There are many unsolved problems in the physics of planet formation and the evolution of their parent disk is expected to play an important role in resolving them. In part I of this thesis, I discuss the evolution of protoplanetary disks under the influence of viscous evolution, photoevaporation from the central source, and photoevaporation by external stars; and explore the consequences for planet formation.

The discovery of hot jupiters orbiting at a few AU from their stars compliments earlier detections of massive planets on very small orbits. The short period orbits strongly suggest that planet migration has occurred, with the likely mechanism being tidal interactions between the planets and the gas disks out of which they formed. The newly discovered long period planets, together with the gas giant planets in our solar system, show that migration is either absent or rapidly halted in at least some systems. I propose a mechanism for halting type-II migration at several AU in a gas disk: the formation of a photoevaporation gap prevents planets outside the gap from migrating down to the star.

The final planet location relative to the habitable zone is often used to discuss the planet habitability. But a planet in the habitable zone may experience large amplitude motion of its rotation axis, which may cause severe climate variations and have major consequences for the development of life. In part II of this thesis, I investigate the true polar wander (TPW) rotational stability of planets. I revisit the classic problem of the long-term rotational stability of planets in response to loading using a new, generalized theoretical development based on the fluid limit of viscoelastic Love number theory. Finally, I explore the time dependent (rather than the equilibrium fluid limit) rotational stability of planets by considering the example of an ice age Earth. I present a new treatment of the linearized Euler equations that govern rotation perturbations on a viscoelastic planet driven by surface loading.