Book Review: Pluto Confidential

Books on controversial issues are usually written by writers representing one side of the issue in question. What sets Pluto Confidential apart is that it is co-authored by two astronomers representing opposing views in the Pluto debate. This makes it an ideal teaching tool and overview of the debate for those seeking a solid, un-biased background on this issue.

The two astronomers who wrote this book have been and still are insiders in the planet definition debate. Dr. Laurence Marschall, the W.K.T. Sahm Professor of Physics at Gettysburg College, is one of the 424 IAU members who voted in Prague, where he supported the current IAU definition. Dr. Stephen P. Maran, who spent 35 years working for NASA, worked on the Hubble Space Telescope, and is now press officer for the American Astronomical Society, opposes the IAU definition and supports planetary status for Pluto specifically and dwarf planets in general.

Thus the word “confidential” in the title refers to the authors’ actual participation in the planet definition controversy, with firsthand experience of the political and scientific exchanges that took place in the inner circles of the astronomy community.

For newcomers to this issue and for those already interested in it and seeking to navigate the intricacies of the debate, Pluto Confidential puts the issue in historical context, with discussions of each planet discovery, beginning with Galileo’s finding of Jupiter’s four largest moons, and the subsequent controversies and wrangling over everything from confirming the new bodies’ existence to naming those bodies.

In each case, both astronomers and lay people were confronted with a discovery much more profound than simply the existence of a new celestial body. Each discovery challenged long held understandings of the universe. In many cases, no words existed to describe the discovery. Galileo, as Marschall and Maran point out, referred to the four jovian moons he discovered as “planets.” In 1781, William Herschel discovered an object not on any star charts, one that showed a “fuzzy disk” and moved against the background stars. He assumed his discovery was a comet, as there was no conception of the existence of any planets beyond Saturn, viewed for millennia as the outer limit of the solar system.

Eventually, studies of this object’s orbit made it clear it was a planet, and it was named Uranus. When Ceres was discovered in 1801, the opposite happened—discoverer Giuseppe Piazzi and other astronomers assumed it to be a planet even though it could not be resolved into a disk. William Herschel proposed the word asteroid for this object and similar ones found between Mars and Jupiter because they appeared “star-like” as points of light when viewed in telescopes.

But were Herschel’s motives in creating this term purely scientific? On page 63, the authors raise the valid point that sentiment may have played a role here as well, when they note Herschel also had a more self-centered motive in coining this term—specifically, assuring he remained the only person alive who had discovered a planet.

Nineteenth-century astronomers ended up being wrong about Ceres anyway. They could not resolve the tiny body into a disk, but 20^th-century telescopes could and did, suggesting that its original designation as planet was likely correct after all.

The authors devote an entire chapter to the discovery of Neptune, a fascinating tale of political intrigue involving France and England as well as a series of miscommunications between young astronomer John Couch Adams and British Astronomer Royal George Airy that ended up costing England the coveted discovery.

As in David Weintraub’s Is Pluto A Planet? a non-existent planet, Vulcan, once believed to orbit the Sun closer than Mercury, gets its own chapter as well. Nineteenth-century astronomers noted that Mercury’s orbit deviated from what it should be according to their calculations and came to the logical but incorrect conclusion that the discrepancy resulted from the influence of a planet even closer to the Sun. The issue was not resolved until Albert Einstein published his theory of relativity. “Planets orbited in the distorted space time around the Sun,” the authors explain on page 106. “…when the orbit of Mercury was calculated using general relativity, its perihelion advance was in perfect agreement with the observations. No intra-mercurial planets were needed.”

The extensive background discussion is meant to illustrate several points. One is that the discovery process is “messy,” complete with false discoveries, personal rivalries, mixed motives on the part of scientists, and usually a significant political dimension. Often amateur astronomers and/or those just starting their careers find their discoveries dismissed by more “established” professionals who may not even themselves realize how jealously they are guarding access to their profession.

And new information is always interpreted in the context of what is already known. If a large piece of the puzzle is missing, such as in the case of Mercury, scientists will attempt to explain enigmas with existing knowledge, which might just be insufficient for what they are trying to accomplish.

Just as early 17^th-century astronomers could not conceive of objects revolving around anything other than the Earth, 18^th-century astronomers could not conceive of any planets beyond Saturn, and early 20^th-century astronomers could not conceive of a universe with billions of galaxies, many 21^st-century astronomers cannot conceive of our solar system possibly having hundreds of planets, of an entirely new class of planets, the dwarf planets, small bodies very much like the big planets but ones that so complicate our understanding of the solar system.

Marschall admits he could have accepted a number schematics other than the IAU definition for which he voted and acknowledges that the IAU definition has no power to influence scientific research and/or cause missions to dwarf planets to lose funding. He even notes in hindsight that the best course of action may have been for the IAU to avoid crafting definitions at all. Though he does not suggest the IAU revisit the issue and possibly suspend the definition, his statements indicate he might be open to such an action.

Unlike a few die-hards who seem to have a personal stake in “killing” Pluto, Marschall is thoughtful and reflective, even noting the positive effect of the IAU vote, specifically an increase in public interest in astronomy. That increase continues to this day.

Maran acknowledges the role of culture in the planet definition debate, but his support for Pluto’s planetary status is grounded in science. He accurately describes the objection to a solar system with hundreds of planets as a problem of “messiness.” “A lot of astronomers don’t want to deal with telling their students about a whole lot of planets in the solar system, and those who write textbooks would like to keep their tables of planetary orbits and physical data to a manageable size.”

He presents an apt analogy regarding Jupiter’s moons, noting that the giant planet had 11 known moons when he began studying astronomy but now has 62. While the small moons are acknowledged to be different from the large ones, “no one says they aren’t moons.” Similarly, he adds, “…it makes little sense to declare that a dwarf planet is not a planet.”

And he also recognizes the existence of a less than scientific motive by some astronomers at the 2006 IAU General Assembly, pointing out “It seems evident that some of those astronomers in the debate and voting at the IAU General Assembly at Prague were emphatic in their insistence that Pluto not only be reclassified as a dwarf planet but be excluded from the category of planets because they resented the public fuss over Pluto.”

Why such resentment? Maran attributes it at least in part to tension between professional astronomers on one side and amateur astronomers and lay people on the other. Pluto has always been a favorite of the latter group. “The lay world of amateur astronomers and the general public was treading on the hallowed ground so jealously guarded in order that only PhDs and professors of astronomy may enter.” When it comes to human motivations, not much has changed since the days of John Couch Adams and George Airy.

Perhaps the most telling statement in this book is Marschall’s quote of astronomer Dr. Marc Buie, who explains that scientists reach consensus on issues by exploring the details involved and using the scientific method as a guide for research. “Perhaps the lack of consensus really tells us that we don’t know enough yet to define what a planet is,” Buie is quoted saying.

The story of Pluto is still being written, and both authors acknowledge the 2015 New Horizons mission will very likely result in major changes to our understanding of this enigmatic celestial body. While we wait the remaining five-and-a-half years for this new data, Pluto Confidential provides a detailed, balanced, and fair must read for anyone seeking a thorough context for this ongoing saga.