The mysterious ninth planet has been in the news recently, and I thought we’d discuss that a bit. But before we do, let’s bring everyone up to speed on the planetary discoveries we’ve made in our own solar system. 

The first civilization to have accurately and mathematically described the planets were the ancient Babylonians living in what is now modern-day Iraq. In the first and second millennia BCE, they wrote down what we know as the “Venus tablet of Ammisaduqa,” a copy of a list of observations of the motions of Venus. This makes Venus probably the first planet to be discovered by humanity. That makes sense, doesn’t it? Venus is the brightest object in the sky, after the Sun and the Moon, so it makes sense that it would be discovered first. 

Later, the Babylonians had similar tablets for the other “classical” planets: Mercury, Mars, Jupiter, and Saturn. These planets would be adapted by the ancient Greeks and Romans, and nothing would change for 3500 years. 

It is worth mentioning that the arithmetic of the Babylonians accurately predicted the motions of the planets. That can not be said of the complicated nature of the Greek universe. The Greeks desired geometric perfection, relying on a mix of circles on circles (called deferents and epicycles). They also held that the planets do not orbit the Sun, as we know now, but rather that they orbit the Earth. This geocentric view would hold sway over Europe for centuries. 

Finally, in the early 1600s, Galileo Galilei broke that hold observationally. With his new telescope that had only been invented very recently, he pointed it at the sky and described what he saw. His observations were so powerful that the geocentric universe virtually fell apart overnight. 

Galileo’s drawing of the Sun with sunspots

On the never-changing Sun, he saw dark spots move across the surface in a matter of days. He viewed the changing phases of Venus, seeing it change from near-full to crescent to black and back again. He discovered the “two ears” of Saturn, and watched in amazement as they disappeared only to reappear again! But the real nail in the coffin was his observations of Jupiter. Galileo peered through his telescope and watched four small points of light orbit not the Earth, but Jupiter! 

These small points of light are what we know as the Galilean moons of Jupiter: Ganymede, Callisto, Io, and Europa, observed on the nights of January 7-8, 1610 by Galileo himself. Suddenly the Solar System became a bit bigger overnight. 

More observations would follow. Dutch astronomer Christiaan Huygens discovered the first moon of Saturn, the clouded world of Titan, only forty-five years later. Then, Giovanni Cassini turned his telescope back at Saturn and found four more moons, Iapetus, Rhea, Tethys, and Dione. Moons were popping up all over the place! 

It would be 100 years before another ground-breaking discovery would be made. 

Before we get to that discovery, we need to talk about a pair of Germans. Independently of each other, Johann Titius and Johann Bode wrote down the same “law.” Both wrote down, in the margins of their books, something along the lines of: 

“This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4+3=7. The Earth 4+6=10. Mars 4+12=16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4+24=28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4+48=52 parts, and finally to that of Saturn by 4+96=100 parts.”

Johann Bode

The Titius-Bode Law, as it came to be known, worked surprisingly well. It seemed that God Himself had designed the Solar System as a precise mathematical law. It was now on the astronomers of the day to find the missing links, that between Mars and Jupiter and predicted planets beyond Saturn. 

Enter Sir William Herschel. Herschel was an English astronomer of the late 1700s. His primary focus was to search for stars that were very close together visually, called “double stars.” The idea was that observing these stars would provide evidence of motion that would allow astronomers of the day to calculate the distance to them from Earth. He would discover over 800 confirmed double or multiple star systems, most of them actually being physically intertwined rather than just an optical illusion. 

It happened that during one of his observing nights in 1781, Herschel noticed an object that appeared as a disk, rather than a point of light. He thought it might be a comet, so he observed it over several nights, allowing its orbit to be calculated. When he found that the nearly circular orbit indicated that it was a planet, and not a comet on a highly elliptical orbit, Herschel was shocked. No one had ever discovered a planet before! 

