  We study the evolution of a protoplanet of a few earth masses embedded in a
protoplanetary disc. If we assume that the atmosphere of the protoplanet, i.e.
the volume of gas in hydrostatic equilibrium bound to the core, has a surface
radius smaller than the Roche lobe radius, we show that it expands as it
accretes both planetesimals and gas at a fixed rate from the nebula until it
fills in the Roche lobe. The evolution occurs on a timescale shorter than the
formation or migration timescales. Therefore, we conclude that protoplanets of
a few earth masses have an atmosphere that extends to the Roche lobe surface,
where it matches to the nebula. This is true even when the Bondi radius is
smaller than the Roche lobe radius. This is in contrast to the commonly used
models in which the static atmosphere extends up to the Bondi radius and is
surrounded by a cold accretion flow. As a result, any calculation of the tidal
torque exerted by the disc onto the protoplanet should exclude the material
present in the Roche lobe, since it is bound to the protoplanet.
