Mass is a property not only of solid objects, but also liquid, gas, plasma, and sub-atomic particles.

"Weight" is sometimes confused with "mass." They are related, but 100 kilograms of mass would weigh nothing in deep space. Though it would weigh 100 kilograms on Earth, it would weigh far less on the Moon, and far more on Jupiter (if a solid platform could be devised on which to perform such a weighing). However, the object would remain 100 kilograms of mass no matter what the location.

The masses of extrasolar planets are typically given in terms of a planet in our own star system — a planet of similar mass like Jupiter, Saturn, Uranus, or Earth. So far, the lightest planets found are comparable to Uranus or Neptune in mass.

Mass · sin (i)
Masses listed for extrasolar planets are likely not the true mass, but only a lower limit to the actual mass. The most widely used method for discovering planets makes use of the distortion the planet makes on the spectrum (light) of its parent star. If we are seeing the orbit of the planet edge-on, then we are receiving the maximum effect of the planet's orbital tug. If the planet's orbit is not inclined toward us at all, then we would never detect it by this method.

Mass for extrasolar planets is usually given in terms of Jupiters, but this is multiplied times the sine of orbital inclination (usually unknown, or only estimated). The inclination of the orbit to the line of sight can be anywhere between zero and ninety degrees, thus the sine of inclination will be between zero and one. Therefore, the mass shown is a lower limit for the planet. With an inclination approaching zero, the calculated mass would tend to approach infinity. Other factors make this an unlikely prospect. For one, beyond a certain mass, the body would not be a planet, but would be a star or black hole, instead.