Motivated by the historical detection of gravitational waves from GW170817, the neutron star and the neutron drop, i.e., a certain number of neutrons confined in an external field, are systematically investigated by ab initio calculations as well as the nonrelativistic and relativistic state-of-the-art density functional theories. Strong correlations are found among the neutron star tidal deformability, the neutron star radius, the rootmean-square radii of neutron drops, and the symmetry energies of nuclear matter at supra-saturation densities. For dense matter composed of nucleons only, these correlations, together with the upper limit on the tidal deformability extracted from GW170817, provides the constraints for the neutron star radii, the neutron drop radii, and the symmetry energy at twice saturation density as R-1.4 M Theta <= 13.4 +/- 0.2 km, R-nd <= 2.41 +/- 0.10 fm, and E-sym (2(rho 0) ) <= 60.7 +/- 10.9 MeV, respectively.
