The mass of a planet is the amount of matter it contains. Since objects attract each other gravitationally, and that gravitational force depends on mass, we can find a planet’s mass by measuring its effect on the motions of other objects passing nearby, such as comets, asteroids, or spacecraft. In this way, Mercury has been measured to have a mass of 3.3 x 1023 kg, which is less than the other planets because Mercury is so small. Actually, for its size, Mercury is unexpectedly massive because much of its inetior is composed of the dense metal, iron. About 18 Mercury’s would equal the mass of Earth.
Compare the mass of Mercury to the other planets here!