The Earth, as seen by the Apollo 17 astronauts. Image courtesy of NASA.
The Earth is the third planet from the Sun, orbiting at a distance of 1 AU once every year.
It is the largest of the rocky planets in the solar system, and also unusual among the solar system's planets in having a moon that is comparable in size to itself.
Although the Moon is significantly smaller than the Earth – it is a little over a quarter of an Earth-diameter across and has less than an eightieth of the Earth's mass – it is much larger than the moons of the other rocky planets.
Mercury and Venus have no known moons at all, and Mars's two companions, Phobos and Deimos, have less than a millionth the mass of their parent.
The now widely accepted theory for how the Earth and Moon formed is the giant impact hypothesis, originally proposed by William Hartmann and Donald Davis in 1975.
According to this theory, early in its history the Earth collided with another large rocky planet, similar in size to Mars and called Theia. This happened only around 100,000 years after the Earth's own formation – around 4.5 billion years ago – but long enough afterwards that the Earth's iron had already sunk to form a heavy metallic core, which was surrounded by a less dense rocky mantle.
The collision had two effects: it melted much of the Earth's crust and mantle, and it placed a sizeable cloud of debris into orbit around the Earth. It was this debris, largely material from the Earth's own mantle, that later conglomerated to form the Moon.
This model explains many of the similarities between the Earth and Moon – for example, the fact that their surfaces are made of very similar materials.
But it also explains some of their differences. The Moon has little magnetic field and is less dense than the Earth, which indicates it has little or no iron core. This matches the predictions of the Hartmann & Davis model, in which only material from the Earth's outer mantle would have been present in the post-collision debris cloud.