In Chapter 7 we were essentially concerned with the first three factors in the Drake equation that were introduced in Section 6.1.1, namely, the rate R at which suitable stars are formed, the probability pp of planets forming around a suitable star, and nE the average number of suitable planets in a habitable zone. We now move on to consider how we could determine the next factor, pl, the probability of life app earing on a suitable planet in a habitable zone.
In this chapter we concentrate on the detection of life that is based on complex carbon compounds and liquid water, i.e. carbon-liquid water life. We thus concentrate on life that resembles life on Earth. In doing so we do not assume that alien life is based on the same carbon molecules as terrestrial life. It might use mirror image isomers of some molecules used by terrestrial life, a carbon compound other than DNA to carry genetic information, or carbon compounds other than proteins to carry out the various functions performed by proteins in terrestrial life. But it is still carbon-liquid water life. Only in Chapter 9 will we free ourselves of this selfimposed (but reasonable) carbon-liquid water restriction. There, in the search for extraterrestrial intelligence (SETI) we will search for evidence of technological civilization regardless of the chemical basis of the life-forms.
In searching for signs of carbon-liquid water life, we are at least looking for something that we know to be possible. Another justification stems from fundamental chemistry. No element other than carbon has anywhere near the same facility to form compounds of sufficient complexity, diversity and versatility to supp ort the many processes of life (Section 1.1.2). Few liquids app roach water in its ability to act as both a solvent and a reactant. Ammonia is a possible alternative to water at low temperatures (at a pressure of 1 bar it is liquid from 195 K to 240 K), but it is pure speculation whether a low-temperature form of life could use ammonia in place of water. A third justification is that we know how to detect evidence of carbon-liquid water life. Apart from SETI, we have a poorer idea of how to detect life that has an entirely different chemical basis from ours, particularly as we are restricted to detection from afar.
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.
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’.
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 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.
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.
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.
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.
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.
‘On Tuesday the 13th of March, between ten and eleven in the evening, while I was examining the small stars in the neighbourhood of H Geminorum, I perceived one that appeared visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet.’
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.
Email your librarian or administrator to recommend adding this to your organisation's collection.