61 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_16, © Springer Science+Business Media, LLC 2010 In August, there was a meeting of the Nomenclature Commission of the International Astronomical Union, held in Prague. I could not go – particularly disappointing because I was, for many years, a member of that Commission, and enjoyed working with it. John Mason did attend, and on return told us what had been decided. In 1930 Clyde Tombaugh, at the Lowell Observatory in Arizona, was carrying out a systematic search for a planet moving beyond the orbit of Neptune using a telescope which had been obtained specially for the purpose. The existence of “Planet X” had been predicted by Percival Lowell, founder of the Observatory, from slight irregularities in the movements of Neptune and (particularly) Uranus. It was not long before Tombaugh found a body not far from the position given by Lowell. It was Chapter 16 The Problem of Pluto Clyde Tombaugh at the blink comparator (Credit: Lowell) 62 16 The Problem of Pluto certainly moving far beyond Neptune, and was thought to be considerably larger than the Earth. Naturally, it was assumed to be a planet, and it was named after the God of the Underworld. (Conveniently, the symbol, PL, also fitted in with Percival Lowell’s name.) From the outset Pluto was an enigma. Its orbit was much more eccentric than those of the other planets, and was also more highly inclined (17°). Its orbital period was almost 248 years, and at perihelion it moved closer-in than Neptune; the last perihelion fell in 1989, and between 1979 and 1999 its distance from the Sun was less than that of Neptune, though its orbital inclination meant that there could be no chance of collision. More worrying was the revelation that it was not only smaller than the Earth but even smaller than our Moon and Triton, the main satellite of Neptune. With a diameter of only 1,444 miles, it simply did not fit in with the general pattern of the Solar System. A satellite, Charon, was found in 1977; its diameter was more than half that of Pluto, and its orbital period, 6.3 days, was the same as Pluto’s axial rotation period, so that the two were tidally locked. To an observer standing on the surface of Pluto, Charon would remain stationary in the sky. Could the pair be regarded as a double planet – and in any case, could the diminutive Pluto really deserve full planetary status? There was no general agreement, but the situation changed in 1992 when another planetary object was found moving further-out than Neptune. For some reason or other it has never been given a proper name, and is still known by its catalogue listing, 1992 QB1. It proved to be the first of many. By now over a thousand Trans-Neptunians are known. Much earlier, the existence of a swarm of asteroid-sized bodies in this remote part of the Solar System had been mooted by the Dutch astronomer Gerard Kuiper, and today we refer to the Kuiper Belt. (In fact, a less positive suggestion had been made previously by Kenneth Edgeworth, in Ireland, and we still sometimes hear it called the Edgeworth-Kuiper Belt.) Pluto is the brightest KBO (Kuiper Belt Object) but it is not the largest. That distinction, so far as we know, belongs to Eris, which is around 1,500 miles across, while others such as Quaoar and Varuna are compa- rable. If Pluto is to be ranked as a planet, then so must Eris, Quaoar and the rest. This seems to make no sense at all. I wish I had been at that IAU meeting – I would have had a great deal to say! The first official proposal was illogical; Pluto was to be retained as a planet and to add three more: Charon and Eris together with Ceres, the largest of the main-belt asteroids, even though Ceres is a mere 600 miles in diameter and Charon is the satellite of Pluto (the excuse here, that the centre of gravity of the Pluto-Charon system lies between the two bodies, was surely irrelevant; after all, the centre of gravity of the Jupiter-Sun system lies above the solar surface). It was fairly clear that the idea of keeping Pluto as a bona fide planet was due to sentiment and tradition. The proposal was put to a general vote, and was defeated. The Commission then came up with a new recommendation: A planet would be a body moving round the Sun, massive enough to assume a spherical form, and to have cleared other bodies out of its orbit. A dwarf planet would be in orbit round the Sun and to have assumed a spherical form, but without clearly its orbit. All others would be lumped together as Small Solar System bodies. 6316 The Problem of Pluto This meant that the only accepted planets would be the familiar ones, from Mercury to Neptune; nobody was likely to quarrel with that. The dwarf planets would be Eris, Pluto and Ceres. The Committee’s proposal was accepted, but to me it seems to muddy the waters. Why should Ceres be a dwarf planet, and Pallas, the second main-belt asteroid, simply an SSSB? My suggestion would have been to class all the minor bodies orbiting the Sun as “planetoids”. But the IAU is the controlling body of world astronomy, and the die is cast. Despite this, there is still considerable resentment about the demotion of Pluto. It is now known to have three satellites, though the two new discoveries, Nix and Hydra, are very small, and a certain amount of surface detail has been made out with the Hubble Space Telescope; we will know much more in 2015 when, if all goes well, the New Horizons space-craft will swoop past it. But insofar as its status is concerned, we have to be logical rather than sentimental, and