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The author and publisher are grateful to the following for the use of photographs in this publication.

National Aeronautics and Space Administration (NASA) European Space Agency CSIRO Archives

While every care has been taken to trace and acknowledge copyright, the author apologies in advance for any accidental infringement where copyright has proved untraceable. He will be pleased to come to a suitable arrangement with the rightful owner in each case.

Chapter 1

Why a new solar system?

This chapter reviews our early and current understandings of the solar system, the discovery of the planets, and some of the earlier space probes used in the exploration of space.

In the last few years our understanding of the solar system has changed. For thousands of years, humans have been fascinated by the movement of the stars and planets across the night sky. They have wondered what these objects are made of, how they move across the sky, and whether these worlds contain other living beings like us.

In ancient times people noted the position of the Sun in the various seasons and its effect on crop growth. They also knew how the Moon affected the tides. And they observed objects called planets moving against a background of stars. The Babylonians even developed a calendar based on the movement of the planets visible to the unaided eye. In fact, the names of the days of our week originate from the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. These objects are the classical objects of our night sky.

The word 'planet' comes from the Greek word meaning 'wanderers'. The Greeks observed that the planets wandered against a background of stars that remained relatively fixed in relation to each other. The band

Figure 1.1 Stars develop everywhere we look in space. In our region of the universe they form mostly in the arms of spiral galaxies. The solar system we live in is part of the Milky Way galaxy. The Milky Way has a spiral structure like that of the galaxy shown in this photograph taken by the Hubble Space Telescope. (Photo: NASA)

Figure 1.1 Stars develop everywhere we look in space. In our region of the universe they form mostly in the arms of spiral galaxies. The solar system we live in is part of the Milky Way galaxy. The Milky Way has a spiral structure like that of the galaxy shown in this photograph taken by the Hubble Space Telescope. (Photo: NASA)

across the sky through which the planets moved was called the zodiac. The star groups or constellations that form the zodiac were given names, such as those of animals; for example, with a bit of imagination, the constellation Leo resembled a lion and Taurus resembled a bull.

Early Western and Arab civilisations and the ancient Greeks believed that the Earth was at the centre of the universe, with the Sun, Moon and the then known planets orbiting around it. This view was challenged by Polish astronomer Nicolaus Copernicus in the sixteenth century, when he suggested that all the planets, including the Earth, orbited the Sun in near-circular orbits. By using a Sun-centred model, Copernicus was able to determine which planets were closer to the Sun than the Earth, and which were further away. Because Mercury and Venus were always close to the Sun, Copernicus concluded that their orbits must lie inside that of the Earth. The other planets known at that time, Mars, Jupiter and Saturn, were often seen high in the night sky, far away from the Sun, so Copernicus concluded that their orbits must lie outside the Earth's orbit.

It was not until early in the seventeenth century that the German astronomer Johannes Kepler showed that the orbits of the planets around the Sun were elliptical, rather than circular. Kepler also showed that a planet moved faster when closer to the Sun and more slowly when further from the Sun, and he developed a mathematical relationship between the planet's distance from the Sun and the length of time it takes to orbit the Sun once. These three proven observations became known as Kepler's laws of planetary motion.

With the invention of the telescope in 1608, an Italian, Galileo Galilei, was able to gather data to support Copernicus's model for the Sun and planets. Galileo discovered four moons orbiting the planet Jupiter; he also observed sunspots moving across the surface of the Sun and craters on the Moon. Galileo's discovery that the planet Venus had phases just like Earth's Moon confirmed that Venus orbited the Sun closer than Earth and provided support for Copernicus's Sun-centred model.

One major problem restricting the full acceptance of Kepler's and Galileo's theories was that it was not known what kept the planets in orbit. People did not know how planets, once they started orbiting the Sun, could keep moving. Isaac Newton proposed the explanation of this motion in the seventeenth century. Newton put forward the idea that the Sun must be exerting a force on the planets to keep them in orbit. This force was called gravity, and it exists between any two masses (such as a planet and a star like our Sun). Using his law of gravity, Newton was able to prove the validity of Kepler's three laws of planetary motion. Newton also developed a universal law of gravitation, which states:

Two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

This law means that the more mass a planet or star has, the greater its gravitational pull. This pull decreases with increasing distance from the object.

