Planet X

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Podcast Transcript

Ever since the discovery of the planet Neptune in 1846, astronomers have noticed that something was not right. The orbit of Neptune was being gravitationally influenced by some other, unknown body that was dubbed Planet X.

In 1930, it was thought that this body had been discovered with the discovery of Pluto, but that couldn’t have been the object that was influencing Neptune because it was too small. 

The search for this mysterious object has continued to this day, and some astronomers think we are finally close to finding it.

Learn more about the hunt for Planet X, aka Planet 9, on this episode of Everything Everywhere Daily.


Planet X sounds like something from a 1950s science fiction novel, but it is not. This episode is not about space aliens from Planet X.


Planet X is a very real thing, or to be more accurate, it is a very real hypothesis. Many top astronomers have been hunting for Planet X for decades. 

The story of Planet X began in the 18th century with the discovery of Uranus. 

If you remember back to my episode on it, Uranus was discovered in 1781 by William Herschel. Finding a planet beyond those known since antiquity was a huge discovery. 

By the time of the discovery of Uranus, the basics of orbits had been firmly established. Once the discovery was announced, other observations were made, and there was a problem. 

The orbital path of Uranus wasn’t quite what it was supposed to be. It didn’t follow the orbit predicted by Newton’s Laws of Motion, which suggested there was some other planet which was acting on it. 

In 1846, French mathematician Urbain Le Verrier calculated the position of this unseen planet using the discrepancies in Uranus’s orbit. Independently, and almost simultaneously, British mathematician John Couch Adams made similar calculations. 

Based on Le Verrier’s predictions, German astronomer Johann Gottfried Galle observed Neptune for the first time on September 23, 1846, just 1° from where Le Verrier had predicted it to be.

The prediction of Neptune before it was discovered was a triumph of mathematics. 

However, there was still a problem. The orbit of Uranus still couldn’t be totally explained by Neptune, and there were deviations in the orbit of Neptune that couldn’t be explained. 

That implied that there was something else that was still out there that had to account for the orbital discrepancies. 

So, astronomers once again set out to find whatever it was that was that was influencing the orbit of Neptune. 

One of the leaders in the hunt was a Boston businessman named Perceval Lovell. He dubbed this mystery planet Planet X. He established an observatory in Arizoan, named the Lowell Observatory, which among its many projects, focused on the discovery of Planet X.

After years of searching for something, in 1930 they found…..something. Astronomer Clyde Tombaugh, after comparing photographic plates, found something moving. That moving object in the distant solar system was dubbed Pluto. 

The initial assumption was that they had discovered Planet X. However, as more was learned about Pluto, there was a problem 

Pluto was far to small to account the orbital discrepancies in Neptune. It was so small it couldn’t even be observed as a disk. I twas just a point of light. 

From the moment it was discovered the estimates of the mass of Pluto was continuously estimated downward as we learned more about it. It was initially thought to be the mass of Earth and now it thought to be only about one fifth of one percent the mass of Earth. 

The Planet X theory fell out of favor in the 1970s and 80s, especially after the Voyager 2 spacecraft was able to make a more accurate  measurement of the mass of Neptune. 

However, the idea of Planet X came back in vogue in the early 2000s with the discovery of Sedna. 

Sedna doesn’t get a lot of attention, but it was an important discovery. Sedna was a large object beyond the orbit of Pluto. It wasn’t as large as Pluto, but it really brought home the issue of what is a planet that eventually led to the demotion of Pluto as a planet. 

It also brought back the idea of Planet X because of Sedna’s extremely eccentric orbit. Pluto had a very eccentric orbit, but it is nothing like Sedna. 

At its closest point the sun, Sedna is 76 astronomical units from the sun. An astronomical unit is the average distance from the Earth to the Sun. 

At its furthest point, however, Sedan is 937 astronomical units from the sun. 

Sedna wasn’t the only transneptunian body that was discovered that had such an exaggerated orbit. In 2012 another body was discovered, known as 2012 VP113. In 2015 an object known as Lele?k?honua was discover that has an even more elongated orbit than Senda. 

At its closest point, Lele?k?honua is 65 astronomical units from the sun, and at its furthest point, it is 2126 astronomical units from the sun.

The problem is that these orbits make absolutely no sense. They can’t be explained by the gravitational pull of the major known planets in the solar system. 

