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Earth is the third planet from the Sun and the biggest of the four Inner Planets. 70% of its surface is water. The other 30% is land. Its atmosphere is made up mainly of nitrogen (77%), with a smaller amount of oxygen (21%) and other gases (such as Carbon Dioxide which keeps the planet's temperature steady). Earth is the only planet in the universe which is known to support life. There are possibilities that life could have existed elsewhere, still exists or will exist one day in the future, but Earth is the only planet where we know life exists for certain. Earth is a planet which orbits the Sun every 365 and a quarter days, taking 23 hours and 56 minutes to turn on its axis. It has one Moon which orbits the planet every 27 days. It is a geologically active planet, meaning that its surface features and landscapes are constantly being reformed by volcanoes, earthquakes, changes in climate (temperature), and weather. Like other planets and moons, it is a target for asteroids and meteorites travelling through space. Most of these objects burn up in the planet's protective atmosphere, although some do survive and leave huge impact craters. The
planet, like Mars, has ice caps at both of its poles.
These are huge areas of frozen water which, if they were melted,
could flood the planet. These polar ice caps are caused by the
planet's axis having a slight tilt, meaning that they spend large
parts of the year receiving almost no sunlight or heat. This tilt
also causes the seasons on the planet. Mars is similar to Earth in
this way, being a planet with a similar tilt.
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