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...

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...

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...

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...

The 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...

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...

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...

The Moon A-Z

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...

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...

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...

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...

Ganymede

Jupiter's Galilean moon Ganymede, at 5268 kilometres (3273 miles) across on average, is the largest moon in the Solar System. It also has the greatest mass of any moon, more than...

Callisto

Callisto is Jupiter’s second largest moon—one of the four Galilean moons. Its average diameter measures 4806 kilometres (2985 miles). For its size, Callisto has more craters...

Io

Io, Jupiter's third largest Galilean moon, has an average diameter of 3630 kilometres (2255 miles). With a surface coloured a vivid mix of yellows, oranges, reds and blacks, Io’s...

Europa

For astronomers searching for life on other worlds in the Solar System, Europa, one of Jupiter’s Galilean moons, is of special interest. Europa measures 3140 kilometres (1950...

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...

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...

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...

Iapetus

Iapetus is the outermost of Saturn's seven large moons (in order of distance from Saturn: Mimas, Enceladus, Tethys, Dinoe, Rhea, Titan and Iapetus). It orbits its parent planet...

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...

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...

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...

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...

Planets A-Z