relegate it from the Premiership of the Solar System. A planetoid, certainly; A remote asteroid, possibly; A KBO, undoubtedly; But a planet it isn’t. 65 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_17, © Springer Science+Business Media, LLC 2010 Earth and Venus have often been regarded as twins. In so far as size and mass are concerned this is true enough, but they are certainly not identical. Drs Fred Taylor and David Rothery joined me to talk about the results from the latest mission there, Venus Express. During the early years of planetary space research Venus was regarded as a prime target, because it did not seem to be really hostile – probably more welcom- ing than Mars. Without going back 80 years to the ideas of Svante Arrhenius, who believed Venus to be in a state similar to that of the Earth during the Carboniferous Chapter 17 Non-identical Twins Venus South polar from Venus express (Credit: NASA) 66 17 Non-identical Twins Period, when the coal measures were being laid down and the lands were covered with lush tropical vegetation, there seemed no reason to doubt that there might be oceans, and that the climate was no more than tolerably hot. The probes of the 1960s and 1970s showed that this attractive picture was very far from the truth; the atmosphere was made up chiefly of carbon dioxide, the surface pressure was around 100 times that of the Earth’s air at sea-level, and the temperature was far too high for advanced life-forms of our kind. Moreover, the clouds were rich in sul- phuric acid. The U.S. Magellan orbiter surveyed the whole surface in detail, and as a potential colony, Venus was ruled out; the main attention swung back to Mars. Venus is a world dominated by vulcanism. There are lava-flows everywhere, and there are craters together with deep valleys – though impact craters are rare; there is overwhelming evidence that the whole landscape is “young”, and has been re-surfaced in what we may call the relatively recent past. Whether the volcanoes are active now is a matter for debate, but most people believe they are. Astronauts may well be able to survey Venus from a safe distance, but certainly not yet awhile, and a manned landing there is obviously quite out of the question. Venus may have been named after the Goddess of Beauty and is a glorious sight when shining down in the evening or morning sky, but conditions there are much more akin to the conven- tional idea of the Inferno. Now we have a new probe, Venus Express, which was launched on 9 November 2005 from the Baikonur Cosmodrome in Kazakhstan, by a Soyuz-Fregat rocket. It reached Venus on 11 April 2006, after a journey lasting 150 days, and was put into closed orbit round the planet. The orbit is eccentric; the distance from Venus ranges between 156 and 41,000 miles (250–66,000 km) and is polar, with a period of 24 h. Transmissions began immediately, and it was clear that the mission was a complete success. It was scheduled to operate until May 2009, but was still working perfectly in 2010. There is no lander; this is purely an atmospheric space-craft. In addition to analysing the atmosphere, and measuring its temperature, Venus Express carries a camera to operate in the visible, ultra-violet and near-infra-red regions of the spectrum. One early surprise was that the polar vortex, already known to exist, is like a hurricane with two “eyes” instead of one; nothing of this kind has ever been known on Earth. There is also a magnetometer. This may seem rather strange in view of the fact that Venus (unlike Mercury) has no magnetic field strong enough to be detected, but the magnetometer should be able to study the interactions between the solar wind and the uppermost part of Venus’ carbon- dioxide “air”. An observer standing on the surface would be able to see the Sun dimly through the clouds, and this leads on to a new theory about the Ashen Light – that is to say the faint visibility of the night side of Venus, seen from the Earth during the crescent stage. Its reality is not in doubt, but its origin has led to many explanations, some plausible and others bizarre. In the nineteenth century Franz von Paula Gruithuisen maintained that they were illuminations lit by the Venusians to celebrate the accession of a new Emperor, while others believed that there were electrical storms in the upper atmosphere, similar in nature to our aurorae but much stronger. Of course, the “Earthshine” on the non-sunlit side of the Moon is familiar 6717 Non-identical Twins enough, but Venus has no satellite. It is now believed that the cause is simply the glow from the fiercely hot surface passing through the atmosphere. Why are Venus and Earth so different? Surely it must be due to Venus’ lesser distance from the Sun, 67 million miles against our 93 million. When the planets were formed from the solar nebula, the two worlds may well have been similar, starting to evolve along similar lines. The Sun then was not as powerful as it is now, and probably both Venus and Earth developed seas, pleasantly warm but no more. But as time went by, the Sun’s luminosity increased. Earth was at a safe distance; Venus was not. The oceans boiled away and the carbonates were driven out of the rocks, so that the atmosphere became thick with carbon dioxide. In a very short time, astronomically speaking Venus was transformed into the inferno of today; there was what may be called a “runaway greenhouse” effect. If life had ever started there, it was ruthlessly snuffed out. This sequence of events may or may not be accurate, but