  It has been recently proposed that giant protoplanets migrating inward
through the disc more rapidly than they contract could be tidally disrupted
when they fill their Roche lobes ~ 0.1 AU away from their parent protostars.
Here we consider the process of mass and angular momentum exchange between the
tidally disrupted planet and the surrounding disc in detail. We find that the
planet's adiabatic mass-radius relation and its ability to open a deep gap in
the disc determine whether the disruption proceeds as a sudden runaway or a
balanced quasi-static process. In the latter case the planet feeds the inner
disc through its Lagrangian L1 point like a secondary star in a stellar binary
system. As the planet loses mass it gains specific angular momentum and
normally migrates in the outward direction until the gap closes. Numerical
experiments show that planet disruption outbursts are preceded by long
"quiescent" periods during which the disc inward of the planet is empty. The
hole in the disc is created when the planet opens a deep gap, letting the inner
disc to drain onto the star while keeping the outer one stalled behind the
planet. We find that the mass-losing planet embedded in a realistic
protoplanetary disc spawns an extremely rich set of variability patterns. In a
subset of parameter space, there is a limit cycle behaviour caused by
non-linear interaction between the planet mass loss and the disc hydrogen
ionisation instability. We suggest that tidal disruptions of young massive
planets near their stars may be responsible for the observed variability of
young accreting protostars such as FU Ori, EXor and T Tauri stars in general.
