing
event.) The mass is lower than that of GJ876d, although the error bars overlap.
Our detection suggests that such cool, sub-Neptune-mass planets may be more
common than gas giant planets, as predicted by the core accretion theory.