In recognition of his great achievement, the reigning king of the time, none other than America’s enemy, King George III, invited Herschel to move to Windsor and paid him to continue his astronomical studies. To honor his new patron, Herschel suggested the name “Georgium Sidis” for the new planet, Latin for the “Georgian Planet.” 


As you might imagine, this suggestion did not go over well outside of Britain. Alternatives were soon proposed. One astronomer suggested we name the planet Herschel, in honor of its discoverer! Eventually, Johann Bode suggested the perfect name: Uranus. He argued that the name should follow the mythology, and it was a perfect sequence: in Roman mythology, Saturn was the father of Jupiter, and Uranus was the father of Saturn. Ultimately, it took seventy years for this to gain enough support. 

The important part was still unanswered: does it fit neatly into the Titius-Bode Law? The law predicted that the planet beyond Saturn would orbit at 19.6 units (or astronomical units, AU) from the Sun. Calculations showed that Uranus orbits at 19.22 AU, which was close enough for Bode and Titius. 

No sooner had Herschel discovered the planet, had he then discovered two moons of Uranus: Titania and Oberon. Herschel also discovered two new moons of Saturn: Enceladus and Mimas. 

A brief aside. Planetary nomenclature is generally Roman (i.e. Mars and Jupiter being Roman gods), while their moons are named after people associated with their Greek counterparts (i.e. Europa being a lover of Zeus). The one exception to this rule is Uranus, since he is a Greek god, the Roman counterpart being Caelus. As such, its moons also are a departure from the trend; they are all named after characters from Shakespeare’s and Alexander Pope’s works (i.e. Oberon being the King of the Fairies from A Midsummer Night’s Dream). 

All these lunar discoveries are great, but there’s still that nagging gap between Mars and Jupiter. That gap would be filled on New Years’ Day 1801 by Italian priest Giuseppe Piazzi. Piazzi was part of a methodical search to find the missing planet and find it he did! His new planet was named Ceres, after the Roman goddess of agriculture. 

Did it fit into the law? Let’s see. The law calculated a planet at 2.8 AU from the Sun, and Ceres came in at 2.77 AU. Once again, close enough for the Germans!


All was not to last. Ceres was discovered on January 1, 1801. A year later, another body would be discovered in Ceres’ orbit. Two years later another object was found. Then another! By 1845, four more objects at the same distance from the Sun as Ceres had been found: Pallas, Juno, Vesta, and Astraea. This was the first nail in the coffin for the Titius-Bode Law. 

Nowadays, we of course know what is between Mars and Jupiter: the Asteroid Belt. Those astronomers weren’t finding planets, but rather the largest and brightest asteroids (or in the case of Ceres, a dwarf planet). NASA tracks hundreds of thousands of asteroids between Mars and Jupiter and some even further in near Earth! 

This was not the end of our favorite law yet. There was still one final test, and it lied in the orbital data for our new planet Uranus. Thanks to our main man Sir Isaac Newton and his laws of motion and gravity, we could calculate where Uranus should be in the future along its orbit. However, doing that and waiting for Uranus to catch up to the predictions showed that something was wrong: Uranus was in the wrong place! Was Newton wrong? Or was something else afoot?

Independently of each other, and at roughly the same time, the British astronomer fresh out of college, John Couch Adams, and the French astronomer trying to work his was into the esteemed French Academy of Sciences, Urbain Le Verrier, proposed a solution. They believed that something was perturbing Uranus’s motion, an unseen eighth body. They both started the laborious task of working Newton’s equations backwards to find the position of this eighth planet. 

In modern times, this sort of problem is an easy one to solve if you have a computer nearby and your favorite computational software. But in the mid-1800s, deducing a mathematical model from observations all had to be done by hand. Adams spent his summer vacation in 1843 working through the first six iterations of differential equations. He wouldn’t finish his work until September of 1845. Evidently, Le Verrier was a more practiced mathematician, for he only took a little less than a year to solve the problem. 


