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Planets and moons


Kepler-22b, an exoplanetKepler-22b, an exoplanetA planet is an object orbiting a star. It can be made of rock, metal, liquid, gas or a combination of these. It does not share its orbit with any other significant objects. It is massive enough to have a rounded (rather than irregular) shape, as a result of its own gravitational pull. At the same time, it is not massive enough for nuclear fusion to take place inside it, as occurs inside stars like the Sun. In our own Solar System, there are eight planets, including Earth, orbiting the Sun, our parent star. Observations of other stars made by astronomers using powerful telescopes indicate that some of these stars, too, have planets, called extrasolar planets or exoplanets. There could be billions of exoplanets in the Universe.

The formation of a planetThe formation of a planet

Origin of the planets

By studying meteorites, scientists have been able to work out the age of the Solar System itself: 4.6 billion years. At that time, a cloud of dust and gas drifted through space. The cloud became a swirling disc of matter, with a centre that became hotter and denser, eventually becoming the Sun. Particles of remaining dust clumped together and became boulders. These built up like snowballs into large balls of rock, called planetesimals, finally becoming planets.


The planets and the Sun to scaleThe planets and the Sun to scale

Saturn’s rings consist of blocks of iceSaturn’s rings consist of blocks of ice

Gas giants

The four largest planets in the Solar system—Jupiter, Saturn, Uranus and Neptune—are known as the gas giants because, unlike our rocky planet, they consist mostly of gases and liquid, chiefly hydrogen and helium. They all probably have rocky cores, but also have oceans beneath their thick atmospheres, consisting of liquid hydrogen, water, and other substances such as ammonia and methane. 

Each gas giant has a number of moons, some of which are larger than Mercury, the smallest planet. They also all have rings, although, other than Saturn’s, they are very faint. Saturn’s bright rings consist of millions of blocks of ice, the largest of which are about the size of small houses.

The four terrestrial planetsThe four terrestrial planets

Terrestrials and dwarfs

The four inner planets—Mercury, Venus, Earth and Mars—are much smaller than the gas giants and made mostly of rock, with iron cores. They are known as the terrestrial planets. Between Mars and Jupiter, thousands of asteroids (also called minor planets) orbit the Sun. The largest, Ceres is about 1000 kilometres (roughly 600 miles) across and has been recently been given the status of "dwarf planet", worlds that orbit the Sun and are a smooth, spherical shape, but not big enough to qualify as true planets. Pluto was once regarded as a true planet, but since 2006 has been reclassified as a dwarf planet. Its orbit around the Sun is more elongated than those of the other planets, so that some of the time it is actually closer to the Sun than Neptune.


The seven largest moonsThe seven largest moons


Moons, also known as natural satellites, are small bodies that circle around, or orbit, the planets of the Solar System. Earth has one moon, of course, known simply as the Moon, but the larger planets have many more (Jupiter has 67). Most moons orbit their parent planet in the same direction as the planet is rotating. A moon that rotates in the opposite way—usually because it has formed elsewhere and was later captured by a planet—has what is called a retrograde orbit. Of the large moons, only Neptune's moon Triton has a retrograde orbit.

Two planetesimals collideTwo planetesimals collide

The moons range in size from the largest, Ganymede, which is 5268 kilometres (3273 miles) across, to a large number that are only a few kilometres in diameter. The large ones are all spherical but the smaller ones are irregular-shaped blocks. They were probably created in different ways. Some are the result of fragments of rock and ice coming together to form a globe. Others are old asteroids that have been "captured" by a planet’s force of gravity.

 Dave Hawksett

See also in Earth

The word "planet" comes from the Greek word planetes, meaning "wandering star".

More than 800 extrasolar planets, or exoplanets, have been discovered in the Milky Way Galaxy, ranging from planets the size of Earth to gas giants larger than Jupiter.

All the planets except Mercury have atmospheres.

Since 2006, five objects in the Solar System have been officially described as dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. Astronomers think there may be hundreds—possibly thousands—of dwarf planets in the Solar System.

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