Space
Planets and moons
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Planets
A planet is an object orbiting a star. Most planets contain rock and metal and are surrounded by atmospheres of gas. Some have liquids on their surfaces, or beneath their crusts. A...
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Mercury
Mercury is the smallest planet in the Solar System, and the nearest planet to the Sun. Mercury can be seen from Earth only with difficulty: when visible at all, it is always low in...
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Venus
Venus is the second planet from the Sun. About the same size as Earth, Venus is sometimes referred to as our sister planet. But these sisters have very little else in common. In...
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Earth
Our own planet, Earth, is the largest of the four inner, or terrestrial, planets. It is the only world where liquid water is known to exist permanently at the surface. About 71% of...
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Moon
The Moon, our nearest neighbour in space, is neither a star nor a planet. It is a ball of rock that travels around Earth, taking about 27 days to complete the circle. The same side...
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What is the Moon made of?
The Moon is made of rock and metal—just like the Earth and the other rocky planets (Mercury, Venus and Mars). The crust, the Moon’s outer shell, is covered by lunar soil, also...
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Why can we sometimes see the Moon in the day?
We think of the Moon as something we see only at night, or as the daylight dims in the evening. In fact, the Moon is visible during the day just as often as it is at night. The...
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Moon A-Z
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Mars
Mars is the fourth planet from the Sun. A barren planet, its reddish colour comes from iron oxide dust (familiar to us as rust). Immense storms occasionally blow up, blanketing the...
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The story of Mars
Mars, along with the other terrestrial planets, began to form 4.6–4.5 million years ago. For millions of years afterwards, Mars was pelted with comets, asteroids and meteorites...
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Jupiter
Jupiter is the fifth planet from the Sun, and, with an average diameter of 143,884 kilometres (89,369 miles) the largest planet in the Solar System. Large enough to contain more...
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Galilean moons
Jupiter’s four largest moons (it has 67 in all) are called the “Galileans” after the Italian scientist Galileo Galilei (1564–1642) who discovered them with one of the first...
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Europa
For astronomers searching for life on other worlds in the Solar System, Europa, one of Jupiter’s moons, is of special interest. Europa measures 3140 kilometres (1950 miles)...
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Saturn
Saturn is the sixth planet from the Sun, and the second largest planet in the Solar System. All four gas giants have ring systems, but Saturn’s, visible through even a small...
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Titan
Titan, Saturn’s largest moon, with an average diameter of 5150 kilometres (3200 miles), is the second largest moon in the Solar System. Half as big again as Earth’s Moon, it is...
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Enceladus
Enceladus, one of Saturn’s moons, is a bright, silvery-white ball measuring only 498 kilometres (309 miles) across. Like a mirror, the surface of Enceladus reflects nearly all...
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Uranus
Uranus is the seventh planet from the Sun. Grouped with Neptune as one of the two "ice giants", it is the third largest planet in the Solar System. It lies about twice as far from...
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Neptune
Neptune is the eighth planet from the Sun—the most distant of the true planets. Grouped with Uranus as one of the two "ice giants", it is the fourth largest planet in the Solar...
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Triton
At 2700 kilometres (about 1700 miles) in diameter, Triton is Neptune’s largest moon, and the seventh largest of all the moons in the Solar System. It orbits its parent planet in...
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Pluto
Thanks to images sent back by the New Horizons space probe in July 2015, astronomers have discovered much new information about Pluto. Once the smallest and most distant planet in...
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Planets A-Z