  In this paper, we address the migration of small mass planets in 3D radiative
disks. Indeed, migration of small planets is known to be too fast inwards in
locally isothermal conditions. However, thermal effects could reverse its
direction, potentially saving planets in the inner, optically thick parts of
the protoplanetary disc. This effect has been seen for masses larger than 5
Earth masses, but the minimum mass for this to happen has never been probed
numerically, although it is of crucial importance for planet formation
scenarios. We have extended the hydro-dynamical code FARGO to 3D, with thermal
diffusion. With this code, we perform simulations of embedded planets down to 2
Earth masses. For a set of discs parameters for which outward migration has
been shown in the range of $[5, 35]$ Earth masses, we find that the transition
to inward migration occurs for masses in the range $[3, 5]$ Earth masses. The
transition appears to be due to an unexpected phenomenon: the formation of an
asymmetric cold and dense finger of gas driven by circulation and libration
streamlines. We recover this phenomenon in 2D simulations where we control the
cooling effects of the gas through a simple modeling of the energy equation.
