The Planet That Wasn’t
For decades stargazers searched in vain for the hidden planet presumed to be causing Mercury’s irregular orbit.
Thomas Levenson’s new book The Hunt for Vulcan is a crisp history of a beguiling scientific episode. Starting in 1859, some scientists believed there must be an additional planet whose gravitational pull would explain a small anomaly in the orbit of Mercury. They named it Vulcan. We now know Vulcan does not exist. But it wasn’t until Einstein developed his theory of general relativity in 1915 that belief in Vulcan finally disappeared, notes Levenson, the head of MIT’s graduate program in science writing.
Einstein showed there was nothing anomalous in Mercury’s orbit when seen through the framework of general relativity—in which gravity follows the shape of space-time rather than acting as an independent force, as classical physics had held. Thus the Vulcan episode neatly reflects (and played a role in) a grand shift from Newtonian mechanics to Einsteinian relativity.
Levenson had been intrigued by the Vulcan story for years without quite figuring out what kind of book he could write about it. He got a boost from an unexpected source: Ta-Nehisi Coates, the journalist and author of Between the World and Me. From 2012 to 2014, Coates was a Martin Luther King Jr. Visiting Scholar at MIT, and as a colleague Levenson was in regular contact with Coates.
One day when the pair was talking at Toscanini’s, the celebrated ice cream parlor near MIT, Coates gave Levenson some firm, time-tested advice about the Vulcan story: just start writing.
“This is a story that wanted to be told,” Levenson says, chatting in his MIT office. So after Coates’s prodding, he finally started writing—and finished in time to have the book published in November 2015, the 100th anniversary of the famous Berlin lectures during which Einstein announced his formulation of general relativity.
Levenson emphasizes that scientific belief in Vulcan was quite reasonable. After all, in the 1840s, the French astronomer Urbain Jean Joseph Le Verrier had claimed that a wobble in the orbit of Uranus must be due to the gravitational tug from an unseen neighboring planet. On this basis, astronomers almost immediately discovered Neptune, in 1846. Surely, many experts thought, the same circumstances would explain the anomaly in Mercury’s orbit.
“When you have a [scientific theory] that is so powerful and works so well, it conditions the way you see the world,” Levenson says. “There was absolutely every reason Vulcan should exist. It wasn’t a crazy idea at all.”
But it also wasn’t true. In that sense, The Hunt for Vulcan is also a cautionary tale, Levenson says. No matter how solid we think our theories and observations are, there may be more powerful explanations waiting in the wings.
“We are not immune to measurement error, and we’re certainly not immune to the capacity for human self-deception,” Levenson says. “Historians 100 years from now will laugh at us just as we laugh at our friends from 100 years ago.”
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By Thomas Levenson, Professor of
Random House, 2015, $26