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Galaxies, stars and planets

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# 4 Our neighbourhood

The Solar System consists of the Sun, eight major planets, some with one or more natural satellites and ring systems, and other minor bodies (dwarf planets, asteroids and comets).

Figure 6 shows the layout of the Solar System. All the planets orbit the Sun in the same prograde direction: anticlockwise when viewed from above the North Pole. Their orbits lie roughly in the same plane and, except for Mercury, are almost circular. In Figure 6 the orbits are viewed from an oblique angle, which distorts their shapes.

Figure 6 Schematic view of the Solar System showing the orbits of the eight major planets, looking obliquely southwards from outside the Solar System. Minor bodies are shown schematically: asteroids between Jupiter and Mars; trans-Neptunian objects in the outer Solar System; and the orbits of two typical comets. Note the scale bar showing a distance of 1000 million kilometres or 1 terametre.

Most of the planets spin on their axes with the same anticlockwise (prograde) sense of rotation. The exceptions are Venus, which spins very slowly backwards (retrograde), and Uranus, which is tipped on its side.

Figure 7 indicates the relative sizes of the major planets. Note the scale bar compared with that in Figure 6. The orbits of the planets cover distances of thousands of millions of kilometres, whereas Jupiter, the largest planet, is only 140 000 kilometres in diameter.

Figure 7 The Sun and the eight major planets showing their true relative sizes. Note the change in scale between the right and left panels.

Table 1 lists the relative sizes of the planets on a scale of 1 cm to 5000 km. On this scale, a model Sun has a diameter of more than 2 m. You can get a feel for the relative sizes of the major bodies in the Solar System by representing each planet as a fruit. Note that an orange (Uranus) is about ten times the diameter of a redcurrant (Mercury), but the volume ratio is much larger - you could fit about 1000 redcurrants into the volume occupied by an orange.

Table 1 also shows the distances to the planets on the same scale. If you made a scale model of the Solar System you would not want to arrange the model planets at these relative distances! This illustrates the vast distances between the planets compared with their sizes.

Table 1 The sizes and distances of the planets on a scale of 1 cm to 5000 km.
PlanetApprox. diameter/kmApprox. model diameter/cmRepresentative fruitApprox. distance from Sun/million kmApprox. model distance/m
Mercury5 0001.0redcurrant58116
Venus12 0002.4cherry tomato108216
Earth13 0002.6cherry tomato150300
Mars7 0001.4blueberry228456
Jupiter140 00028water melon7781600
Saturn120 00024pumpkin14302900
Uranus51 00010orange28705700
Neptune49 0009.8orange45009000
• The data in Table 1 are for a model on a scale of 1 cm to 5000 km. How much bigger is the real Solar System than the model?

• One metre is 100 centimetres, and 1 kilometre is 1000 metres, so there are one hundred thousand centimetres in a kilometre (i.e. 1 km = 100 000 cm). In 5000 kilometres there are five hundred thousand thousand centimetres - in other words five hundred million centimetres - so the actual Solar System is five hundred million (500 000 000) times bigger than the model.

The planets form two groups: the four closest to the Sun (the terrestrial planets) are similar in size to the Earth and have rocky surfaces, whereas the outer four planets (gas giants) are much larger with deep dense atmospheres.

The orbits of all planetary satellites lie close to the plane of their planet's equator and most travel in the same prograde direction as their planet's spin. The largest are comparable in size with the planet Mercury, whereas the smallest are little more than giant boulders. The largest of the minor bodies (asteroids, comets and trans-Neptunian objects) are more than 1000 km in diameter and are large enough to have their shapes (roughly spherical) determined by their own gravity - they are called dwarf planets and include the former planet Pluto as well as the largest asteroid Ceres.

For astronomers, the Sun is fascinating because it is our nearest star. By studying the Sun, they can gain an insight into the workings of the other millions of stars that are visible in the night sky. Learning that the Sun is a star can be a little surprising. After all, the Sun is a brightly glowing, yellow object - so bright that it is dangerous to look at it directly, and so hot that its radiation can be felt warming the whole Earth. Stars, on the other hand, are mere pinpoints of light that are visible only against the darkness of the night sky and with no discernible heating effect on Earth. How can they possibly be the same sort of object? The key to the answer lies in their distances.

In astronomical terms, the Sun is relatively close, being only about 150 million kilometres (1 astronomical unit) from Earth. As you have seen, the stars that are visible at night are so much further away that they appear as just faint points of light. Imagine looking at a glowing light bulb first from very close up and then from a much greater distance. Close up, you would see the shape of the bulb but, from far away, it would be just a point of light.

Although it is a very ordinary star, the Sun dominates the Solar System. With a diameter of 1.4 million km it is about ten times larger than Jupiter and more than a hundred times larger than the Earth. Its mass is over three hundred thousand times that of the Earth. The combined mass of the planets is less than 0.2% of the mass of the Sun. For this reason the Sun dominates the Solar System in several ways. The Sun's gravitational force controls the motion of bodies within the Solar System. It also distinguishes the Sun (as a star) from planets. The temperatures and pressures near the centre of this massive body are sufficiently high to sustain the nuclear reactions that power the Sun and result in its prodigious output of energy in the form of electromagnetic radiation. The planets, with their much smaller masses, cannot support these reactions. They are generally observed as a result of reflected or absorbed and re-emitted sunlight.