In the Solar system, the radioisotope $^{26}$Al exists on the surface of celestial bodies due to nuclear spallation by low-energy cosmic rays. Meteorites sampled on the Earth and lunar samples allow us to measure current $^{26}$Al decay rate in laboratories. Based on these decay rates, we investigate the expected $^{26}$Al gamma-ray line signals from solar system bodies, specifically asteroids in the main belt and the Moon. We find that the signals from the main belt are too weak to be detected by future planned MeV gamma-ray missions. However, due to its proximity and mass, next generation MeV gamma-ray mission such as the Compton Spectrometer and Imager (COSI) satellite will be able to detect $^{26}$Al signals from the Moon. A future measurement of $^{26}$Al will help us to understand lunar geology.