The mass unit of the International System is the kilogram.
For planets (extrasolar or not), masses are given in terrestrial masses, Jupiter masses, or solar masses
!!Important!! Mass and weight are different things, even if, when we are on the surface of the Earth, it is difficult to tell the difference! Weight is equal to mass multiplied by "g", a value almost constant on the surface of the Earth. Weight is related to the gravity force. Mass measures the amount of matter, which depends on the number of atoms and on the mass of these atoms.
On Earth, your weight is the attraction between your mass and the mass of the Earth (see gravitation) at a distance equal to the radius of the Earth. In the interplanetary medium, you wouldn't have any weight (you would be in a state of weightlessness), but you would keep the same mass. On the surface of the moon, your weight would be 6 times lower than your weight on Earth.
Des poids et des masses