Musing over the comments posted on my (slightly ironic) blog item about exoplanetary ennui, I remembered a piece I wrote for the Times Literary Supplement last summer so I dusted it off, chopped it up, and updated it for presentation here because it expands a bit on the earlier contribution.
If the Sun were the size of a golf ball, then the Earth would be a speck of dust a few metres from it and the nearest star would be hundreds of kilometres away. And this is what it is like in the relatively crowded environment of the Milky Way. The unimaginable scale of our Universe means that astronomy has never really become an experimental science, but has largely remained an observational one, having more in common with, say, archaeology than chemistry or other laboratory-based disciplines. Consequently, even though it is perhaps the oldest science, it is also in some respects the least mature. The absence of the traditional interplay between theory and experiment, the inability to perform repeated experiments under slightly different conditions, and the sheer difficulty of measuring anything at all have stunted its development compared to younger fields. For this reason, one often finds in astronomy certain tendencies that other subjects have largely grown out of, such as an unhealthy mania for classification and nomenclature.
Taxonomy has its place within the scientific method: modern chemistry owes much to Dmitri Mendeleev‘s periodic table; botany could not have progressed without Linnaeus; and the theory of evolution was founded on Charles Darwin‘s painstaking studies on the Galapagos Islands. But arranging things in groups and giving them names does not in itself constitute scientific progress, no matter how systematically it is done. The great experimental physicist Ernest (Lord) Rutherford dismissed this kind of activity as not science but “stamp collecting”.
This brings us to the grand debate that took place in Prague in the summer of 2006 under the auspices of the International Astronomical Union. One of the problems before the IAU’s 26th General Assembly was what to do about the fact that recent investigations have revealed the presence of a number of objects orbiting the Sun that are ostensibly at least as worthy of the name “planet” as Pluto, which in our current textbooks is the ninth one out. Obviously, which objects should be called planets depends on how you define what a planet is. The solar system contains objects of all shapes and sizes, from tiny asteroids to immense gas giants such as Jupiter and Saturn. Where should one draw the line? The original proposal was to increase the number of planets to twelve by admitting some lowly new members to the club, but in the end the IAU decided to demote Pluto to the status of a “dwarf” planet thus restricting the number of true planets to eight. This was a controversial decision, at least in the United States, because the vital vote was taken on the last day of the meeting when most of the US delegates had to take flights home. Pluto was discovered by an American, Clyde Tombaugh, in 1930, so the decision deprived the nation of its only planet-discoverer.
The “no” decision hinged on the adoption of three criteria: that the object be round, i.e. have a shape determined by internal gravitational forces; that it should have cleared its own orbit of debris; and that it should be orbiting our own star, the Sun. None of these has any special scientific value; the resulting decision was therefore pretty arbitrary. Moreover, deep-space observations have led to the discovery of literally hundreds of planetlike objects orbiting other stars. These exoplanets offer much greater prospects for scientific progress into the general theory of planet formation than the few objects that happen to have formed in our particular vicinity, so why are they excluded from the definition? In any case, what have we learned scientifically from the new nomenclature? Pluto is still the same object that it was before August 2006, and astronomers still don’t understand what one can infer from its own particular properties about the general process of planet formation.
So is Pluto a planet?
Who cares? In this case there really is nothing in a name. When I was asked this question on the telephone by a reporter I gave precisely that answer and he was shocked. I’m sure he thought that all that astronomers do is look at things and give them names. There are some that do that, of course, but most of us prefer doing proper science.
In the field of exoplanet research we are seeing real signs of maturity, although current studies are still firmly rooted in the “discovery” and “classificatuion” stage. Witness last weeks press interest in the first directly imaged exoplanets. I am well aware of the immense potential that those pictures have for stimulating interest in science, but there is still a long way to go before this field reaches its prime. That probably makes it an excellent area for young scientists to work in. But ultimately this youthful exuberance should give way to something a bit more serious, which is to go beyond what these discoveries are in themselves and ask what deeper questions they might answer.
One can see many other parallels in the history of astronomy, such as the discovery of quasars in the late 1950s. The first few of these must have generated a huge amount of excitement because they were not at all understood. Within a few years hundreds had been detected by radio observations but their nature remained unknown. The subsequent identification of redshifted hydrogen emission lines in the spectra of these objects led to them eventually being identified as very distant extragalactic sources of immense intrinsic power. By the 1980s quasars were identified as a particular type of active galaxy and placed within a general classification scheme that also involved blazars, Seyfert galaxies, and so on. Nowadays we have samples of tens of thousands of quasar spectra and the interest evolves around how the activity in their nucleus relates to the process of galaxy formation in an expanding Universe and how we can use these objects to map out the large-scale distribution of matter. To an outsider these tasks may seem less glamorous that the early days of quasar research, but that’s what science is like.
At the extreme end of the distance scale of astronomical investigation lies my own field of cosmology, the scientific study of the Universe as a whole. The scale of the solar system is challenging enough, but the cosmos is really big. Until recently, cosmology was so lacking in reliable observational input that it was thought of as a flaky offshoot of astronomy, more a branch of metaphysics than a proper scientific discipline, a paradise for theoreticians whose wildest speculations stood no chance of ever being tested with real measurements. Over the past twenty years or so, however, staggering advances in astronomical instrumentation have allowed astronomers to probe the darkest depths of space, capturing light that has travelled for almost 14 billion years on its way towards us. Theories are now so tightly constrained by these observations that there is very little room for manoeuvre. From this interplay between conjecture and refutation has emerged a cosmological framework that accounts, at least in a broad-brush sense, for how the Universe is constructed and how it is evolving.
There are some important gaps, including some puzzling anomalies, and the precise nature of many of its constituents is yet to be understood, but the establishment of the “concordance model” is a sign that cosmology really has come of age.