Heavenly bodies, nebulous concepts

Theoretical study of our solar system is at a cusp. Up until a couple of years ago, it was the only solar system we had, and astronomers as well as other types of planetary scientists assumed that many of its features were required. For example, it became canonical that a solar system would contain small, solid planets close in and giant, gaseous planets far out. After all, the solar wind would sweep the lighter elements away from the inner planets, which are in a hotter part of the solar system, while hydrogen/helium atmospheres could survive in the cold farther out, where the low temperature merely means that the atoms are moving relatively slowly and so have less chance of escaping a planet's gravity. But Michel Mayor and Didier Queloz of the Geneva Observatory and Jeff Marcy of the universities of San Francisco and of California at Berkeley and Paul Butler, now of the Mt Stromlo Observatory, started bringing us knowledge of solar systems other than our own. The nearest, reported recently, is around a small star only 15 light years from us, so close that such planets must exist all over our galaxy.

The biggest surprise is perhaps that giant planets are found so close to their parent stars, closer even than Mercury is to our own sun. Is it truth or mere wishfulness that our understanding of a solar system survives the recent proclamations that these giant planets must have formed in the outer parts of these other solar systems and then changed orbits to their current ones? The perspective brought by Stephen G. Brush to the study of solar-system formation over the last couple of hundred years is valuable for assessing the major and rapid changes now in store.

Brush, in three nicely produced volumes, starts the story with the theories of William Herschel, respected since his discovery of the planet Uranus in 1781, and Pierre Simon Laplace. Their nebular hypothesis, individually arrived at, dominated 19th-century thought about the origin of the solar system. Brush describes a series of ideas and how they evolved, with biographical paragraphs as footnotes and occasional photographs of the leading actors. The books are as readable as books so detailed can be, though his choice of quoting H. W. Olbers at such length in the original German and Laplace for three full pages in the original French, without translation, makes me wonder if more attention to the reading audience would have been useful.

Volume one of Brush's work, Nebulous Earth, traces the evolution of the nebular hypothesis, that a cooling, shrinking, spinning nebula condensed to form a central sun with surrounding rings condensing to make planets. Volume two, Transmuted Past, discusses the controversies over the age of the Earth, now accepted to be 4.55 billion years. Volume three, Fruitful Encounters, traces the nebular hypothesis through its rejection by this century's theory of Thomas C. Chamberlin and Forest R. Moulton that gravity from a passing star drew off gas from the sun that then condensed to make the planets, and its revival in current form. The tension between the "monistic" theories like the nebular hypothesis, with only our own sun and solar nebula required, and "dualistic" theories like tidal theories that require a second star, is a theme of the set of books. Since the chance of life arising elsewhere in the Universe than our Earth seemed much more likely in monistic than in dualistic theories, given the expected rarities of encounters, the scientific results had important philosophical implications.

A major research project like the one reported in these books must obviously dominate a career for a period of time, yet it is disappointing that Brush essentially ends his story in 1985. References to subsequent papers do not seem adequate. Still, volume three brings the story through the important data gleaned from the Apollo moon landings, which led, at a seminal 1984 conference, to the rejection of all the then current theories for the formation of the Moon and the genesis of the current favourite. Our best current theory is that a collision of an asteroid-sized body with the Moon over four billion years ago (a type of event astronomers are now worrying more about, though not at the level described in some recent major motion pictures) sent off a spray of material into space that coalesced into a ring around the Earth from which the Moon condensed. Brush correctly resists the temptation through the volumes to assess all past ideas in terms of whether they are "correct" or not according to the current theory, yet the abrupt ending to the book after the "Whence the Moon?" discussion leaves one wishing for more evaluation. And one occasionally wants further updates, such as current thinking about the idea that Saturn's retrograde moon Phoebe was captured, instead of merely statements from about 100 years ago of the importance of Phoebe's orbit for the nebular hypothesis.

In these days when "cosmogony" is often used to mean the origin of the solar system, as opposed to "cosmology" for the study of the whole Universe, Brush discusses "planetogony", for the origin of the planets. From time to time, Brush refers to methods of science, citing that "the strongest case for the existence of a single shared paradigm in planetogony was made by George Wetherill (1988), who claimed that the community had now arrived at 'a more normal type of science, which is not simply pursued by eccentric, elderly gentlemen who fight with one another's theories.'" Brush concludes that Thomas Kuhn's theory of scientific revolutions "has limited usefulness in understanding the recent history of planetary cosmology because there has been no fundamental revolution in planetary science, despite an immense increase in the quantity of empirical data and the emergence of several new theoretical concepts in the past 50 years." Sometimes, Brush makes the same point or definition at two different places in the same book, such as his citing of the falsification theory of Karl Popper. Similarly, his discussion of how much credit James G. Baker should get for his hypothesis of a giant impact for lunar formation appears repetitively in different parts of the same chapter.

