  Overcoming type I migration and preventing low-mass planets from spiralling
into the central star is a long-studied topic. It is well known that outward
migration is possible in viscous-heated discs relatively close to the central
star because the entropy gradient can be sufficiently steep that the positive
corotation torque overcomes the negative Lindblad torque. Yet efficiently
trapping planets in this region remains elusive. Here we study disc conditions
that yield outward migration for low-mass planets under specific planet
migration prescriptions. Using a steady-state disc model with a constant
alpha-viscosity, outward migration is only possible when the negative
temperature gradient exceeds ~0.87. We derive an implicit relation for the
maximum mass at which outward migration is possible as a function of viscosity
and disc scale height. We apply these criteria, using a simple power-law disc
model, to planets that have reached their pebble isolation mass after an
episode of rapid accretion. It is possible to trap planets with the pebble
isolation mass farther than the inner edge of the disc provided that alpha_crit
>~ 0.004 for discs older than 1 Myr. In very young discs the high temperature
causes the planets to grow to masses exceeding the maximum for outward
migration. As the disc evolves, these more massive planets often reach the
central star, generally only towards the end of the disc's lifetime. Saving
super-Earths is therefore a delicate interplay between disc viscosity, the
opacity profile and the temperature gradient in the viscously heated inner
disc.
