Venus is a bigger and closer world to us than is Mercury. Looking like a small sunspot slowly poking its way across the Sun, Mercury would be tough to detect at sunrise. Transits of Mercury are rare; last century there were only 13. I missed transits like the one on May 9, 1970. The last one, in November 1999, was visible from our home in Arizona but we were on the Mediterranean Sea hoping to see a bright display of Leonid meteors. Finally, since this year's transit would be completed before sunrise from Arizona, I had to travel toward the east coast of North America to see it.

On the evening of Monday, May 5, a group of visitors on a tour of Arizona stopped by at our home for an evening of observing. A few hours later, with three hours of sleep early in the morning of Tuesday, May 6, I headed off to Montreal. Although the weather seemed good most of the way there, the plane landed in a steady drizzle that persisted well into the evening.

Though Pluto, and the far-flung depths of the Solar System, is the focus of this book, it is essential that Pluto is placed in the context of the planetary system that it inhabits – our Solar System. In the first place, this is because Pluto is just one of a large and varied number of bodies that orbit the Sun, and cannot be treated as an isolated body in space. Secondly, much of the material in this chapter is needed to support and enhance your understanding of subsequent chapters.

But before we get to the Solar System, I start by examining its cosmic neighbourhood: a vast assemblage of stars called the Galaxy, which we see in the sky as the Milky Way.

A JOURNEY INTO OUR GALAXY

The Sun, which is at the centre of the Solar System, is one of about two hundred thousand million stars that make up the Galaxy. From extensive observations made from Earth it is clear that it has a beautiful form that, face-on, is something like that in Figure 1.1.

The stars, of various kinds, plus tenuous interstellar gas and dust, often woven into stunning forms, are concentrated into a disc highlighted by spiral arms (Figure 1.1). In our Galaxy the disc is about 100 000 light years in diameter (see Box 1.1), and most stars are in a thin sheet about 1000 light years thick – roughly the same ratio of diameter to thickness as a CD. This sheet is called the thin disc.

So far, I've said very little about the surfaces and atmospheres of Pluto and its satellites, and for interiors I've given only the mean global densities and a broad indication of global compositions. In this chapter the compositions will be discussed in more detail, and internal models will be introduced.

I start with surfaces and atmospheres, which are clearly of intrinsic interest, but also because they provide clues and constraints about the interiors of Pluto and Charon. Very little is known about Nix and Hydra, therefore almost nothing is said here about these tiny satellites.

In Chapter 6 you will see that what we learn about Pluto and its satellites helps us to learn about other objects in the outer Solar System, notably the Kuiper belt objects.

First, some basic science. How do we obtain information about the surfaces and atmospheres of distant bodies? The answer is through measuring their albedos (reflectivities), which has already been discussed in Section 3.1, and by measuring their electromagnetic reflection and emission spectra.

REFLECTION AND EMISSION SPECTRA

The albedo of a body gives us information averaged over a wide range of wavelengths in solar radiation, particularly visible wavelengths. The reflectance spectrum is the reflectivity versus wavelength, and provides much more detailed information about a body. A reflectance spectrum is obtained with a device called a spectrometer, the essential components of which are illustrated in a simple way in Figure 5.1.

Pluto is a very tiny, distant world. It orbits the Sun beyond the giant planet Neptune, the outermost of the other eight planets in the Solar System. In inward order from Neptune these planets are Uranus, Saturn, Jupiter, Mars, Earth, Venus and Mercury. Pluto has a diameter a little less than one fifth of the diameter of our planet, which itself is a long way from being the largest planet in the Solar System. That title belongs to the giant planet Jupiter, with a diameter just over 11 times that of the Earth.

Why should a book be devoted to such a tiddler among the planets? There are three main reasons. First, the discovery of Pluto in 1930 is a fascinating episode in our quest to discover whether the Solar System beyond Neptune is devoid of planetary bodies. Second, ever since its discovery, controversy has been rampant about what sort of body Pluto is. Is it deserving of the status of planet, or is it too small for that? The classification of Pluto is an excellent example of the role of classification in all branches of science: classification not only comes with great advantages but also with difficulties. Third, Pluto is the closest large member of the Edgeworth-Kuiper belt, a great swarm of small bodies that orbit the Sun beyond Neptune. Though the existence of such a belt had been predicted in the 1940s, it was not until the 1990s that discoveries of other trans-Neptunian bodies were made.

