(Apologies all, I have been very busy, and this blog has been somewhat neglected. I am not quite sure what its future will be at this point. But, while I decide that, here is a popular science article I wrote for the University of Manchester Planet Bee magazine).
You have probably heard of the planet Vulcan. But you haven’t heard about this Vulcan. This one is the planet in our Solar System that makes Mercury wobble. The one that Einstein made vanish with a flick of his pen. This is the Vulcan that helped put a French scientist, Le Verrier, on the Eiffel Tower.
Our story begins in the 19th century. Scientists were still marvelling at the “clockwork” Universe described by Sir Isaac Newton, where planets (and apples) fell according to a simple set of rules. The cosmos was one of order and, armed with Newton’s equations, astronomers were taking control of the heavens, computing the orbits of the planets and showing that they matched the observations. But, not everything was quite moving the way it was supposed to.
Onto this stage steps Urbain-Jean-Joseph Le Verrier (1811-1877), a French mathematician, who was working and teaching at the Paris Observatory. In 1845, Le Verrier started looking at the orbit of Uranus. There were known irregularities in Uranus’ orbit, and Le Verrier calculated the effect of each of the other known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn) on the orbit of Uranus about the sun. His calculations were more detailed than any that had been done before, and he realised that he could solve the problems with Uranus’ orbit if there was an extra planet in the Solar System, further from the Sun than Uranus, which was pulling on Uranus and causing the irregularities.
In August 1846, he published his calculated position for this new planet. At the time, the Paris Observatory was not the most efficient organisation, and he failed to convince any French astronomers to look for this new planet for him. Undeterred, Le Verrier wrote a letter to the German astronomer Johann Gottfried Galle in Berlin, who took the letter seriously. Galle looked for Neptune the very night after receiving the letter, and found it within a degree of the position predicted by Le Verrier. Success!
This finding catapulted Le Verrier to fame, if not popularity. He became a member of the French Legion of Honour and, in 1847, a “chair of celestial mechanics” was specifically created for Le Verrier at the Sorbonne. In 1854, he became director of the Paris Observatory. Disappointed that he had had to write to a German astronomer to find Neptune, he set about making the Paris Observatory into a much more efficient institution. In the process, he upset a lot of the staff, and was even described as “detestable”.
The discovery of Neptune further built confidence in Newton’s clockwork Universe: here were laws so true, and so powerful, that just by calculating with a pen and paper, a new planet had been found. Whilst a doubter may have wondered if the irregularities of Uranus’ orbit pointed to a problem with Newton’s laws, true believers were justified in their faith by the discovery of Neptune.
Le Verrier didn’t stop here though. His ambition was to make a single work that described the entire planetary system, thus bringing “everything into harmony”. To this end, he started looking at the orbit of another planet, Mercury. In 1859, he was the first person to show that the precession of Mercury’s orbit (the ellipse of Mercury’s orbit moves around a bit each year) could not be explained by Newton’s laws and the known planets. Someone else might have worried about whether a chink had finally been found in Newton’s armour, but not Le Verrier. After all, he had solved this problem before. In September 1859, he proposed the planet Vulcan. A new planet, closer to the Sun than Mercury, whose existence explained the irregularities in Mercury’s orbit. Incidentally, Vulcan was the Roman god of fire (including volcanos) and the forge, which is indeed a fitting name for the planet closest to the fiery heat of the sun.
All that remained now was to find Vulcan. Le Verrier’s fame meant that his prediction of Vulcan was taken seriously, and over the next few decades there were many claims that Vulcan had been found. Nonetheless, none of them were ever truly verified, and its existence remained controversial. By the turn of the century, the idea of Vulcan had mostly fallen out of favour, even though astronomers still didn’t understand Mercury’s orbit.
Le Verrier died in 1877 believing that Vulcan existed, and never found out the truth about Mercury’s orbit. Nonetheless, his contributions to astronomy and “discovery” of Neptune immortalised him. In 1889, a statue of him was erected in the courtyard of the Paris Observatory, and he is one of 72 scientists whose name is on the first floor of the Eiffel Tower. An asteroid, a lunar crater and (appropriately) a ring of Neptune are all named after him. But what happened to Vulcan?
Only a giant has the reach to pluck a planet from the heavens, and so it proved with Vulcan. The giant in this case was an intellectual giant, Albert Einstein. In 1915, Einstein published his General Theory of Relativity. This theory ripped apart the vision of the cosmos espoused by Newton. It replaced the absolute time and space of Newton by “space-time”, the four dimensional fabric of the Universe. This “spacetime” could be curved and distorted by massive objects, and the gravitational “force” of Newton’s laws was replaced by the altered trajectories of objects travelling through this distorted spacetime.
Einstein calculated that his new theory of gravity correctly predicted the orbit of Mercury using only the known planets, with no requirement for Vulcan, thus casting it from the sky. Upon realising that his theory correctly predicted this, Einstein is reputed to have said he was so excited he couldn’t work for three days. Today, Einstein’s gravity is our best description of how gravity works in the Universe, and the calculation of Mercury’s orbit is one of the key successes of this theory. NASA spacecraft have investigated the Solar System inside the orbit of Mercury, and found no “vulcanoids” larger than 5.7km. Vulcan now only exists in Star Trek.
The juxtaposition of the two stories of Neptune (the planet that was found, saving Newton’s laws) and Vulcan (the planet that was lost, killed by the same theory that killed Newton’s laws of gravity), is an interesting lesson for scientists. When should one take the existing laws seriously, and thus posit the existence of extra things in order to make them work? And when is a problem so large, or so hard to solve, that it necessitates abandoning the established laws of the Universe and finding better ones?
Never before has this story been as relevant as it is now, with our current model of the Universe. If one “keeps the faith” with Einstein’s theory of gravity, then we require 95% of the Universe to consist of otherwise undetected substances (called dark matter and dark energy, or the “dark sector”), in order for our gravitational calculations and observations to match. Just like Neptune… but on a much grander scale.
The problem is that we’ve been trying to find dark matter particles for decades. Many theorists, like Le Verrier before them, have come up with different hypotheses for what this dark matter might be (and how we might find it), but to no avail. Whilst many scientists would put their money on finding dark matter in the next decade, it could just be that the dark matter (and dark energy) we are looking for is not a “Neptune”. Instead, maybe it is a “Vulcan”: a hypothesis trying to save a theory of gravity whose time is up. If the dark sector does turn out to be the next Vulcan, who will be the giant who reaches up and plucks it from the sky, thus overturning the view of the cosmos established by Einstein over a century ago?
Dark Matters 
