Book Review - The Neptune File

Last spring I was given a copy of The Neptune File. It’s been quietly sitting on my shelf, but I finally picked it up a few weeks ago.

As anyone that’s familiar with the history of science should be able to tell you, it’s not the quiescent field of fact finding that it often appears to be. Rather, new discoveries can often cause contentious situations (although such situations are not necessarily indicative of discoveries).

I’ve written about one such situation before in which there was a large debate over the so called “spiral nebulae” and the subsequent realization that these were actually “island universes” or, as we call them today, galaxies.

Neptune File illustrates another situation of this sort: The discovery and aftermath of the discovery of Neptune.

The story as it is told here, starts with an even earlier discovery: that of Uranus by William Herschel in 1781. At that time, the big thing for astronomers to do was to determine precise orbits of known planets, comets, and other celestial bodies. Knowing where they should be allows one to determine their location on the Earth, so having precise coordinates was especially important to seafarers long before the invention of GPS satellites.

Before precise measurement techniques were available, rough orbits were good enough, but eventually, astronomers realized that the planets weren’t behaving as predicted by the simplistic models. What was needed was to add in the gravitational effects of the other planets (namely Jupiter and Saturn since they’re the most massive by far).

But even adding in these perturbations from the innermost gas giants couldn’t quite account for why Uranus’ orbit would slowly drift off from predictions. Many hypotheses were given to explain this, such as some sort of ether, differential gravity, or even comet impacts, but none of them stood up to any scrutiny.

It slowly became clear that the only possible explanation was that there was another planet yet to be discovered, that was exerting gravitational influences. But how to find it? Uranus’ discovery was somewhat serendipitous when Herschel stumbled upon it almost by accident. Hoping to find another that way, especially one that should be further away and thus move slower, be smaller, and fainter, seemed somewhat unlikely.

Undergraduate astronomer John Couch Adams realized that it should be possible to predict this planet’s position mathematically. Quietly, he began working out the location and sent his results to Britain’s chief astronomer, George Airy. But Airy dismissed the predictions, replying to Adams with an inquiry as to whether or not his hypothetical planet could also account for some other errors in Uranus’ orbit. Instead of replying immediately, Adams decided to work on refining his calculations.

In the meantime, French astronomer Urbain LeVerrier began his own calculations and published them in a series of papers. When Airy caught word that a second person had predicted a location for the hypothetical planet very close to that of Adams, he secretly directed the observatory at Cambridge, under the direction of James Challis, to begin a methodical search for it by scanning an area of the sky the two predicted, and looking for any objects that moved.

The problem with this is that this required determining the positions of a large number of stars repeatedly for long periods of time. Doing positions a single time for that large an area alone would take weeks, but having to do it repeatedly meant the search would take months.

While Airy conducted his search, LeVerrier worked to find someone to search for the planet. He finally convinced Johann Galle to conduct a search. Galle compared recent star charts with a high degree of accuracy to observations at the eyepiece. He would describe the positions of the stars to student Heinrich d’Arrest who would make sure the stars Galle saw were on the map. Any star that wasn’t would presumably be the moving planet.

On their first night of hunting, they discovered an object less than 1º from LeVerrier’s predicted position that did not correspond with any known star. They had just beautifully confirmed the mathematical predictions made by LeVerrier and, unknowingly, Adams.

Since Adams’ work had not been published, LeVerrier was initially given all the credit. But Airy, eager to keep the French from getting all the credit, revealed that Adams had also predicted the new planet’s position quite accurately. Furthermore, the Cambridge search under Challis had actually observed the planet on several occasions. But since Challis had not compared his observations with one another yet, the English lost out on the chance to make the discovery first.

The result of this was an intense nationalistic rivalry in which each country accused the other of trying to steal credit. Eventually the dispute died down and the consensus was that Adams and LeVerrier should receive equal credit.

This introduced an entirely new era of attempting to look for planets through their gravitational influences. It led to the prediction of the infamous “planet X” due to problems with orbits of Uranus and Neptune. However this was eventually resolved to be due to inaccurate estimations of the mass of these planets. It was also thought for a time that there may be another planet between the Sun and Mercury due to some strange properties of the innermost planet’s orbit (which was resolved with the incorporation of relativistic motion). But despite the failed predictions of those two planets, the technique has been refined and similar ones are now used to detect planets around distant stars.

Overall, this book is quite an entertaining read. It’s a very quick read (took me 3 days) and has almost no technical jargon (the most technical bit is the use of arcseconds which are defined early in). I’d highly recommend this book to anyone interested in the field as a wonderful reminder of how amazingly well science can make predictions, even if the politics gets somewhat muddled.