Comparing the masses of the given planets: - Neptune: approx. 1.02 x 10^26 kg - Jupiter: approx. 1.90 x 10^27 kg (most massive in the solar system) - Mars: approx. 6.42 x 10^23 kg - Mercury: approx. 3.30 x 10^23 kg Comparing these values, Mercury has the smallest mass. In fact, Mercury is the smallest planet in the solar system both in terms of mass and radius.