Likewise, it couldn’t be galactic tides. These are gravitational pulls that come from the gravitational fields of a galaxy. 

If they had been formed in their current location, they would have had a circular orbit like the planets. This is because of how accretion works when smaller objects form into larger ones. If there were massive differences in velocity or direction, then they would never have formed. 

That means that something else must have caused the extreme orbits of these bodies. 

There have been several theories put forward as to what might have caused this. 

The first theory is that early in the birth of the solar system, the sun may have been born in what is known as a star nursery. During this period, another star may have come close to the sun, relatively speaking, and that would have been enough to influence the orbits of these distant objects.

The second theory is that Sedna and other similar objects didn’t even originate around the sun. During the formation of the sun, the objects may have been captured by the sun, having been jettisoned from some other solar system.

The other theory, and the one that had gained the most support, is that there is a distant massive object in the solar system  that is responsible for causing their highly elongated orbits. 

Basically, a return to the Planet X hypothesis that existed back in the late 19th and early 20th centuries. 

There is more to it than the fact that these objects have highly elliptical orbits. 

Their orbits also tend to be clustered. Instead of each object clearing out a space, you see objects clumped together. 

Finally, most of these objects have highly inclined orbits. The eight planets are all basically in what is known as the ecliptic plane. These extreme transneputnian objects go well outside of the ecliptic plane at steep angles to the rest of the planets. 

The Planet X hypothesis has gained traction because it explains all of the things about the orbits of these objects that I’ve just listed. 

The quest to find Planet X has been a much more advanced version of the discovery of Neptune. Neptune was discovered with mathematics before it was ever observed with a telescope. 

As more transneptunian objects are discovered and their orbits calculated, its possible to use computers to calculate data about possible orbits for Planet X. 

Computer simulations have been able to recreate similar results to what we see today with a heavy planet in a very distant orbit. 

As a result, many astronomers believe they are narrowing down possible locations where Planet X might be found….if it exists. 

So what would Planet X look like if it does indeed exist? 

It would probably be something larger than Earth, but smaller than Neptune. The current best guess is that it would be 6.2x the mass of Earth. 

It would have an orbit that would be slightly elliptical. More elliptical than the planets, but much less so than Sedna and other similar objects. Likewise, it would be about 11 to 20 degrees inclined to the ecliptic plane. Again, more than the planets, but much less than other objects at that distance.

Moreover, its orbit would take it anywhere from 240 to 500 astronomical units from the sun. 

So, if Planet X can explain many of the questions regarding Sedna and other transnepunian objects, and if we can compute its possible orbit, and even its mass, then the big question is, where is it?

The problem with trying to find a planet that far away is that it is almost impossible to see. 

Planets, unlike stars, do not produce light. They can only reflect light. 

Planets that are closer to the sun reflect more light. That is why Venus looks so bright in the sky, almost as if it were a star. 

However, once you are beyond the orbit of Neptune, it is hard get enough light. From the distance that Planet X would have to be it would be extremely dim. In fact, the sun would appear as nothing other than brightest star in the sky.

With so little light reaching the planet and much less light being reflected back. Planet X would be extremely difficult to observe. It would be difficult to detect amongst all of the background stars. 

It would require viewing photographic images of the same point in the sky at different times and seeing if anything had moved. Even if we could identify it, it probably would appear as nothing more than a point of light, or maybe a small disc with no details, from even the most powerful telescopes. 

The good news that astronomers think that they are getting close to finding Planet X, or at least, they are narrowing down where to look for it. However, in all fairness, there are also still astronomers who are skeptical and they don’t think that there is a Planet X. 

So, what if we find Planet X? With the recent controversy surrounding the demotion of Pluto as a planet, would Planet X be considered to be an actual planet?

Well, it would depend on what we can find out about it. If it is more massive than Earth, then it probably would be considered to be a planet. The reason why Pluto was demoted is because it wasn’t massive enough to clear out its orbit of other debris. 

The only reason why it might not be considered a planet despite its large mass would be because of its very long orbit. 

The hunt for Planet X has been going on for decades now. While there is optimism in some circles that the planet will be found, that optimism is not universal. Nonetheless, one day, you might wake up to find that there are once again nine planets in the solar system.