Size: 12,700 km Orbit: 149,597,887.5 km (1 AU)‏ Earth Size: 12,700 km Orbit: 149,597,887.5 km (1 AU)‏

Life Earth is the only place in the universe where we know life exists. The planet formed 4,540,000,000 years ago, and life appeared on its surface within a billion years. Since then, Earth's life has changed the atmosphere and other things on the planet. The world is expected to continue supporting life for another 1.5 billion years, after which the death of the sun Sun will kill the Earth's plants.

Water About 71% of Earth's surface is covered in salt water oceans. Earth is the only place in the universe where liquid water is known to exist. The other 29% is made of rocky land in the shape of continents and islands.

What's it made of? Earth is a terrestrial planet. It has an outer, solid rock layer called the crust. Everything that lives on Earth is on top of the crust. Underneath that is a layer of thick, liquid rock called the mantle. Under that is a thin liquid layer called the outer core and then the solid iron inner core. The thickness of the crust changes. On land the average is between 30-50 kilometres thick. Under the oceans in some places it is only 6 kilometres thick. The inside of the Earth is very hot, the outer core may be as much as 7000 degrees.

Atmosphere The Earth's atmosphere is made of about 78% nitrogen, 21% oxygen and small amounts of other gases. The atmosphere also protects the earth from meteors and small asteroids. Some gases including carbon dioxide and methane act like a blanket around the Earth, keeping the Earth warm.

Temperature Earth has very different temperatures in different places. Coldest: −89.2 °C (in Antarctica)‏ Hottest: 57.8 °C (in northern Africa)‏

Size: 3,474.8 km Orbit from Earth: 384,400 km The Moon Size: 3,474.8 km Orbit from Earth: 384,400 km

How far away? 1.25 light-seconds

Lunar Maria

Regolith The surface of the moon is covered in rock powder called regolith. Large rocks were broken into tiny pieces by many, many meteorites.

Craters The moon is covered in craters of all sizes. These craters come from meteorites hitting the moon. Since there is no weather, craters never change. There are more than 500,000 craters over 1km across.

Exploration First probe: Luna 2, which made a hard landing on September 14, 1959 First person: Neil Armstrong, July 21, 1969

What's Next? There were 65 moon landings, but after Luna 24 in 1976 they suddenly stopped. Russia started studying Venus and space stations and the U.S. on Mars and beyond. Nobody has walked on the Moon in 30 years. The European Space Agency launched a small, low-cost lunar orbital probe called SMART 1 on September 27, 2003. The People's Republic of China launched the Chang'e 1 robotic lunar orbiter on 2007-10- 24. India launched Chandrayaan-1, a lunar orbiter, on October 22, 2008.