Vulcan: the Solar System’s Ghost Planet

We have been celebrating the first Neptunian year since the discovery of the eighth (and as far as we know last) planet in the Solar System. Neptune’s discovery was a famous triumph of nineteenth century astronomy. Less well-known is the aftermath, when a ninth solar planet was discovered. This planet was called Vulcan. Once thought to be a certainty to exist, our Solar System’s Vulcan is now known to be as much a fantasy as the Star Trek one (which is said to orbit 40 Eridani A). Here is the cautionary tale of Vulcan’s discovery.

Image of Kepler planet

Vulcan would have been very hot, and perhaps would have resembled this artist’s impression of exoplanet Kepler 10b (Image credit: NASA)

 

The discovery of the seventh planet Uranus in 1781 thrilled astronomers. Observing this distant orb and plotting its course around the Sun were on the cutting edge of science two hundred years ago. Uranus was found to take about 84 years to plod through an orbit. As the decades passed, astronomers found that the planet was exhibiting some unexpected behaviour, deviating slightly from where it ought to be. This was utterly mysterious as by the early nineteenth century gravity and orbits were well understood (or were they?) There was a possible answer to this puzzle. Perhaps Uranus was being tugged from its expected position by the gravitational field of some unseen body. There could be an eighth planet to discover!

In the early 1840s the brilliant young mathematician Urbain Le Verrier (1811-71) spent months carrying out painstaking calculations (by hand with ink and paper), analysing the position of Uranus to pinpoint where the hypothetical planet’s effect on Uranus began. (At the same time as he was investigating the anomalous motion of Uranus in the distant outer darkness, he was also studying the innermost Solar System which had a mystery of its own.)

Image of Urbain-Le-Verrier

Urbain Le Verrier: astronomer and snappy dresser (image credit: via wikimedia.org)

 

By 1846, Le Verrier completed his calculations, which showed that the planet ought to lie between Aquarius and Capricorn, and posted them to the Berlin Observatory (no French observers were interested, such was the novelty of Le Verrier’s reasoning). Within a day of receiving Le Verrier’s missive German astronomers led by Johann Galle (1812-1910) used his calculations to discover the no longer theoretical planet on 23 September 1846. It was with 1° of the calculated position. The planet was duly named Neptune and Galle and Le Verrier were lauded as the planet’s co-discoverers. At the time and ever since, some have manufactured a controversy around this: in 1843, John Couch Adams (1818-92) also calculated that Uranus was being influenced by an undiscovered eighth planet. Should Adams receive some of the credit? From recent research it seems the answer ought to be no, his work was less complete than Le Verrier’s.

Le Verrier’s fame was assured, especially in his native France, and he was regarded as “the man who discovered a planet with the point of his pen.” Buoyed by this triumph, he further investigated the inner Solar System anomaly alluded to earlier. Sometimes Mercury wasn’t where it should be! The orbits of the planets could be predicted perfectly- apart from Mercury’s.

Planetary orbits are not circles, but ellipses and Mercury’s orbit was the least circular (or most “eccentric”) known. Mercury varies from as close as 46 million km from the Sun (“perihelion”) before climbing as far as 70 million km from the Sun when it is at the opposite side of its orbit (“aphelion”). This was all in accordance with the laws of planetary motion discovered by Kepler and explained by Newton. The problem was that Mercury kept reaching the low point of its orbit early and the high point late. The discrepancy was tiny (so tiny as to invoke awe for the observational skills of the astronomers of the period who measured it) but disconcerting. Could Newton have been wrong?

Le Verrier said not, but after thoroughly studying observations of Mercury going back to the 1600s, he also proved that Mercury’s odd shifting orbit was not due to the gravitational pulls of Venus, Earth and Mars. Instead a previously unseen planet between Mercury and the Sun must be to blame. He began the task of predicting its location so that it could be found in a telescope. Even once its position was known, observing it would be difficult. This planet would be smaller than Mercury and close to the Sun, so it would be hard to see. The best chances to see it would during a total eclipse of the Sun or whenever the tiny planet passed across (“transited”) the Sun’s face as seen from Earth.

Image of simulated Mercury transit

Image of simulated Mercury transit, Vulcan would look like this but smaller. (Image credit: ESO)

 

In 1859 Le Verrier published his calculations and proposed that once found (a mere detail he was willing to leave to others) the planet should be named Vulcan. Vulcan was the Roman god of fire and it seemed appropriate for a world that would be roasted by its proximity to the great furnace of the Sun. History seemed set to repeat itself with Le Verrier successfully predicting another planet’s position. At the great observatories, astronomers set to work to find the new world.

Vulcan was discovered transiting the Sun in March 1859 by a physician and amateur astronomer named Edmond Modeste Lescarbault (1814 -94). Le Verrier had some misgivings about Lescarbault’s non-professional status and about the insignificant size of the object he had reported ( it was estimated to have only 0.06 of Mercury’s mass, too small in fact to account for the deviations of Mercury’s orbit) However once Le Verrier  had paid a visit to Lescabault  at his little home-built observatory, he satisfied himself that the observation was legitimate. Le Verrier formally announced Vulcan’s discovery in March 1860. In France Le Verrier and Lescarbault were hailed as heroes (Lescarbault was awarded France’s highest medal, the Legion d’honneur). The Solar System was complete! Vive la France!

Sadly this was too good to be true. Within weeks astronomers were pointing out that the body ‘seen’ by Lescarbault would actually be quite prominent. It was difficult to see how it could have been missed by other observers with superior equipment. Then there was the annoying fact that most astronomers who used Le Verrier’s orbital parameters for Vulcan could not see it at all. Those who did report it claimed positions irreconcilable with the supposed orbit. Vulcan was quickly recognised as being much less substantial than Neptune. Vulcan was spotted occasionally over the next 40 years, sometimes by very reliable observers (but so were the Martian canals, and so too are some UFOs today) but by 1900 its existence seemed unlikely. Photography had by then revolutionised astronomy yet no photographic plate had captured the tiny planet. The problem of Mercury’s orbit was unsolved.

Image of mercury by messenger

Vulcan’s surface conditions would be harsher than those on Mercury seen here from the MESSENGER probe. (Image credit:NASA)

 

It took an even greater giant of science than Le Verrier to solve the mystery. In 1915 Albert Einstein (1879-1955) published his ground-breaking General Theory of Relativity. This postulated that space and time were linked and that the mass of very large objects could bend both. Einstein was not overthrowing Newtonian physics but was showing that there was more going on in extreme gravitational fields than previously thought. Applying his reasoning to Mercury’s orbit, Einstein showed that at its perihelion Mercury was moving through space that was being ‘bent’ by the Sun’s gravity, effectively lengthening the distance Mercury was travelling (by a miniscule amount). The predicted discrepancy was a good match to the measured value. This theory neatly solved the mystery of Mercury’s strange orbit- and Vulcan, sadly for its discoverers retreated into myth.

Were he to be aware of Vulcan’s fate from some celestial Valhalla, I expect Le Verrier would be disappointed to lose one of his planets but then intrigued and delighted by the Universe’s unexpected wonders!

(Article by Colin Johnston, Science Communicator)