Scientists at one time thought the Solar System had an extra planet. And it wasn’t just a brief error; this expectation persisted for many decades despite some of the greatest minds at the time working on the problem. The missing planet was predicted to have been even closer to the Sun than Mercury. And despite the lack of any reliable, consistent observational evidence, it remained there until Einstein shot it out of the sky.

As the other planets were all named for Roman gods, the new planet was fittingly called Vulcan to complete the pantheon. Vulcan was the god of the forge, the metalworker responsible for creating the weapons of the gods (after whom volcanoes take their name). The intense heat of an orbit so close to the Sun makes it an appropriate name. Or it would have, if the planet existed.

It can be tempting to think of the story of Vulcan as a blunder, a laughable embarrassment in the history of science. Or worse, as evidence that science isn’t all it’s cracked up to be in general. But as Thomas Levenson shows in his new book The Hunt for Vulcan: How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe, the story’s not that simple.

The Hunt for Vulcan isn’t just about the titular search for a theoretical planet that didn’t pan out. At its heart, it’s also an examination of how science really works. A quick and engaging read, the book explores uncomfortable questions about how the field deals with contradictions between data and otherwise successful theories.

Prediction

It seems the trouble with Vulcan started on the heels of the discovery of the planet Neptune, a remarkable triumph for science and for the Newtonian framework in general. Star scientist Urbain Le Verrier was responsible for the Herculean calculations that predicted the existence of Neptune based on observed irregularities in Uranus' orbit. He went on to make an exhaustive account of the Solar System using the same Newtonian physics. In the process, he predicted another planet—Vulcan.

The logic was unshakable. There was a slight irregularity in Mercury’s orbit, similar to the one seen in Uranus’ that led to the prediction of Neptune. Specifically, Mercury’s orbit was undergoing precession, a process in which the planet’s orbit—in the shape of an ellipse—shifts a little bit with each revolution. The ellipses ultimately traces out a flower-petal pattern around the Sun.

A variety of contributing influences (such as the other planets) made some amount of precession inevitable. But Mercury was precessing at a slightly higher rate than the known influences could account for. Therefore, the reasoning went, there had to be an extra contributor to its precession, something causing its orbit to diverge by a tiny amount from predictions.

There was no way around the problem. Measurements consistently showed Mercury’s departure from predictions no matter how they were reworked or how much detail was taken into account.

A planet within Mercury’s orbit would solve the problem neatly just as Neptune solved the issues with Uranus’ orbit. There was some reason to be skeptical, as a planet so close to the Sun would likely have been observed before. But the prediction was made by Le Verrier, and there was an obvious way to test it—look for the planet.

Looking for the planet but finding it’s not there could have taken some wind out of Vulcan’s sails right from the get-go, hinting that something else is going on. But that’s not what happened. Instead, Vulcan was observed. Repeatedly.

Discovery?

One such observation was made by Dr. Edmond Modeste Lescarbault, a country doctor and amateur astronomer. He spotted Vulcan right where Le Verrier said it should have been. While there was a good deal of skepticism at first (especially since Lescarbault was an amateur), Le Verrier argued that his discovery was credible.

Other astronomers now started digging through old records and finding observations that had previously been written off as comets or sunspots. A number of them matched the timing and location of Vulcan's expected location, or at least they were close enough.

Multiple, independent lines of evidence is a hallmark of a successful model in science. But despite all the amateur sightings that came pouring in, most professionals failed to find it. While making observations was difficult, if Vulcan was there, it should have showed up consistently. Yet, it consistently failed to do so.