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How Telescopes Made Earth a Planet: 400 Years Since Galileo

A session co-sponsored by Section D (Astronomy) and Section L (History and Philosophy of Science) at the annual meeting of the American Association for the Advancement of Science, Chicago, 14 February 2009.

Just about 400 years ago, Kepler and Galileo became the 15th and 16th Copernicans in the world, holding the opinion that a heliocentric rather than geocentric system was veridical as well as a reliable way to describe motions of the sun, moon, and planets. Incidentally, in order of publication, the first Copernican was his student Rheticus, and Copernicus himself only second or third. The session, co-organized and chaired by Saeqa Vrtilek (Harvard) included five speakers, whose time frames were the 16th (Copernican) century (Dennis Danielson, University of British Columbia); the Galilean, 17th century (Maurice Finocchiaro, University of California, Santa Barbara); the 17th to 19th centuries of increasing astronomical precision (Alan Hirshfeld, University of Massachusetts, Dartmouth); the 20th century in which Mars was gradually eliminated as an abode of advanced life (David DeVorkin, National Air & Space Museum); and the most recent 15 years of exo-planet searches and discoveries (Geoffrey Marcy, University of California, Berkeley).

Galileo himself, whose early astronomical applications of the telescope in 1609-10 are commemorated in the International Year of Astronomy in 2009, is remembered as much for his philosophical point of view and conflict with the Catholic Church as for his astronomical and other scientific achievements. Finocchiaro began with astronomical discoveries (mountains on the moon, moons of Jupiter, phases of Venus, resolution of portions of the Milky Way and some nebulae into stars, spots on the sun, and solar rotation), pointing out that, although most were seen by others in the same time frame, Galileo had more fully appreciated what they meant, saying, among other potentially inflammatory ideas, that scripture should not be regarded as a guide to science. While the astronomical issues have long been settled, the question of whether scientists ought to adhere to standards other than scientific truth has not.

Recent critics of “the tyranny of truth” have included Koestler and Feyerabend, criticized in turn by Finocchiaro, who ended by urging a high standard of behavior in the continuing debates between science and other ways of looking at the world, such as, he felt, both Galileo and his critics had upheld.

The contemporaries and successors of Copernicus were, Danielson pointed out, as distressed by the removal of the sun from among the planets as by the placing of the earth among them (remember Pluto at the 2006 IAU, he remarked!). Also lost was the “two storey universe” of heaven and earth. And, though Copernicus made a point of honoring the sun (in Chapter 10, Book 1 of De Revolutionibus), his ideas were slow to catch on. Among his geocentric contemporaries, Regiomontanus and Apian emphasized the importance of triangles, geometry, and gnomons (including obelisks!) at least as much as what goes at the center. The application of earth geometry to the sky perhaps helped pave the way for a more planet-like earth and a central, stationary sun. Of the early Copernicans, the ones you are most likely to have heard of are Harriott (who drew what he had seen of the moon through a telescope before Galileo, but never published the result), William Gilbert (who said the earth was like a sphere of lodestone), and Giordano Bruno (who has a statue on the site where he was burned, though the Vatican statue of Galileo has yet to be erected).

Accustomed as we are to regarding data as paramount, participants were much surprised by the rant of Flamsteed (first Astronomer Royal; and the current one was by then in the session room) against ill-conceived and ill-executed observations and experiments. But the rest of Hirshfeld’s talk dealt with these observations and experiments and the increasing precision of the instrumentation that enabled them. He began with a 17th century “to do” list, on which finding parallax was the most critical item, along with the sizes and motions of the planets, satellites, and comets. The speaker noted that Galileo had looked for parallax; not found it (small wonder with his flawed and poorly-mounted refractors); and not advertised the non-detection. We met the long, skinny telescopes of Hevelius, the zenith instruments of Hooke and Bradley (who set a roughly 1 arc-sec limit to parallax, but found the 29 arc-sec aberration of starlight in the process, an equally firm demonstration of earth’s orbital motion), the mural quadrant of John Bird, Troughton’s transit circle, and Piazzi’s 5” transit circle (built by Jesse Ramsden and still on site in Italy), en route to the 9” Great Refractor of Dorpat and the 4” heliometer of Königsburg, built by Fraunhofer for Struve and Bessel respectively. These yielded, along with a Troughton-like instrument used by Henderson at the Cape, the first stellar parallaxes, all indeed less than 1 arc-sec (Bessel for 61 Cyg, Struve for Vega, and Henderson for Alpha Centauri).

