Pressure P inside the pebbles vs. depth s. The continuous line shows the tensile strength, S, bonding a homogeneous aggregate of dust grains. The dashed line represents P at T_s = 48 K and ∇T = 12 K cm⁻¹, which crosses S at s_m = 0.12 mm and s_M = 8 mm. The dotted line shows P at T_s = 40 K and ∇T = 4 K cm⁻¹ and is tangent to S at s_m =  s_M = 3 mm. For every value of T_s, the nucleus at the depth s_M maintains a temperature T_s − ∇Ts_M = 38.5 K.