2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/193696We consider general metric $f(R)$ theories of gravity by solving the field equations in the presence of a spherical static mass distribution by analytical perturbative means. Expanding the field equations systematically in $\cO(G)$, we solve the resulting set of equations and show that $f(R)$ theories which attempt to solve the dark energy problem very generally lead to $γ_{PPN}=1/2$ in the solar system. This excludes a large class of theories as possible explanations of dark energy. We also present the first order correction to $γ_{PPN}$ and show that it cannot have a significant effect.4 pages; v2: added references, modified abstract and introduction, conclusions unchangedAstrophysicsGeneral Relativity and Quantum Cosmologytext