Mass / Density
The masses of planets vary due to what they're composed of and how much of the substance there is throughout the planet. The planets we know most about are those within our solar system. Mercury has a mass of 330, 104, 000, 000, 000, 000, 000, 000 kg. It's large mass is due to its large iron core, making it a very heavy planet, though its rather small, the smallest of the 8 planets in our solar system. Because of this large iron core, it also has a large density, which is 5.43 g/cm3. Venus has a mass of 4, 867, 320, 000, 000, 000, 000, 000, 000 kg, much heavier than Mercury, though is less dense due to its larger size, with a density of only 5.24 g/cm3. Earth has a mass of 5, 972, 190, 000, 000, 000, 000, 000, 000 kg, and has a density even larger than that of Mars, with a large 5.52g/cm3. Mars is considerably smaller than Earth, with a mass of 641, 693, 000, 000, 000, 000, 000, 000 kg and a density of 3.93 g/cm3. The next planet, Jupiter, is the largest in our solar system, with a mass of 1, 898, 130, 000, 000, 000, 000, 000, 000, 000 kg, the heaviest of all the planets and not, however, the most dense, with a density of only 1.33 g/cm3. Saturn has a mass of 568, 319, 000, 000, 000, 000, 000, 000, 000 kg. However, it only has a density of .69 g/cm3. So, Saturn can float on water! Weird, right? Uranus has a mass of 86, 810, 300, 000, 000, 000, 000, 000, 000 kg, and its density is just over 1 at 1.27 g/cm3, so it doesn't float on water. Neptune has a mass of 102, 410, 000, 000, 000, 000, 000, 000, 000 kg and a density of 1.64g/cm3.
Other planets outside of our solar system vary in mass; however, to be considered a planet, their mass must be below the mass required for nuclear fusion to occur - if it were above or equal to that mass, it would be what's known as a brown dwarf, or a star that isn't massive enough to produce power by fusing hydrogen atoms, which is what stars do to produce light and energy. A planet must have a mass lower than around .08 solar mass (1.9891 nontillion kg) to be considered a planet instead of a star. So, the maximum mass a planet can have is about 159.128 octillion kg, though this number is subject to change as we continue to discover more in space. At the time, with so few planets discovered, a minimum mass has not been defined, though it can't get too small or it could be considered a dwarf planet, as a small mass can lead to a smaller volume, making the body too small to be considered a proper planet. As for density, the density of planets varies on the type of material the planet is composed of and how large the planet is, as density is mass/unit volume. So, smaller planets composed of heavier elements will have a very large density, while larger planets with lighter elements will have a very small density, and anything in between will also vary. Once we know what makes up the planet, its mass, and how large it is, we can determine its density through mathematical equations and reach the conclusion of the planet's density