Earth

Earth is the third solar system planet from the Sun and the solar system's largest rocky planet. Its temperature and atmospheric pressure are such that liquid water can exist on the surface, which is mostly covered with water. Its atmospheric makeup, with substantial oxygen, is influenced by life on the planet. It is also geologically active, exhibiting plate tectonics. It generates some of its heat internally from radioactive uranium and thorium (radiogenic heating). Some characteristics:

• 6371 km or 3959 miles - radius.
• 5.972×10²⁴ kg - mass.
• 5.5 g/cm² - mean density (cf. water 1 g/cm², rock ≈ 2.5 g/cm², and iron 7.9 g/cm²).
• 287 K or 57° Fahrenheit - mean surface temperature.
• 1 AU, 150 million km, or 93 million miles - orbital radius (semi-major axis).
• 30 km/sec or 67000 MPH - orbital speed.
• 0.0167 - orbital eccentricity.

Earth's single moon is notable for being nearer to Earth's size and mass (1/4 the radius and 1% the mass) than the moons of the other planets (excluding dwarf planet Pluto and its largest moon, Charon).

As the only planet known to harbor life, observable characteristics of Earth are considered for their potential as biosignatures, e.g., for the study of extra-solar planets. Earth is plainly in the Sun's habitable zone, for which estimates are refined over time as more information is available and considered. Some recent determinations put Earth near the inner edge.

The term Earth is also used for classes of exoplanets that resemble the Earth (Earth analogs), such as similar mass and/or constituents.