it does seem plausible. Venus Express will help us to solve some of the problems which still puzzle us, but no further specialised probes there have been funded as yet, though in 2008 Messenger is due to make some observations as it flies by Venus on its way to Mercury. We may not be able to go there, and perhaps this will never be possible, but we must surely be deeply interested in the Earth’s non-identical twin. 69 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_18, © Springer Science+Business Media, LLC 2010 Some time ago we did present a “musical” programme, but that was conventional music on cosmic themes. Our last programme of 2006 was different – the music of the stars themselves. I was joined by two leaders in the field of astroseismology, Drs Don Kurtz and Yvonne Elsworth. Chapter 18 The Sounds of the Stars Coronal loops (Credit: NASA SOHO) [...]... string; pluck the string, and you will hear a musical note In the cosmic version, the sound waves generated travel in the Sun and are linked with vibrations, which can be tracked Among the results from helioseismology is the revelation that the Sun spins in a rather unexpected way Of course, it has long been known that we are dealing with differential rotation; at the equator, the rotation period is... relegated the programme to two o’clock in the morning On the previous month he had forgotten to schedule the Sky at Night at all, with the result that the programme was transmitted a week late Mr Dixon has been deleted from my Christmas card list At least he could not stop the Piers Sellers programme repeats on BBC4 and BBC2 I will say no more! The first space-man was a Russian, Yuri Gagarin Since then,... recall that our ideas about Mars have changed so dramatically over the past few decades Opinions oscillated wildly to and fro First, the polar caps were snowdrifts, then they were solid carbon dioxide, then they were due to hoar-frost no more than a millimetre thick, then they were back to snowdrifts, made of water ice mixed with CO2 ice; the dark areas were old seabeds coated with vegetation, then made... then, many astronauts have flown; all nationalities have been represented, and that includes Britons Our first true astronaut was Michael Foale; the second was Piers Sellers, who did me the honour of joining me for the first Sky at Night programme of 2007 He would not have been able to join me for my first programme, in 1 957 , as he was then at P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_19,... right through the Sun, and the inner regions of the Sun rotate in the way that a solid sphere would do 18 The Sounds of the Stars 71 The Sun is a normal star, and other stars may be expected to show the same acoustic phenomena – and so they do, provided that they are not too hot or too cool On star which has been studied with particular care is the brightest component, of the Alpha Centauri system... anticipated men on the Moon before 1970, Lunar Bases by 1980 and the first trips to Mars well before the end of the twentieth century He was right about the first of these predictions, but not, alas, the other two (I was even less correct; I doubted whether men would reach the Moon much before 1990.) According to Piers, the ISS will last until 2020 at least; its main function has been to show that habitable... shallow the layer of atmosphere is Thunderstorms give the impression of extending about half-way up from the ground Aircraft condensation trails over the Atlantic could be made out, and the scene was always changing I could not resist asking Piers what he expected to happen in the foreseeable future I wondered whether he would be more accurate than Arthur Clarke was in a very early Sky at Night programme... Venus Also, there is no longer the slightest doubt that Mars was once much wetter and much warmer than it is today Rivers flowed – and the great volcanoes were active too The Red Planet was anything but changeless and sterile Not that it is quite changeless at the present moment; small new impact craters have been found, and there is strong evidence that water occasionally gushes out from below the surface,... Alpha Centauri shines as the third brightest star in the sky, inferior only to Sirius and Canopus; it is too far south to rise over Europe (our two brightest stars are Sirius and Arcturus) At a range of 4.3 light-years, Alpha Centauri is the nearest of our stellar neighbours; there are two bright components, one rather more luminous than the Sun and the secondary (B) rather less so The third member, Proxima,... communications, but can also prove really dangerous to astronauts who are outside the protective shield of the Earth’s atmosphere If the new methods can locate an active area on the far side, we can easily tell when the rotation will bring it on to the Earth-facing side – and astronauts can make sure that they are protected It takes an acoustic wave only one and a half hours to travel right through the . helioseismology is the revelation that the Sun spins in a rather unexpected way. Of course, it has long been known that we are dealing with differential rotation; at the equator, the rotation period. programme to two o’clock in the morning. On the previous month he had forgotten to schedule the Sky at Night at all, with the result that the programme was transmitted a week late. Mr Dixon has been. Sun, and the inner regions of the Sun rotate in the way that a solid sphere would do. 7118 The Sounds of the Stars The Sun is a normal star, and other stars may be expected to show the same acoustic