  A new class of relativistic stellar structure equations which include the
effects of an energy-dependent Newton coupling is presented. Significant
modifications in the mass-radius relation for neutron stars are possible only
if the running of the Newton coupling due to quantum gravity occurs at low
energies. A new Buchdahl limit is derived and its physical implications are
discussed. In particular, sub-Planckian self-gravitating objects with
arbitrarily small radii are possible.
