Pebble isolation mass M_iso (Eq. (5), solid lines) and the disc’s aspect ratio H∕r (dashed) for different disc ages. As the disc cools in time, the pebble isolation mass is reduced. However, in the inner few AU of the disc, the pebble isolation mass stays constant at around 4–5 Earth masses, which is too low for efficient envelope contraction. Here α_disc = 0.0054 has been used to calculate the pebble isolation mass. The age of 0 Myr corresponds to a disc that already evolved for 2 Myr in the Bitsch et al. (2015b) disc model and corresponds to our starting disc age.