From ancient times there were seven known planets. These were bright objects in the sky that wandered around the fixed stars. The name “planet” is Greek for “wanderer.” Some were amongst the very brightest stars and some were very faint.
Listing them in order they are Mercury, Venus, Mars, Jupiter, and Saturn. Wait, that’s only five. Well throw in the Sun and the Moon — very bright objects, and you get the magic number seven. You see, these ancients were also superstitious numerologists. They considered the number seven to mean completeness. Note there are seven days in the week and seven creative days in Genesis.
Around the 16th and 17th centuries, things started to change in a big way. The telescope was invented and Galileo observed moons on Jupiter and rings on Saturn. The heavens became messy. Kepler’s laws and Newton’s explanation of these laws with formulas and even a new form of math (Calculus) to solve the equations and science had a whole new view of the heavens.
Soon another planet was discovered: Uranus. If you’ve got really good eyesight and amazingly dark skies, you can see Uranus without a telescope. It’s only possible with the right conditions, and if you know exactly where to look.
Since it’s possible to see Uranus with the unaided eye, it’s amazing that it went undiscovered for almost all of human history. Uranus was only discovered in March 1781 by Sir William Herschel.
Actually, that’s not entirely true. There were several observations of Uranus before that, but in every case, it was mistaken as a star since it moves so slowly in the sky. The first recorded sighting was in 1690 by John Flamsteed, who spotted it at least 6 times. He cataloged it as the star 34 Tauri. The French astronomer Pierre Lemonnier also observed Uranus between 1750 and 1769.
Herschel’s original plan was to name this new planet after King George III of England. But in the end, British astronomers decided to name the new planet Uranus, after the father of Saturn in Roman mythology.
After the discovery of Uranus in 1781, astronomers noticed that the planet was being pulled slightly out of its normal orbit. John Couch Adams of Britain and Urbain Jean Joseph Leverrier of France, used Newton's laws and mathematics to predict that the gravity from another planet beyond Uranus was affecting the orbit of Uranus. They figured out not only where the new planet was, but also how much mass it had. A young astronomer, Johann Gottfried Galle, decided to search for the predicted planet and observed Neptune for the first time in 1846.
That means that Neptune was the first planet to be discovered by using mathematics. Further demonstration of the power of Newton’s law of gravity.
Neptune was the god of freshwater and the sea in Roman religion. He is the counterpart of the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto (another god we will hear about soon); the brothers presided over the realms of Heaven, the earthly world, and the Underworld.
Recall the planet Uranus was found to have strange movements that could only be attributed to another body. Neptune's discovery in 1846 somewhat accounted for the orbit, but there were still discrepancies that led scientists to conclude yet another planet existed.
In 1894, businessman Percival Lowell built Lowell Observatory in Arizona near Flagstaff to study Mars. In 1905, he turned the telescope toward the search for the elusive Planet X, though he died before the new planet could be found.
When Clyde Tombaugh was hired in 1929, he joined the search for the missing planet. The telescope at the Lowell observatory was equipped with a camera that would take two photographs of the sky on different days. A device known as a "blink compactor" rapidly flipped back and forth between the two photographs. Stars and galaxies essentially remained unmoving in the images, but anything closer could be visually identified by its motion across the sky. Tombaugh spent approximately a week studying each pair of photographs, which contained over 150,000 stars, and sometimes nearly a million.
On Feb. 18, 1930, Tombaugh noticed movement across the field of a pair of images taken a month beforehand. After studying the object to confirm it, the staff of Lowell Observatory officially announced the discovery of a ninth planet on March 13. This ninth planet was named “Pluto” for the god of the underworld.
Of course, Pluto has now been demoted and is no longer officially a planet. It was a Mickey Mouse place anyway. No really. It’s orbit actually crossed that of Neptune and for some part of it’s revolution around the Sun it was the eighth planet. For that and other reasons it got kicked out of the club.
