The total mass of Earth, the largest and densest terrestrial planet of the solar system [1] , is about 6 Zt (6 zettatonnes, or 6 × 1021 t), with a volume of 1.1 trillion km3 and an average density of 5.5 t/m3 [2] , causing a gravitational acceleration of 9.8 m/s2 at the surface. About 90% of the mass is made up of only four elements: iron, oxygen, silicon (see key elements), and magnesium, whereby iron and oxygen alone account for more than 60 %.
In recent years, more than 3000 exoplanets have been detected based on indirect evidence (such as movements and dimming of the parent star). The vast majority are gas giants (normally larger than Jupiter or Saturn) in close orbit to their parent star. Earth-like planets in the habitable zone are very difficult to detect; so far, about 20 candidates (1% of total discoveries) have been identified by NASA's Kepler spacecraft.
Earth's density (quotient of total mass and volume) happens to be equivalent to the average of the densities of peridotite (3.2 kg/m3, typical rock of Earth's upper mantle) and iron (7.8 kg/m3, constituent of Earth's core).