A century ago, a significant subset of observers thought they had seen straight, dark features on Mars, connecting the polar caps with dark surface areas, and that these were canals, built by the intelligent inhabitants of a dying planet. DeVorkin suggested, reluctantly, possible explanations for the illusion and the delusion. Even before that, William Herschel had reported changes in the surface appearance; Huggins had claimed absorption lines in the spectrum of Mars not seen in moonlight and which were strongest at the limb; and changing polar caps and haziness at the limb had been recorded by many observers. All of these things are true, but the surface changes in absorption lines are not aggressive enough to have been seen when first claimed. Efforts to understand the surface and to detect water and/or oxygen in the Martian atmosphere were pursued at Lick and Mt. Wilson observatories as well as at Lowell’s own facility in Flagstaff (and, noted DeVorkin, anybody who worked there had better see the canals! Indeed Lampland even managed to photograph them with the Amherst 18” refractor in Peru).

Lowell died proclaiming that because his site and his eyesight were so much better than Yerkes, Mt. Hamilton, and Mt. Wilson his results should be accepted. But the spectroscopic limits on water and oxygen crept down from at most a quarter of the terrestrial value (Campbell and Keeler at Lick) to 10-3 (Dunham and Adams at the 100” in 1932) to, finally, a detection at 0.03% of terrestrial by Kaplan, Munch, and Spinrad with the 100”. Their numbers helped to guide construction of instruments for Mariner IV, which found a very inhospitable Mars. This was a sort of relative minimum, and DeVorkin pointed out, briefly, that recent evidence strongly favors (without absolutely proving) a much wetter past Martian climate.

But, after centuries of uncertainty and decades of searching, we now know that our solar system is not the only game in town. Marcy began by noting that the University of California, Berkeley astronomy department is still to be found in W. W. Campbell Hall (seismically unsound, it is to be torn down in the next year and replaced by something that will probably also be called Campbell Hall). He then took us back to 1995, when his group and another in Switzerland reported a Jupiter-mass, short orbit period planet orbiting 51 Peg. By 1996 there were three, and the number of stars with planets now exceeds 300, including 27 multiple systems. Most have been found by careful tracking of changing radial velocities of the host stars, a dozen or so by transits across their stars, and a handful in direct images (some of which still need confirmation).

About 100 planets with masses less than 10 times earth are in the current inventory. Detectable masses will be pushed down and orbit sensitivity into the “habitable zone” by upgrades of current ground-based systems, the Kepler (transit) mission launched 6 March, the Allen Telescope array near Mt. Lassen, and the more distantly future SIM (astrometry) mission.

Earths are, in other words, within reach, and are likely to be common, given 200 billion stars in the Milky Way with at least 15% of them (in our neighborhood) with one or more planets. Marcy believes that, given the ubiquity of water, organic molecules, and energy, simple life forms should also be fairly common, perhaps like the bacteria and algae that manage to live in the near-boiling, very alkaline waters of Yellowstone. Should at least one in a million of these have intelligent life? If so, then we need an answer to the Fermi question, “where are they?” Marcy suggested several. These include: (1) the bracketing risks of deserts and water worlds (the latter unfavorable to technology – where do you put your car keys?); (2) evolution not favoring intelligence (none of the mesozoic fossil assemblages included chess boards; but, says the discussant, there was a good deal of encephalization through the Mesozoic, reaching the late Cretaceous Stenonychosaurus); and much the scariest, short lifetimes of civilizations.

Among the items that came up in the discussion period was the question of whether the eccentricities of exo-planets (many of which are large by solar system standards) are correlated with host star ages in the sense of gradual circularization. Not noticeably, said Marcy, but nearly all the interesting dynamics takes place in the first 10 to 100 million years, and most of their systems are much older. The others items discussed were largely historical. When was the Copernican model generally accepted? What about the near relationship between the sun and other stars (which Kepler did not accept)? Was the significance of Bradley’s aberration generally understood, given that it is hard, even now, to explain it to students? And would Bessel, Struve, and Henderson have received Nobel Prizes if the prizes had existed then? Yes, said Marcy (echoing contemporary praise by John Herschel), even though, by then, noted Hirshfeld, neither the existence nor the small numerical values for parallax were a surprise.

– Virginia Trimble, University of California, Irvine and LCOGT

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