Probing Ultralight Scalars through Compact Stars and Precision Tests of Gravity

Ultralight CP-even scalars coupled to the Standard Model via electromagnetic and electrophilic interactions can induce observable imprints on compact and planetary systems, independent of whether the scalar constitutes dark matter. We derive bounds on these couplings using precision astrophysical and gravitational observables, including pulsar spin-down luminosity and surface magnetic field from magnetized neutron stars and magnetars, deviations in the gravitational force in the Sun-planet systems, and corrections to geodetic (de Sitter) precession of orbiting gyroscopes. We further quantify prospective sensitivity reach from differential photon-redshift and non-gravitational potential measurables through atomic clock precision. These results demonstrate that compact star and solar-system observables provide competitive and highly scalable probes of ultralight scalar interactions with the visible sector.