As well as Charon, Pluto has two other satellites, both very tiny. Pluto is thus the largest member of a small family. In this chapter the emphasis is on the family as a whole.

The two tiny satellites were discovered in May 2005 by H A Weaver and colleagues, with the Hubble Space Telescope during studies of Pluto in relation to NASA's New Horizons spacecraft, now on its way to Pluto (Chapter 8). The discovery team proposed the Greek names Nyx and Hydra, but as Nyx already appeared in the names of two asteroids, the spelling was changed from the Greek Nyx to the Egyptian Nix. In mythology Nix (Nyx) is the goddess of darkness and light, very appropriate for a satellite orbiting the god of the underworld. Nix is also the mother of Charon – rather a small mother for such a large child! Hydra is a nine-headed serpent, a suitable name for the ninth planet Pluto. In June 2006 the International Astronomical Union approved these names.

Figure 4.1 shows an HST image of Pluto and all three satellites obtained on 15 February 2006. Pluto and Charon are grossly overexposed so that the very faint Nix and Hydra can be imaged. Nix and Hydra are the two dots, Hydra being the further from Pluto.

What an extraordinary question! Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune are planets, so why not Pluto? Pluto's planetary status had been questioned by some astronomers from not long after its discovery, on the basis of its small mass and eccentric, inclined orbit. But the crunch came in 2006. It was in that year that, after much debate and several votes, the International Astronomical Union, at its triennial General Assembly in Prague in August, which I attended, classified Pluto as a dwarf planet. This short chapter is devoted to Pluto's classification, which is an ongoing issue. But first let's consider the wider issue of the role of classification in science.

THE ROLE OF CLASSIFICATION IN SCIENCE

In science, classification provides an economy of description, a tool for structuring knowledge, and can also lead to deeper understanding.

A simple example is provided by crystals. All crystals share two attributes that define the class:

• the basic unit, be it an atom or a molecule, is arranged in one of a variety of repeating patterns in space

• they are solids, i.e. they retain their external form and do not flow like liquids.

The economy of description is that, in place of saying ‘one form of water ice is a solid with its component molecules arranged in a repeating pattern in space’, one just says ‘crystalline water’.

Before I tell you the story of Pluto's discovery, it is both instructive and relevant to the discovery of Pluto for you to learn, briefly, about the discovery of Uranus and, in more detail, about the discovery of Neptune. Between them, these three planets were found by strikingly different means.

THE DISCOVERY OF URANUS

Until 1781 just five planets were known: Mercury, Venus, the Earth, Mars, Jupiter and Saturn, all readily visible to the unaided eye. Then, on 26 April of that year a scientific paper was read to the Royal Society that opens as follows.

This is the opening of a paper written by the Germano-British astronomer William Herschel (1738–1822). It was read to the Royal Society by the British physician William Watson (1744–1825). Thus was announced to the world the discovery, not of a comet, but of what was soon shown to be a planet. Its true nature was clear by May 1781 after its large, low eccentricity orbit had been established.

What would it be like to stand on Pluto: what would we see, what would we feel? Would Pluto be useful as a launch pad for spacecraft to go to other Kuiper belt objects, to the Oort cloud and even to the stars?

TO STAND ON PLUTO

The sky

The distances from Pluto to the stars are so very much greater than from Pluto to the Earth, that the same constellations will appear in Pluto's sky as in the Earth's sky, and the same Milky Way, all with the same relative brightnesses. The retrograde rotation of Pluto means that the stars will rise in the West and set in the East. The solar day on Pluto, as on the Earth, is the time between successive noons (at noon the Sun is at its maximum altitude). On Earth this interval is one (solar) day. Pluto's axial rotation period is longer than that of the Earth, and therefore the solar day is longer, 6.387 Earth days. The Sun, planets, and stars thus move considerably more slowly across Pluto's sky than across our skies.

