Why the long-lost planet Vulcan holds lessons for Planet Nine

In the late 1800s, French mathematical astronomer Urbain Jean Joseph Le Verrier announced he’d discovered a new planet. He didn’t find it with the help of a telescope, however.

His discovery was based on a series of mathematical calculations. He decided to name the new planet Vulcan.

The only problem? Vulcan didn’t exist. 

There was nothing wrong with Le Verrier’s math. In fact, a few decades before, Le Verrier had conjured up Neptune using Newtonian physics to predict the exact location of our eighth planet, before ever observing it.

“Nature,” says Thomas Levenson, professor of Science Writing at MIT in Cambridge, Massachusetts, “has many more ways to fool us than we have ways to figure things out.”

It wasn’t until Albert Einstein came along that someone was able to explain the physics behind why Le Verrier’s math did not prove Vulcan’s existence.

Now, with the recent announcement from Caltech astronomers Michael Brown and Konstantin Batygin that there is likely a ninth planet beyond Neptune, astronomers, physicists and mathematicians are reviewing just what cautionary lessons the story of Vulcan may hold for today’s planet-hunters.

“I really hope they're right and if I were a betting man I'd bet at least lunch money that they are,” says Levenson, who has written a book called “The Hunt for Vulcan … And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe.”  

Levenson knows that our understanding of the laws of physics is constantly changing, evolving. And, in the case of Vulcan, Le Verrier's mistake came from the fact that the understanding of the laws of phsyics was not as advanced as it is now. The French mathemetician had observed a slight wobble in Mercury's orbit that, according to Newtonian physics, could be caused by the presence of Vulcan. 

“Everything made sense. Newton's theory had worked in every application. The calculations that Le Verrier did everybody knew,” Levenson says. “But in science a single brute fact is powerful enough to overturn the most beautiful theory.” 

It wasn’t until many decades later that a physicist came up with a theory that could explain the wobble in Mercury’s orbit. 

“Finally, along comes this, you know, a patent clerk. He's still a patent clerk when he gets started on this problem — Albert Einstein,” Levenson says, “And he starts working on a different problem. Not the problem of explaining Mercury's orbit, but the problem of reconciling Newton's whole theory of of motion with his then Special Theory of Relativity.”

Einstein applied his theory to the real world problem of Mercury’s orbit. “Mercury’s orbit behaved exactly as Le Verrier said it did, but … it was just rolling down the shortest path it could travel in curved space-time,” Levenson says. “You didn't need another planet, you didn't need some mysterious effect on the sun, you didn't need to play with the fundamental constants of nature. That's just the way it was. And it was a revelation.”

Levenson is excited about the possible existence of Planet Nine. But he’s a bit more hesitant than some to believe it's out there. 

“The thing that's beautiful about Planet Nine is, you know science advances, people do things differently now than they did in the 1850s. The mathematics that Batygin and Brown brought to bear … are much more sophisticated mathematics than Le Verrier had at his disposal. Just as Le Verrier had much more sophisticated mathematics than Newton had at his disposal,” Levenson says. “But underneath it all, the argument is exactly the same. There's stuff out there in the universe that is doing something that we can't quite fully explain.”

This article is based on an interview that aired on PRI's Science Friday.