Thanks to refined astronomical techniques we now have clear evidence that there are planets elsewhere in our galaxy. Some of these planets are "free" in interstellar space, occurring either singly or in clusters. By 2004, rather more than a hundred had been discovered in orbit around nearby stars. "Free" planets are very unlikely to house life because their surface temperatures can be only a few degrees above absolute zero, far too cold for the complicated physical and chemical processes that must be required for any imaginable form of life. Planets that orbit stars are more plausible candidates.
There is no firm definition of "planet", but although this is a significant issue in astronomy it need not impinge on our discussion here. At the lower end of the size range there is no absolute distinction between planet, planetessimal and asteroid; these are names for different portions of a size continuum. At the upper end, there is no clear way of discriminating between planets and superplanets, or superplanets and brown dwarfs. (A brown dwarf is a very small star that emits no light.) However, asteroids are unlikely to house the components necessary for any kind of life, and the
35 There are stimulating and scholarly books, for instance those by Francis Crick and Paul Davies, that argue powerfully for the position opposed to ours. Many of these works are easily accessible and we urge the interested reader to consult them (see "Further Reading").
same applies to large planets comparable to the gas giants of our Solar System. Gas giants consist very largely of hydrogen, which is no basis for forming complex molecules of any sort. Also, their gravitational fields are huge. So far as we can suppose, therefore, a planet must be substantially bigger than an asteroid and substantially smaller than a gas giant to be potentially life-bearing.
Fig. 15-1: numbers of extrasolar planets known in 2004. In the left hand diagram, the numbers of extrasolar planets orbiting stars that had been discovered before August 2004 are categorised according to their relative masses. Each unit on the horizontal axis corresponds to the mass of Jupiter. Taking account of technical limitations, the two lowest (left-hand) values are probably underestimates. These two values are therefore ignored in the right hand diagram, where the natural logarithm of planet number is plotted against mass. This plot assumes the simplest mathematical model (exponential decay) to fit the data. The fit is imperfect (the correlation coefficient is -0.89), but the data do not justify a more sophisticated model. Given the gradient of the best straight line (-0.272) and taking "Earth-sized planets" to have masses in the range 1/250-1/400 that of Jupiter, the model predicts that the extrasolar planetary systems so far observed contain around 70 "Earth-sized planets". Observation of such planets is beyond the limitations of current astronomical methods.
Extrapolating from limited data is dangerous, but it is interesting to plot the numbers of known extra-solar planets against planetary mass. Because of the limitations of current methods (measurement of dynamic effects, transits, photometric variables, microlensing and so on), relatively low-mass planets have a greater chance of being overlooked than larger ones. Therefore, the numbers at the lower (left-hand) end of the mass scale on Fig. 15-1 are likely to be underestimates. Allowing for this, the data approximately fit an exponential decay curve. If this curve is extrapolated backwards, then for every 56 planets in the range 0.5-2.0 Jupiter masses (the number presently known) we predict 70-80 roughly Earth-sized planets. (Jupiter has about 300 times the mass of the Earth.) This ratio resembles that in our own Solar System, which contains five small planets and four gas giants36. If this prediction is valid, then roughly Earth-sized planets are fairly common in those parts of the galaxy that have so far been studied; and by inference, common throughout the galaxy and presumably other galaxies. However, this is a questionable calculation: current techniques are incapable of detecting planets smaller than 6-8 earth masses, and Fig. 15-1 extrapolates from very limited evidence. But suppose the inference were broadly correct. Would it mean that life is widespread in the universe?
The process of planet formation is not fully understood. It seems that young stars are formed from clouds of material comprising 99% gas, mainly hydrogen, with traces of water ice, dust and simple organic molecules such as methanol (see chapter 14). When a star has formed, the remainder of this cloud continues to circulate around it in the form of a protoplanetary disc. The material of this disc is (probably) unevenly distributed. In its denser parts it forms lumps that grow by gravitational accretion. The bigger the lump becomes, the more of the surrounding disc material it draws into itself. The growth is self-limiting because the lump - the nascent planet - gradually clears the neighbouring space of matter until there is nothing left for it to attract37.
Beyond the "snow line" of the developing solar system, that is, beyond the point at which radiation from the star can keep water above its freezing point, ice in the growing lump might foster the formation of giant planets. This might have been the case in our own Solar System; the four planets nearest the sun are small, but the next four, beyond the "snow line", are gas giants. Organic molecules might help to "glue" the rubble of a growing planet together. But this account is incomplete. Some extra-solar giant planets are very close to their stars, so close that their orbital transits take only a few days, so ice cannot have fostered their formation. Such "hot Jupiters" would swallow a nearby Earth-sized planet as a powerful vacuum cleaner might swallow a baby gerbil.
In our Solar System the planetary orbits are almost circular. In other systems this is not the case. Some large extra-solar planets have orbits that
36 We have included Pluto in our count of small planets, but in fact the Kuiper Belt beyond the orbit of Neptune contains about 100,000 mini-planets (planetessimals), of which Pluto is just one relatively large example.
37 After planets have formed, the outer parts of the disc might remain. Even beyond the Kuiper Belt on the outside of our own solar system lies the Oort Cloud, the remnant of our protoplanetary disc. It is in the Oort Cloud that comets are formed.
are markedly elliptical and highly eccentric. Smaller planets close to any part of such an orbit would be wiped out. An Earth-sized planet with such an eccentric elliptical orbit would, even if it survived destruction by a gas giant, suffer extreme surface temperature fluctuations during the course of its "year", well beyond the buffering capacity of any Gaia effect. A planet like that is very unlikely to support life, probably regardless of the chemistry involved.
Many stars are large and short-lived. Others are small and prone to storms of high-energy radiation that would eliminate life on any nearby planet: life as we know it, certainly; other forms of life, probably. But there are plenty of stable sun-sized stars in our galaxy and probably elsewhere in the universe. However, even if most of these have orbiting planets, how many of those planets have the right size and chemistry for developing life, and how many of these have near-circular orbits, and avoid destruction by gas giants? We have no sound basis for answers, but judging from the limited evidence we have and the foregoing reasoning, potentially life-bearing planets would seem to be quite rare.
There is another difficulty: how important was the moon for the development of life on Earth? Having a single relatively large satellite has helped to stabilise the orientation of the Earth's axis for long periods of time, inhibiting rapid fluctuations of climate. Mars was not so lucky, one likely reason why Mars is now sterile. How many approximately Earth-sized planets describing near-circular orbits around stable sun-sized stars, free of gas giant interference, boast a large stabilising satellite? Given the extraordinary circumstances under which the Earth is believed to have acquired its moon, the answer seems likely to be a low number. Therefore, potentially life-bearing planets are probably rare.
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