The hunt for “Planet Nine”, or “Planet X”, is nothing new. From the mid-nineteenth century right through until 100 years ago this week there was a common belief that a planet was orbiting closer than Mercury and lost in the Sun’s glare. There were scientific reasons to believe in this planet, nicknamed “Vulcan,” but ultimately, Albert Einstein and those that tested his theory of relativity put paid to it.
Theorized “extra” planets have come and gone down the decades. So why is Vulcan back on the scene?
What is Einstein’s theory of general relativity?
The theory of general relativity kills off gravity as some kind of natural force. Instead, it describes gravity as a result of spacetime being curved by the presence of massive objects, such as stars and planets. Planets orbit the Sun because spacetime is curved; the planets are actually falling towards the Sun.
Why did scientists think there was a planet Vulcan?
French astronomer Urbain Le Verrier realized in 1859 that he could not explain the eccentric orbit of Mercury using the laws of motion set down by British astronomer Sir Isaac Newton, and theorized that there could be a planet (or planets) in between Mercury and the Sun whose presence could explain the anomaly. Since Le Verrier had used similar calculations about the orbit of Uranus to correctly predict the position of a new planet, Neptune, in 1846, the scientific world was ready to believe him. The predicted planet was called Vulcan, the god of fire, since it must have a very hot surface temperature.
“There was a belief in many quarters that there was a planet that was between Mercury and the Sun, but which was usually lost in the sun’s glare,” says Daniel Kennefick, author of No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein’s Theory of Relativity and physics professor at the University of Arkansas. “Some people had thought they had seen Vulcan during total solar eclipses and it was very common to look for Vulcan at those events because the sun’s glare was eliminated so you would have a much better chance of seeing any possible planet.”
Does Einstein’s theory of general relativity explain Mercury’s orbital motion?
Yes, and Einstein himself realized that his own general theory of relativity could explain Mercury’s strange orbit not by proving the existence of Vulcan, but by calculating how the Sun’s mass curves space-time around it. That was an important moment for Einstein, but there is still work being done to prove him 100% correct. A May 2018 paper called New General Relativistic Contribution to Mercury’s Perihelion Advance from Will Clifford at the Department of Physics, the University of Florida argues that Mercury’s orbital path shifts a degree every two billion years because of relativity.
Clifford thinks that the planets’ curving of spacetime mean that the orbits of all them are slightly altered over very long periods. It could soon be proven correct by BepiColombo, a space probe now on its way to Mercury, where tiny changes will be more easily detected than on planets further away.
Why did it take so long to disprove the existence of Vulcan?
Have you ever seen Mercury? The closest planet to the Sun is very hard to observe from Earth. Although it can occasionally be glimpsed very close to the horizon, scientists in 1859 theorized that even if there was an unknown planet between Mercury and the Sun, it would be impossible to see at any time other than during a total solar eclipse or a transit of the planet across the Sun. Or, of course, it could be explained away by a grand scientific theory. However, even such a grand theory as Einstein’s general theory of relativity also required a total solar eclipse to prove its accuracy.
Who was looking for Vulcan?
“Vulcan was supposed to explain the anomalous perihelion shift of Mercury, and then Einstein came along and his theory explained that very nicely,” says Kennefick. “In 1918 some Americans tried to test Einstein’s theory during a total solar eclipse, but they were also still looking for Vulcan.” However, when British astrophysicist Sir Arthur Eddington, secretary of the Royal Astronomical Society, and his assistant Edwin Cottingham, traveled to Príncipe Island off the west coast of Africa for the total solar eclipse of May 29, 1919, they ignored Vulcan. “They didn’t think that it was there, and went specifically to test Einstein’s theory,” says Kennefick. “There is a certain logic there. If you try to find out that Einstein’s theory is correct then you don’t have to worry about Vulcan.”
How was Vulcan finally disproved?
Despite cloudy skies, Eddington and Cottingham, along with colleagues in Sobral, Brazil, elsewhere on the path of totality, successfully photographed the stars around the Sun during totality. Those photographs of some of the stars in the Hyades open cluster, in the constellation of Taurus, showed that the stars were in the position predicted by Einstein’s general theory of relativity, thereby proving the theory, and by extension, disproving the existence of another inner planet.
Where is the “new” planet Vulcan?
Of course, long after Eddington and Einstein, Vulcan made a comeback as the fictional home of Spock in Star Trek. It was said by Star Trek creator Gene Roddenberry to be orbiting around 40 Eridani (also called HD 26965), a triple star system in the constellation of Eridanus “the river” in the southern hemisphere just 16 light years distant. In September 2018, astronomers at the University of Florida in Gainesville found a “super-Earth” exoplanet orbiting exactly where Vulcan was said to be.
Officially it’s called HD 26965 b, but is anyone not going to call it Vulcan?
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