Mass-radius relations of homogeneous cold spheres are obtained for six solid materials commonly found in terrestrial planets. An additional degeneracy in the (exo)planet profiles is discussed together with their properties concluded from our findings in the framework of Palatini f(R) gravity. Moreover, a new test of gravity has been proposed: The results presented here will allow us to test and constrain models of gravity by the use of seismic data acquired from earthquakes and marsquakes.