Finally, on September 24, 1846, at the urging of Le Verrier, the German astronomer Johann Galle in Berlin investigated the region of the sky deduced by Le Verrier’s mathematics. After less than an hour of searching, the eighth planet was found, less than the width of a full moon from where Le Verrier said it would be. 

What of Adams’ math? The observatory to which he sent his work ignored it, and it got buried under papers. When they eventually dug it up after Le Verrier’s discovery, Adams’ math pointed them away from Uranus, with twelve times less accuracy than Le Verrier’s math. 

In the eternal war of France versus England, France won this round. 

Thus, it was up to Le Verrier to pick a name. After going through the same hassle Herschel went through when the public suggested they name Uranus after him, Le Verrier instead suggested the name Neptune, the Roman god of the sea. 


But what does this hold for our fancy law predicting the positions of the planets? Titius and Bode predict the eight planet to be at 38.8 AU from the Sun. With contemporary mathematics, Neptune clocked in at… 30.11 AU. This was too big of an error for the Germans to handle, and their eponymous law began to crumble. 

It is worth it to mention that two weeks after Neptune’s discovery, astronomers were already finding moons around it. They discovered Triton, a frozen world and one of the largest moons in the Solar System. 

Many more moons would be discovered in the coming years, but where we rejoin the story is at the beginning of the nineteenth century. Up until now, astronomy was done by-and-large in Europe. Americans would soon rectify that. 

Just as astronomers could compare the predictions and observations of Uranus’s orbit to deduce the existence of Neptune, they continued to do the same with Neptune. To their astonishment, there was still a discrepancy between the data and the math! And so the hunt for a possible ninth planet was on. 

In 1894, Percival Lowell, himself a wealthy Bostonian, moved out west to the new frontier away from increasingly blinding city lights to found his own observatory in Flagstaff, Arizona. Shortly afterwards, he started the search for this missing ninth planet, which he called “Planet X.” With the help of the Harvard astronomer William H. Pickering, they had calculated several possible spots on the sky where this planet could be, and so it was left to Lowell and his staff to find it. 

Sadly, Lowell died in 1916, before any planet was found, but his widow fought for the observatory to continue this search to honor his legacy; that fight caused the search to be delayed some time until 1929 when it started anew. 

This time the new observatory director gave the job to twenty-three year old Clyde Tombaugh, a farmer from Illinois with a knack for amateur astronomy. His task was a daunting one: he had to systematically image the night sky in pairs of photos, then examine each pair and determine if any objects had shifted position. 

Like the search for Neptune and the complicated differential equations, this task is easily done by computers nowadays, and so the average astronomer (or lucky graduate student) only has to press “go” and the computer will do the work for him. In the early days of the twentieth century, this task was accomplished using a machine called a “blink comparator.” This device would have slides for two images, and a viewing tube, similar to a microscope. The device would then rapidly light up the images, back and forth, allowing the eye to perceive any small change in the images. 

After nearly a year of looking through this contraption, on February 18, 1930, Tombaugh discovered a possible moving object on pictures he had taken a few weeks before. With more confirmation, the news was telegraphed to the Harvard College Observatory on March 13, 1930. 

This discovery made headlines around the globe. With such an astounding discovery, Lowell Observatory received more than 1,000 suggestions for names, including one from an eleven year old schoolgirl in Oxford, England, Venetia Burney. She suggested the name Pluto for the planet, after the Roman god of the underworld. The astronomers at the observatory voted on the names they had received, and Pluto won every vote. 

It is worth mentioning that the name Pluto was chosen also in part because the first two letters of “Pluto” are the initials of Percival Lowell, honoring once again the man who started the search. The name really started to stick, at least in the hearts of Americans, when Walt Disney named his favorite cartoon mouse’s dog Pluto as well. 

Once Pluto was found, astronomers needed to answer the question that started the search: did its mass account for the observational discrepancies in Neptune’s orbit? At the time of its discovery, its mass was calculated to be about the mass of the Earth. But more and more calculations reduced its mass and size even more, until in 1978 the discovery of its largest moon Charon allowed for a precise determination of its mass: about 0.2% of Earth’s. It turned out that the discrepancy in Neptune’s orbit was simply caused by not knowing Neptune’s mass to an accurate degree. Thus, the mystical Planet X disappeared overnight. 

