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!