Physical properties
The Earth was formed about 4.54 billion years ago. About 71 % of the Earth's surface is covered by salt-water oceans, which corresponds to 149 of a total of 510 million square kilometers.
The mass of the Earth is approximately 5.98 x 10^24 kg, which is more than 81 times the mass of the moon. Among others things, it consists of 32.1 % iron, 30.1 % oxygen, 15.1 % silicon and 13.9 % magnesium.
Evolution of life
Earth is the only place in the universe where life is known to excist. Other planets tend to be either to hot or cold, or lack necessary substances like water and oxygen.
Kilder:
http://en.wikipedia.org/wiki/Moon
http://en.wikipedia.org/wiki/The_Earth
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