Elastic properties of Fe alloys are critical in constraining the compositions of planetary bodies by comparing to the planetary observations. The sound wave velocities and density of an Fe5Si (9 wt% Si) alloy in body-centered cubic (bcc) structure were measured by combining an ultrasonic technique with synchrotron X-ray radiography at pressure (P) and temperature (T) conditions of 2.6–7.5 GPa and 300–1173 K, respectively. At room temperature, it is observed that adding Si to bcc-Fe increases the compressional wave velocity (νP) but decreases the shear wave velocity (νS). At high temperatures, we observed a pronounced effect of pressure on the νS-T relations in the Fe5Si alloy. The νP-density (ρ) relationship of the Fe5Si alloy is found to follow the Birch's law in the P-T range of this study, whereas the νS-ρ relation exhibits complex behavior. Implications of these results to the lunar core and the Mercurian core are discussed. Our results imply that adding Si to a pure Fe lunar core would be invisible in terms of νP, but exhibit a decreased νS. Including Si in a sulfur-rich lunar core would display an increased νP and a decreased ρ. Our density and sound wave velocity model provide lower and upper limit for a Si-bearing lunar core if 1–3 wt% Si content of enstatite chondrite is taken as compositional analog. A Si-rich (>9 wt%) Mercurian core model is derived to satisfy newly observed moment of inertia values by Messenger spacecraft.