The rotation axis on Earth is directed at a point in the northerly sky near the fairly bright star Polaris (the Pole Star), which is a little under 1° from the exact point around which the sky appears to rotate. On Pluto the corresponding point is about 15° East from the bright star Altair.

With the discovery of Pluto, was the Solar System complete? No, far from it!

WHY SEARCH FOR MORE TRANS-NEPTUNIAN OBJECTS?

In 1930, soon after Pluto's discovery, the American astronomer Frederick C Leonard (1896–1960) wondered whether Pluto was the first of many trans-Neptunian objects awaiting discovery, but he does not seem to have acted on his prescient speculation. In July 1943, in the Journal of the British Astronomical Association, the Anglo-Irish polymath Kenneth Essex Edgeworth (1880–1972) stated that beyond Neptune the solar nebula was too thinly dispersed to have made planets, but instead many smaller bodies were present, some of which become comets. He expressed similar views in 1949 in an edition of the Monthly Notices of the Royal Astronomical Society (MNRAS). This was a year before the Dutch astronomer Jan Hendrick Oort (1900–1992) proposed the existence of a distant spherical shell of small icy bodies that enveloped the Solar System and supplied long period comets to its inner regions – the Oort cloud (Section 1.2). Such a cloud is not the belt of trans-Neptunian objects that Edgeworth had proposed; his belt was not so far away and was largely confined to the planes of the planetary orbits.

Whether there was such a belt of trans-Neptunian objects was also considered by the Dutch-American astronomer Gerard Kuiper (1905–1973) in a 1951 edition of the journal Astrophysics, but he concluded that such a belt no longer existed! This was based on the then belief that Pluto was about the mass of the Earth and would thus have scattered the bodies away.

As soon as Pluto was discovered, astronomers were eager to learn as much as possible about this remote world. What type of body was it that lurked at the outer edge of the Solar System? The most fundamental properties are size and mass. These give the mean density by dividing the mass by the volume; the mean density in turn constrains Pluto's composition.

PLUTO'S SIZE

If Pluto could be seen as a disc then, with its distance known, its size could be estimated from its measured angular diameter, as described in Section 1.6. You might think that with a sufficiently large telescope a disc would have been seen. For telescopes at the Earth's surface this is not the case. There are two reasons for this, given in Box 2.1, reasons that I slightly enlarge upon here.

First, there is the intrinsic limit of the optics (the diffraction limit), the larger the main lens or mirror, and/or the shorter the wavelengths detected, the smaller the fuzzy disc image of a point of light and the better the telescope's resolution. Visible light covers the wavelength range of about 0.38 millionths of a metre (a micrometre), to about 0.75 micrometres. The human eye is most sensitive at about 0.55 micrometres, which we see as green. At this wavelength, a telescope with a perfect main lens or mirror a metre in diameter produces a fuzzy disc such that two points of light separated by about 0.14 arcsec (each imaged as a fuzzy disc), could just be distinguished.

We would clearly learn a lot more about Pluto, its three satellites, the E-K belt, and anything else beyond Pluto, were a spacecraft targeted to investigate this far flung region of the Solar System. As yet no such spacecraft has visited Pluto and beyond, but one is on its way, New Horizons.

THE LONG PATH TO NEW HORIZONS

Table 8.1 lists the spacecraft that have already visited the outer Solar System, and reached their targets. You can see that Pluto is the only one of the original nine planets that has not been visited by a spacecraft. Why? One reason is surely that when each of the missions in Table 8.1 was being selected for development, no other KBOs were known, the first to be discovered was the small body 1992 QB1 in 1992 (Section 6.2), and therefore Pluto and its satellite Charon was regarded as just a small, isolated system. Consequently it was of considerably less interest than it is now, with its numerous companions in the E-K belt. Attention was instead focused on the rich domain of the four terrestrial planets and the four giant planets plus their numerous satellites.

As our knowledge of Pluto grew, so did interest in sending a spacecraft there, such that in the late 1980s a small number of planetary astronomers began to campaign for a mission to Pluto. The campaign was aided by the 1989 flyby of Neptune by Voyager 2.