Fractional difference for the sound speed in the sense (Sun) – (Model). ‘Sun’ refers to the sound-speed profile as inferred from helioseismic observations at standard gravity according to the results of Rhodes (1997). Models GS98 and A09 correspond to theoretically computed calibrated solar models at standard gravity (𝒴 = 0) with the two distinct, and most popular metallicity mixtures for the Sun (see also Table 2 and Sect. 3). For comparison, we also show two computed models with the fifth force turned on (𝒴 = ±0.01) based on the GS98 mixture. We note that the fifth-force effect peaks in the radiative zone which will be key for constraining the fifth force with helioseismic observations.