1859

Search for Vulcan

Urbain LeVerrier (1811–1877)

The French mathematician Urbain LeVerrier was still basking in the glory of his 1846 mathematical “discovery” of Neptune when he took on his next theoretical challenge. Astronomical observations of the rapid motion of Mercury (which orbits the Sun in just 88 days) over many decades had revealed some inconsistencies in its motion, resulting in inaccurate predictions of solar transits and other observational phenomena related to the planet. Kepler’s and Newton’s Laws of Motion worked incredibly well for the other planets, moons, and asteroids of the solar system except Mercury. What was going on?

LeVerrier guessed that the source of Mercury’s orbital inconsistencies was the same as the source of the orbital inconsistencies seen for Uranus more than a decade earlier: another planet must be exerting an occasional gravitational tug on Mercury, perturbing its orbit. Another planet was waiting to be discovered! LeVerrier did some calculations and came up with a prediction for the unseen planet’s location, which he figured must be very close to the Sun, with an orbital period of only about 20 days. He even named it: Vulcan, after the Roman god of fire. He announced his “discovery” to the French Academy of Sciences in 1859, and the search for Vulcan was on.

Mercury is exceedingly hard to observe with a telescope because it never gets more than about 20 degrees away from the Sun in the sky, so astronomers are always battling the Sun’s glare to observe the planet. The search for Vulcan was even more challenging, as it was predicted to never get farther than about 8 degrees from the Sun. Still, professionals and amateurs searched. LeVerrier pursued a few reports of small objects in roughly the right place transiting the Sun, or reports of objects of about the appropriate brightness being observed during eclipses, but none of the reports could be verified by follow-up observations. LeVerrier died in 1877 thinking that Vulcan was still out there, waiting to be found.

Astronomers never did find Vulcan, because Mercury’s orbital motions were eventually found to be an effect of Einstein’s theory of general relativity and the curvature of space-time that close to the Sun. Still, LeVerrier’s search lives on in a way; modern astronomers are now searching for a hypothesized population of small asteroids interior to Mercury’s orbit called, fittingly, vulcanoids.

Artist’s impression of a possible vulcanoid asteroid orbiting close in to the Sun.