In search of Vulcan (and Vulcanoids) to Explain Mercury’s Odd Orbit


Home / In search of Vulcan (and Vulcanoids) to Explain Mercury’s Odd Orbit

Although the planet Vulcan does not orbit between Mercury and the Sun, it is believed a family of asteroids called the Vulcanoids may exist. Pictured: the asteroid Ida. Image courtesy of NASA

You’ve no doubt heard of the planet Vulcan, Mr. Spock’s homeworld in the sci-fi universe of “Star Trek”. But did you know that Trek’s fictional Vulcan was named after a hypothetical planet astronomers once thought orbited close to the Sun?

Vulcan: The Hot Planet

The search for a real planet Vulcan began in earnest during the 19th century.

In the year 1859, the discoverer of the planet Neptune, the celebrated French astronomer Urbain Jean Joseph Le Verrier (1811-1877), learned of a report about a mysterious transit of an unknown object across the face of the Sun. Collecting data from the observer, Le Verrier scribbled the transit calculations on an envelope; the calculations demonstrated the very unusual nature of the planet Mercury’s orbit around the Sun.

The hot planet’s excess precession (its direction change in rotation around the Sun) didn’t quite follow the rules of Newton’s laws of celestial mechanics, according to the great astronomer.

Precession Problem

Le Verrier published a thorough study of Mercury’s motion. This was based on a series of meridian observations of the planet as well as 14 transits… During Mercury’s orbit, its perihelion advances by a small amount each orbit, technically called perihelion precession. The phenomenon is predicted by classical mechanics, but the observed value differed from the predicted value by the small amount of 43 arc seconds per century,” says Richard Baum, author of In Search of Planet Vulcan.

Of course, when the great Le Verrier spoke everyone listened—after all, the gentleman had discovered the planet Neptune in 1846. Surely, such genius must not be ignored.
So, what caused Mercury’s puzzling precession? Ah, that was the rub. La Verrier was determined to solve the orbital mystery.

The Mercury Problem – and Solution: Voilà!

As the calendar pages of 1859 slipped away, the world’s scientific community was all ears in anticipation of Le Verrier’s final solution to the Mercury problem.

Finally, on the morning following New Year’s Day 1860, after months of mathematical doodling, with figures scribbled upon even more envelopes stained with drops from his morning tea, La Verrier announced that he had the solution: A solution both he and the astronomical community would come to embrace (even though in hindsight, the solution required a large dollop of crème brulée to go down easy).

Voilà! There is a new, undetected planet causing Mercury’s slow precession,” La Verrier explained to the world, as told in Baum’s book. “It orbits our Sun inside the orbit of Mercury.
Being so close to the Sun, La Verrier’s planet was thought to be a very hellish place.
Even no one had observed Vulcan through a telescope, the proud French astronomer, riding the popularity wavecrest of his Neptune discovery 13 years earlier, boldly named the planet Vulcan, after the ancient Roman god of fire.

How can we observe Vulcan now, astronomers asked La Verrier? “Bah! Observation? That’s a mere detail. The math is all you need. Trust me, mon ami.” was the great one’s response, according to Baum.

Le Verrier’s Solution:

What follows was Le Verrier’s solution:
•Vulcan has a nearly circular orbit at a distance from the Sun of 13,048,795.036 miles or 0.14 astronomical units (1 A.U. equals 93 million miles).
•Vulcan’s greatest elongation from the Sun is 8 degrees (as seen from Earth, that is, if the planet could be seen at all).
•Vulcan’s has a period of revolution of 19 days and 17 hours; its orbit is inclined to the ecliptic at 12 degrees, 10 minutes.

Unfortunately, a planet such as Vulcan wasn’t the only way to explain Mercury’s slow precession. So, a few astronomers began asking themselves whether a slight oblateness of the Sun could be to blame.

No Vulcan but Vulcanoids?

After more than a decade, there was still no evidence for the existence of Vulcan.
When Le Verrier died in 1877, he went to his grave convinced he had discovered two planets: Neptune and Vulcan. But with nothing to see, most astronomers gave up the search.

Mercury’s Odd Orbit

By the early 20th century Einstein’s theory of relativity handily explained Mercury’s perturbations: They were the effect of the Sun’s gravitational field.

Although Vulcan does not exist as a planet in our solar system, a few astronomers continue the search for smaller, Vulcanoid asteroids; a Vulcanoid object might be as large as 5.7 km (3.5 mi) in diameter. So far, a high-tech search for these asteroids, employing imagers aboard NASA’s twin STEREO/Solar Terrestrial Relations Observatory spacecraft, has found nothing.


Baum, R. In Search of the Planet Vulcan: The Ghost in Newton’s Clockwork Universe. (2003). Basic Books.

Steffl, A. J. et al. A Search for Vulcanoids with the STEREO Heliospheric Imager. (2013). Icarus. Accessed October 8, 2013.

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