Meanwhile, back at the slide rule, scientists were noting some unusual perturbations or shifts in the orbit of Mercury, the innermost planet. With the success of the use of math to find Neptune and Pluto, it was natural to start looking for a planet inside the orbit of Mercury to explain its variation from the mathematical expectations. Possibly this new planet was so close to the Sun that we failed to find it in the solar glare.
In the late 1800s, French mathematical astronomer Urbain Jean Joseph Le Verrier announced he’d discovered a new planet. He didn’t find it with the help of a telescope, however. He used math. He even named his new discovery calling it Vulcan for the Roman god of fire, which would be very appropriate for a planet this close to the Sun.
There was nothing wrong with Le Verrier’s math. Recall that, a few decades before, Le Verrier had conjured up Neptune using Newtonian physics to predict the exact location of our eighth planet, before ever observing it.
The clues seemed there. However, the object observed by Verrier was not a new planet. The search continued for the cause of Mercury's subtle shifts.
It wasn’t until Albert Einstein came along in 1915 that someone was able to explain the physics behind why Le Verrier’s math did not prove Vulcan’s existence.
You see, Einstein discovered that Newton was wrong. His equations were wrong. Well, at least they required a correction. These corrections are required in the case of strong gravity or very fast motion. As the inner-most planet (as we now know) Mercury is both most affected by the Sun's great gravity and is also the fastest moving planet in its orbit. Einstein’s corrections, once applied to Mercury’s orbit, removed any need for another planet to explain the orbit. Newton’s ideas worked well for the rest of the solar system, but produced some errors when applied to the innermost planet.
What lesson is there to be learned from this tale? Now, with the recent announcement from Caltech astronomers Michael Brown and Konstantin Batygin that there is likely a ninth planet (not counting Pluto) beyond Neptune, astronomers, physicists, and mathematicians are reviewing just what cautionary lessons the story of Vulcan may hold for today’s planet-hunters.
Thomas Levenson in his book “The Hunt for Vulcan … and How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe,” explains, that our understanding of the laws of physics is constantly changing, evolving. And, in the case of Vulcan, Le Verrier's mistake came from the fact that the understanding of the laws of physics was not as advanced as it is now.
Everything made sense. Newton's theory had worked in every application up to then. The calculations that Le Verrier did everybody knew. But in science a single brute fact is powerful enough to overturn the most beautiful theory.
It wasn’t until many decades later that a physicist came up with a theory that could explain the wobble in Mercury’s orbit.
Albert Einstein starts working on a different problem. Not the problem of explaining Mercury's orbit, but the problem of reconciling Newton's whole theory of of motion with his Special Theory of Relativity. His answer was to show how space (or more precisely, space-time) is warped by massive objects. This improvement is his General Theory of Relativity.
Einstein applied his theory to the real world problem of Mercury’s orbit. Mercury’s orbit behaved exactly as Le Verrier said it did, but … it was just rolling down the shortest path it could travel in curved space-time. You didn't need another planet, you didn't need some mysterious effect on the sun, you didn't need to play with the fundamental constants of nature. That's just the way it was. And it was a revelation.
Levenson is excited about the possible existence of Planet Nine. But he’s a bit more hesitant than some to believe it's out there.
“The thing that's beautiful about Planet Nine is, you know science advances, people do things differently now than they did in the 1850s. The mathematics that Batygin and Brown brought to bear … are much more sophisticated mathematics than Le Verrier had at his disposal. Just as Le Verrier had much more sophisticated mathematics than Newton had at his disposal,” Levenson says.
Underneath it all, the argument is exactly the same. There's stuff out there in the universe that is doing something that we can't quite fully explain. We may be jumping to conclusions. Somewhere some scientist may be trying to explain something entirely different and may end up modifying Einstein’s modifications of Newton.
That’s how science works. Nothing is sacred. No magic numbers. Just corrections and adjustments to allow for new data. Pretty exciting times I’d say. We now know much better than ever before just what Pluto looks like. Planet or not, it has some mysteries to be solved. Whether it has any companions out in the distant reaches of the solar system is yet to be confirmed. Who will find the answers?