As someone who has followed various changes in the field for my own textbook's five editions (over the period 1979-98), I find it interesting to see the evolution of several topics. Brush traces the idea, for example, that the aluminium-26 isotope means that the collapse of the solar nebula to form our solar system was begun by a nearby, external supernova, and how this thought came into favour, then was found unnecessary, and now is in partial favour again.

These volumes are obviously the result of long and careful thought, and Brush's reputation for reliability is one I have counted upon in using his evaluations of several interesting topics in the history of sciences in which he has worked. Still, I wondered why he did not get quite right the names of Fred Whipple's institutions. And I quite disagree with his giving credit to the Princeton astrophysicist Henry Norris Russell: "in 1929 Russell, confirming an earlier finding of Cecilia Payne I showed that hydrogen is by far the most abundant element in the Sun's atmosphereI" The story is now widely discussed, especially by those seeking to make sure that women are given their due credit. In this case, it seems that Payne's PhD thesis, in which she found how important hydrogen is in stars, was very publicly dismissed by Russell, whose enormous influence in American astronomy of the time led to a negative impact on Payne's career, which persisted even when she was proved right. It took several years before Russell conceded that Payne had been right. Though these past problems were not apparent to me when as a first-year student I first met Cecilia Payne-Gaposchkin in 1959 in her roles as professor of astronomy and chairman (as it was then termed) of the astronomy department at Harvard. I have since learned that she was the only female professor in the Harvard faculty of arts and sciences at the time, that the promotion had been recent, and that she had not been allowed since the 1920s to go on in the theoretical stellar field in which she was so talented. At the very least, "Mrs G." (as she was then called in those politically incorrect times) should be given the major credit for her seminal idea, now at the basis of stellar astronomy.

While volumes one and three of Brush's work are fundamentally connected to the origin of the solar system and its planets, volume two is more a history of geological ideas. It traces Earth's history from the uniformitarianism of Charles Lyell, who argued in 1830 that not catastrophes but rather forces or causes that now operate on the Earth were the source of past terrestrial evolution, to our current acceptance of occasional catastrophes, like the asteroid thought now by most (but not all) astronomers and a large but smaller percentage of geologists to have been important for the demise of many species, including the dinosaurs, 65 million years ago.

Brush digresses to contrast the working methods of scientists and of historians. He writes, "As a historian of science I still keep a foot in each world, with one office in a physical science institute, another in a history department, and an uneasy truce with the university bureaucracy after a long argument about where I should park my car." Brush reports "some striking differences in the behaviour of physicists and historians; these turn out to be clues to more fundamental methodological differences." For example, historians rarely "talk shop" at lunch while scientists often do; his physics department has a daily coffee hour, while historians are working alone at that time; historians literally (and boringly) "read" a seminar paper while scientists chat about their slides; physicists frequently collaborate while an editor of a historical journal is cited as not being able to comprehend how two people could write a paper together; physicists send out preprints and seem not afraid, as historians are, of others stealing their ideas during that interval; and physicists credit intermediate sources while historians tend to rely only on primary sources. Brush goes on to contrast the methods of particular geologists - first Lyell and then Archibald Geikie - with particular historians - first Leopold von Ranke and then G. M. Trevelyan.

Brush finds backing for the "Two Cultures" theory of C. P. Snow. He emphasises the similarities between historical and geological investigations, but finds that "the most striking difference" is that "the latter are constantly interacting with each other whereas the former seem to work in isolation. He encourages historians "to engage in vigorous debate with each other on specific questions of fact and interpretation, and, more importantly, to change their conclusions as a result of such debate."

Brush cites the shift within astronomy of the focus from "the solar system to stars and galaxies during the first part of the 20th century". I agree with him that the relative importance of solar system astronomy declined. But now the vigour of the astronomy of distant galaxies is joined by the renewed vigour of solar system astronomy, with current Mars landings and orbiters (Mars Pathfinder and Mars Global Surveyor, most recently), with Galileo in orbit around Jupiter and Cassini en route to Saturn and to its giant moon Titan, with the discovery of objects in the Kuiper belt in the outer reaches of the solar system and of asteroids whose orbits come near to the Earth in the inner parts, with comparisons of the atmospheres of moons of outer planets with the atmosphere of the Earth (such as "Global Warming on Triton," a recent Nature paper by James Elliot and others, including me, using an occultation of a star by Triton to find out about effects of solar heating on polar caps in a simpler system than the Earth), and with close-up photography and rendezvous missions under way to asteroids and comets, planetary science can proudly hold up its own end of the battle for astronomical significance. Further, the knowledge of a dozen other solar systems brings studies of solar-system formation to the fore. Brush's History of Modern Planetary Physics is a basic work that should be in all libraries where those working in the field can have access to the historical fundamentals of their science.

Jay M. Pasachoff is professor of astronomy, Williams College, Massachusetts, United States.