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50 years ago, the population of the Solar System included one central star, nine planets, 31 satellites, and thousands of comets and asteroids. However, since the arrival of the Space Age and the development of ever more sensitive ground‐based instruments, the inventory of objects has swollen remarkably.

Today, the astronomical community recognizes eight planets and five dwarf planets, the tally of planetary satellites has passed 150, and the number of identified small objects is climbing rapidly as increasingly sensitive searches discover thousands of Sun‐grazing comets and icy Kuiper Belt objects that orbit beyond Neptune.

In terms of numbers, the Solar System is dominated by debris, in the form of comets, asteroids, meteorites, and dust. These are the leftovers from the formation of the planets, 4.5 billion years ago. The main asteroid belt, between Mars and Jupiter, is populated by millions of rocky objects that are shepherded by the powerful gravity of the nearby gas giant. They are thought to represent planetesimals – small planetary building blocks – that were unable to accrete due to the gravitational interference of Jupiter.

Beyond the orbit of Neptune are two more swarms of small objects, this time largely made of ice (Figure 1.9 and Figure 1.10). The inner region, known as the Kuiper Belt, is where short‐period comets originate. Pluto and Eris are the largest known inhabitants. The orbital periods of Kuiper Belt objects range from 200–400 years for objects such as Pluto to 1,000 years or longer for those which follow very elliptical orbits that take them far from the Sun.

Figure 1.9 The size of the Solar System. The scale bar is in astronomical units, with each marked distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 150 million km. The Kuiper Belt, which extends beyond Neptune from about 30 to 55 AU, is not shown. Two distant stars are also shown (right).

(NASA/JPL‐Caltech)

The Kuiper Belt poses a serious challenge for theories of planet formation, since it contains less than 1% of the mass of the protosolar nebula. If the Kuiper Belt objects formed like the terrestrial planets, growing by accumulating smaller objects as they orbit the Sun, the shortage of local building material means it would take longer than the age of the Solar System to make one KBO!

Even further out – indeed, so far that none of the objects have ever been observed in situ – is the postulated Oort Cloud, the home of most long‐period comets.

The basic characteristics of the Solar System are straightforward to describe. Close to the Sun, where temperatures are higher, there are four quite small, but dense, “terrestrial” planets that are composed largely of rock (Figure 1.11 and Table 1.1). Beyond Mars, where temperatures are always well below zero, is the realm of the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune.

As noted above, the orbits of the major planets are approximately circular, and close to the ecliptic plane. All of the planets and main belt asteroids circle the Sun in the same direction – counterclockwise as seen from above the Sun's north pole. This is also the direction of the Sun's rotation. However, the beautiful symmetry breaks down when it comes to the smaller members of the Solar System. Comets can arrive from any direction, and the orbits of the Kuiper Belt objects have no particular orientation, suggesting that there is a spherical swarm of these objects surrounding the Sun and major planets.

Of the four inner planets, Venus and Earth both possess dense atmospheres – though they are very different in nature – while Mercury is too lightweight to have retained a substantial gaseous envelope. Whereas the most common gas on both Venus and Mars is carbon dioxide, Earth is something of an oddball, with an atmosphere dominated by nitrogen and oxygen. This latter gas can be accounted for by the fact that Earth is – as far as we know – the only abode of life in our Solar System, and it is those life forms that pump oxygen into the air. Satellites are rare: Earth is orbited by the Moon, while Mars has two small companions that are generally considered to be captured asteroids.

As their name suggests, the gas and ice giants are characterized by their large size – tens to thousands of times bigger than Earth – and low bulk densities which can be accounted for by the dominance of hydrogen and helium in their interiors. All four of the giants have ring systems composed of dust, ice, and rocky debris, and their gravitational influence is such that they retain dozens of satellites – most of them captured billions of years ago.

Since they are relatively close to the Sun, all the terrestrial planets have high orbital velocities with periods of less than two Earth years (see Box 1.2: Kepler's Third Law). In contrast, their axial rotations are slow and their axial inclinations are very different.

Figure 1.10 These four panels show the scale of the Solar System as we know it today. At top left are the orbits of the inner planets and the main asteroid belt. Top right shows the orbits of the outer planets and the Kuiper Belt. Lower right shows the orbit and current location of Sedna, one of the most distant known objects in the Solar System. Lower left shows that even Sedna's highly elliptical orbit, which takes it nearly 1,000 AU from the Sun, lies well inside the proposed Oort Cloud (shown in blue). This spherical cloud contains millions of icy bodies orbiting at the limits of the Sun's gravitational pull.

(NASA/JPL/R. Hurt, SSC‐Caltech)

Figure 1.11 In general, a planet's surface temperature decreases with its distance from the Sun. Venus is the exception, since its dense carbon dioxide atmosphere traps infrared radiation. The runaway greenhouse effect raises its surface temperature to 467°C. Mercury's slow rotation and thin atmosphere result in the night‐side temperature being more than 500°C colder than the dayside temperature shown above. Temperatures for Jupiter, Saturn, Uranus, and Neptune are shown for an altitude in the atmosphere where pressure is equal to that at sea level on Earth. Earth lies in the center of the “habitable zone,” where water can exist as a liquid and conditions are favorable to life.

(NASA / Lunar and Planetary Institute)

Table 1.1 The Planets: Relationship Between Solar Distance and Mean Density

Planet	Distance from Sun (AU)	Mean Density (g/cm3)
Mercury	0.3871	5.43
Venus	0.7233	5.24
Earth	1.0	5.52
Mars	1.5237	3.91
Jupiter	5.2028	1.33
Saturn	9.5388	0.69
Uranus	19.1914	1.29
Neptune	30.0611	1.64

Mercury's axis is almost at right angles to its orbit. It takes 58 days to rotate once, or about two‐thirds of the time it takes to orbit the Sun. Venus resembles a top that has been knocked completely upside down. As a result, it rotates in a retrograde direction that takes 243 Earth days, longer than its orbital period. Earth and Mars have very similar days and seasons – at least in the present epoch – since their sidereal periods of axial rotation are both around 24 hours and both axes are inclined about 24–25° to their orbits (Figure 1.12).

The motions of the outer planets are very different. Their large distances from the Sun require modest velocities to maintain their orbits. Orbital periods range from almost 12 years for Jupiter to about 165 years for Neptune. However, despite their swollen spheres, they all spin much faster on their axes than their terrestrial siblings, with sidereal periods in the range of 10–20 hours.⁵ However, there is considerable variation in their axial tilts. Jupiter is almost upright, Saturn and Neptune are inclined more than Earth and Mars, while Uranus spins on its side so that the polar regions alternately point toward or away from the Sun.

The orbits and axial inclinations of the planets (and satellites) are not fixed, e.g. the axial tilt of Mars changes dramatically over millions of years.