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, it has been found that Mercury is rather “light” compared to other planets, with a mass of 3.3 x 1023 kg. This means that about 18 Mercury's would equal the mass of Earth.
Compare the mass of Mercury to the other planets here!