Polytropic spheres in Palatini f(R) gravity
  We examine static spherically symmetric polytropic spheres in Palatini f(R) gravity and show that no regular solutions to the field equations exist for physically relevant cases such as a monatomic isentropic gas or a degenerate electron gas, thus casting doubt on the validity of Palatini f(R) gravity as an alternative to General Relativity. 
Enrico Barausse, Thomas P. Sotiriou and John C. Miller