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| OUR SUN | MERCURY | VENUS | EARTH | MARS |
| JUPITER | SATURN | URANUS | NEPTUNE | PLUTO |
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EARTH
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Our planet circles the Sun every 365 days, and spins once every 23 hours 56 minutes about an axis tilted on average 23.5 degrees from the perpendicular to its orbital plane. This inclination assures that there are 4 well-defined seasons in most locales on the Earth.
The radius of the Earth is 6,371 km. Seismic data indicate that it has a layered structure, the basic components of which are a crust, a mantle and a metallic core. Part of the core is fluid and maintains the Earth's magnetic field.
Planetologically, the Earth is unique. It is the only planet on which water covers a large fraction of the surface, a fact that has profound implications on the geological processes that occur. Earth is very active internally, and as far as we know, is the only planet on which the crust is being renewed by the process of plate tectonics with its associated manifestations, mid-ocean ridges, island arcs, and folded mountain belts. The distribution of elevations on Earth is bimodal, the average ocean floor is 3,900 metres below sea level while the average continent rises to 860 metres. The highest elevation is Mt. Everest, 9 km above sea level, the deepest ocean trench is 11 km below.
The Earth's atmosphere consists of 78 percent nitrogen and 21 percent oxygen, and is believed to be the result of a long history of evolution involving geological, chemical, and biological processes. It is believed to have arisen by gradual outgassing from the interior. Various gases were involved, but carbon dioxide and water vapour predominated.
Since the Earth is located at just the distance from the Sun where liquid water can exist, the water has ended up in the oceans. It has also dissolved most of the carbon dioxide, which was eventually locked up in carbonate rocks. With the water and carbon dioxide removed from the atmosphere, nitrogen remains the major constituent. There is strong evidence that the level of oxygen in the atmosphere has been increasing and that this increase can be attributed to biological activity. It is estimated that 600 million years ago the amount of oxygen in our atmosphere was 100 times less than present today.
As far as is known, the Earth is the only planet in the solar system that harbours life.
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