An introduction to exoplanets
An introduction to exoplanets

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An introduction to exoplanets

1.5  Other objects in the Solar System

There are plenty of objects in the Solar System that are neither planets nor dwarf planets. Objects that orbit a planet rather than the Sun are called moons, or satellites. Mercury and Venus are the only planets in our Solar System that don’t have satellites, while the giant planets – Jupiter, Saturn, Uranus and Neptune – have over 150 between them.

The montage in Figure 11 shows Jupiter’s Great Red Spot and its four largest moons, the Galilean moons. From top to bottom, these are Io, Europa, Ganymede and Callisto. Their relative sizes and the size of the Great Red Spot are accurate.

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Figure 11  Jupiter and the Galilean moons

Dwarf planet Pluto has five moons, of which the largest, Charon, is almost as large as Pluto itself (Figure 12).

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Figure 12  Colour-enhanced images of Pluto (bottom) and Charon (top)

The rest of the Solar System is far from empty. Apart from the planets and moons, the Solar System contains asteroids (smaller, irregular lumps of rock) and comets (irregular lumps of ice and dust). Asteroids mostly congregate in the region between Mars and Jupiter known as the ‘asteroid belt’. Comets tend to have very stretched, or elliptical, orbits around the Sun, often coming from regions beyond Pluto to pass close to the Sun. The most famous of these, Halley’s comet, returns to the inner Solar System about once every 75 years – it will visit us again in 2061.

When a comet approaches the Sun, the heat and radiation destroys some of the ice, creating a huge cloud of gas and dust, called a ‘coma’, which surrounds the comet. Some of the material streams out in a long tail behind the comet, giving the comet its distinctive appearance. Some comets return after hundreds of years, but some never come back at all. The unlucky ones, such as comet ISON in 2013, fail to survive their encounter with the Sun (Figure 13).

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Figure 13  Comet ISON on 24 September 2013

Activity 3  Planet, dwarf planet, moon, or something else?

Timing: Allow about 5 minutes

Using the internet to help where necessary, identify each of the following objects as either a planet, dwarf planet, moon or something else.

Figure 14

1. Vital statistics: approximately half the size of Earth, rocky, round, orbits Saturn.

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Answer

Titan, Saturn’s largest moon.

Figure 15

2. Vital statistics: approximately one-third the size of Earth, rocky, orbits the Sun, found in the distant region of the Solar System called the Kuiper Belt (like Eris).

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Answer

Haumea, a dwarf planet. Although Haumea is rounded it is less spherical than other dwarf planets because it is rapidly spinning.

Figure 16

3. Vital statistics: a few kilometres across, icy, orbits the Sun.

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Answer

Comet 67P, on which the Philae spacecraft landed in 2014.

Figure 17

4. Vital statistics: a few tens of kilometres across, rocky, orbits the Sun.

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Answer

Ida, an asteroid (larger body) and Dactyl, a baby asteroid that orbits the larger body like a mini-moon (smaller body).

The planets in our Solar System vary greatly in size, although they are generally much smaller and fainter than stars, as Video 2 shows.

Video 2  The Solar System planets, Pluto, the Sun, Vega (a bright star visible to the human eye) and a giant star (please note this video has no spoken audio)
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