Pluto was not done being diminished yet! From 1992 onward, many more bodies were discovered orbiting in the same area as Pluto. Astronomers seemed to have fallen into the same trap they had done with Ceres nearly two centuries earlier! Many people began questioning whether Pluto should be considered a planet, including those astronomers at the California Institute of Technology when they announced the discovery of a more massive object in Pluto’s orbital neighborhood: Eris, named for the Greek goddess of chaos and discord. 

And chaos did she cause. 

Pluto may not be a planet anymore, but it still loves you!

In August 2006, the International Astronomical Union officially defined the term “planet,” something that had not been done since the ancient Greeks. Officially, there are three conditions for an object in the Solar System to be considered a planet: 1) the object must orbit the Sun; 2) the object must be massive enough to be spherical; and 3) the object must have cleared the neighborhood around its orbit. Pluto failed the third condition; its mass is much less than the combined mass of the other objects in its orbit. Thus, Pluto, like Ceres, was renamed a dwarf planet. 

Change is always hard. But change had to be made. Famously, Neil deGrasse Tyson, the Director of the Hayden Planetarium in New York City, received tons of angry letters from third graders when his museum exhibit put Pluto with the other Kuiper Belt objects (as they’re now called) and not with the other planets. Personally, I agree with Tyson’s decision. Pluto belongs with its friends, not with the vastly more massive gas and ice giants of the outer solar system. 

And plenty more objects like Pluto have been found! And they have much cooler names than the Greek and Roman gods of old. Just look at Haumea, named for the Hawaiian goddess of childbirth, or Sedna, named for the Inuit goddess of the sea, which is also one of the most distant-known objects in the Solar System! 

That is until we find Planet Nine. You may ask, why do we need another planet? Didn’t we just get rid of one? 

Well, as scientists, we have to go where the evidence points us. And right now, there is some compelling evidence for a massive planet hiding in the furthest reaches of our Solar System. 

The evidence is pretty difficult to explain, but let’s try our best. One of the puzzle pieces can be seen in the orbits of the objects beyond Neptune’s orbit. These objects are (creatively) called trans-Neptunian objects, or TNOs. Those TNOs that are more than 250 AU from the Sun are extreme TNOs, or eTNOs. So many fun acronyms! 

Orbital clustering of eTNOs

These eTNOs all have orbits that appear to be clustered to one side of the solar system, as well as lying in the same plane. You may say, so what? There is only a 0.007% chance that this combination of alignments is coincidence. 

A smaller portion of the eTNOs also orbits high off of the ecliptic plane, the plane that all of the major planets orbit in to some extent. Their orbits take them almost perpendicularly up, away from the Sun. This can be caused by Planet Nine’s gravitational influence on them. 

With these two puzzle pieces, and many more that haven’t been mentioned, astronomers can calculate some of the properties of this planet. It probably has an extremely elliptical orbit, taking between 10,000-20,000 years to orbit the Sun. Planet Nine rests at an average distance of 700 AU, twenty times further from the Sun than Neptune. This would make it extremely cold, but combined with its predicted mass of 10 Earth masses, it would be an ice giant like Uranus and Neptune. 

Where did it come from? It was probably ejected from closer in the solar system in our early history. Perhaps it was captured from a passing star. Could it be a rogue planet, a planet that had been cast from its home star system, wandered the universe for who knows how long, and been caught by our Sun? Or maybe it formed out there and we have much more to learn about solar system formation. All of these are questions we can only have the answer to once we find it and send a probe to it. 

Right now, teams of astronomers are searching for it, primarily through existing images. In fact, many of them are using the same techniques Clyde Tombaugh used to discover Pluto! 

Will we find it? I don’t know, but the evidence is compelling. 

It’s out there somewhere!

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