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 \sim 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 } \gtrsim 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 .