Which units of mass are used in Astronomy? We're familiar with kilograms (kg), but once you get to the size of planets, the number of kilograms is just unimaginably large. That's why astronomers often use the masses of planets as their own standard. For instance, the smaller, rocky planets are often described in terms of "Earth Masses." The larger gas giants are often spoken of in terms of "Jupiter Masses." And finally, we use the mass of our own sun ("Solar Masses") as a benchmark for the mass of other stars.