Discovering new planets

Towards the end of the eighteenth century, only six planets were known -Mercury, Venus, Earth, Mars, Jupiter and Saturn. In 1781, the British astronomer William Herschel accidentally discovered the seventh planet, Uranus. In 1846, Urbain Leverrier in France, and John Adams in England,

Figure 1.2 Astronomers use optical and radio telescopes to explore the universe. Pictured is the Parkes Radio Telescope in Australia. (Photo: CSIRO Archives)

independently used Newton's gravitational laws to predict that variations in the orbit of Uranus were caused by the influence of an eighth planet. Soon after, astronomers at the Berlin Observatory found the predicted planet and named it Neptune. In the early twentieth century, Percival Lowell and William Pickering predicted that another planet should exist beyond Neptune. In 1930, Clyde Tombaugh found a body, which was named Pluto, close to where Lowell and Pickering predicted it to be. Between 1930 and 2006 Pluto was regarded as the ninth planet of the solar system. However, in 2006 a meeting of the International Astronomical Union (IAU) decided on a definition of a planet that excluded Pluto, instead classing it as a 'dwarf planet', along with some other newly discovered bodies. Then, in 2008, the new category of 'plutoid' was established for Pluto and similar bodies. As a result we now have what many call 'the new solar system'.

What is a planet?

Traditionally, a planet has been regarded as a spherical body that orbits a star and is visible because it reflects light. The spherical shape is only possible when the object has enough mass for gravity to be able to pull it into a spherical shape. All planets, and many large moons and large asteroids, are spherical.

In August 2006 the IAU decided on the following definition of a planet:

To be a planet a body must:

1. be in orbit around the Sun,

2. have sufficient mass for self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

3. have cleared the neighbourhood around its orbit.

What made a definition suddenly critical was the discovery of a number of objects beyond Pluto in the outer solar system. Before the new definition was accepted, there could have been as many as 50 objects classed as planets orbiting the Sun.

The IAU introduced two new classifications: 'dwarf planet' and 'plutoid'. A plutoid is a body that is in orbit around the Sun beyond Neptune, has sufficient mass for self-gravity to have pulled it into a spherical shape, has not cleared the neighbourhood around its orbit, and is not a satellite. A dwarf planet is one that orbits more closely around the Sun, has sufficient mass for self-gravity to have pulled it into a spherical shape, has not cleared the neighbourhood around its orbit, and is not a satellite. All other objects orbiting the Sun are collectively known as 'small solar system bodies'. Currently, there are a number of bodies regarded as plutoids, and more are expected to be added to the list over the next few years when more is known about them. Examples of plutoids include Pluto and Eris, which is further out from the Sun than Pluto. The former asteroid Ceres is the only dwarf planet. Plutoids and dwarf planets are not considered to be true planets mainly because they have not cleared their orbital path of other material.

As of mid-2008, there are eight major planets in the solar system. In order of distance from the Sun they are Mercury, Venus, Earth, Mars,

Figure Solar System
Figure 1.3 Major bodies in the solar system (not to scale).

Jupiter, Saturn, Uranus and Neptune. There are also one dwarf planet (Ceres) and three plutoids (Pluto, Makemake and Eris), but more bodies may be classed as plutoids in the future.

Many of the planets in the solar system have natural satellites or moons orbiting them. The planets Jupiter and Saturn have the most moons. True moons are large enough for gravity to have pulled their mass into a spherical shape, while smaller moons do not have enough mass and gravity and are irregular in shape. To be a moon, a body must be naturally orbiting a planet and be smaller than the planet. Most moons have been captured by planets during the formation of the solar system.

The four largest planets (Jupiter, Saturn, Uranus, Neptune) are also orbited by planetary rings of varying size and complexity. These rings are composed primarily of dust or particulate matter. The origin of such rings is not known, but they may be leftover debris from moons that have been torn apart by tidal forces.

Table 1.1 Major planets, dwarf planets and plutoids in the solar system (as of 2008)



Average distance


Number of

Ring system

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  • sayid
    Has cleared the neighbourhood around its orbit?
    3 years ago

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