Planet Nine

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Podcast Transcript

The planet Neptune was discovered in 1846. 

Ever since then, astronomers have felt there had to be another planet beyond the orbit of Neptune.

Everyone assumed they found it with the discovery of Pluto in 1930, however, something still wasn’t quite right. 

90 years later, the mystery planet hasn’t been found, but astronomers might be getting close. 

Learn more about Planet Nine, sometimes known as Planet X, on this episode of Everything Everywhere Daily.


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Before we get too far into this discussion, let me address what many of you are probably thinking. Didn’t we already have a ninth planet? Pluto was a perfectly fine planet and then they demoted it. Shouldn’t we be talking about a tenth planet??

If you remember way back, I previously did an episode where I went through the entire Pluto saga and the reasons behind its demotion from ‘planet’ to “trans-Neptunian object”. 

Pluto, ever since its discovery has always been the odd planet out. Its orbit, its mass, and everything about it was different 

The problem with Pluto being a planet is that we began discovering a whole lot of things beyond the orbit of Pluto which looks a lot more like Pluto than anything else. 

We went through the same thing in the 19th century with Ceres. Ceres is the largest object in the asteroid belt and it was originally considered a planet. However, we then began finding a whole lot of other stuff in that region that looked more like Ceres than anything else, and it too was demoted as a planet.

So, for the purposes of this episode, Pluto is not the ninth planet. If it were, we’d have at least 12 or 13 planets.

With that being said, this story starts in the 18th century with the discovery of the planet Uranus, excuse me, URanus. 

Uranus was discovered in 1781 by William Herschel the old-fashioned way of someone looking up at the night sky via a telescope. 

Once its existence was known, its orbit was calculated and something was found to be off. Its orbit was being affected by some larger object beyond its orbit. 

This lead to the eventual discovery of Neptune in 1846. It wasn’t discovered the old-fashioned way. It was predicted based on the orbital eccentricity of Uranus, and then the prediction was confirmed via observation. 

However, something still wasn’t quite right. Neptune’s orbit it seemed was also being influenced by some other body beyond its orbit. 

This planet was dubbed Planet X. 

As with Neptune, this lead to another search for a planet which eventually led to the discovery of Pluto back in 1930. Planet X had been found and champagne could flow in celebration. 

However, there was another problem. It was determined in 1978 that Pluto simply wasn’t big enough to cause the gravitational influence observed on Neptune.

Pluto couldn’t be Planet X. 

The Planet X hypothesis fell out of favor after Voyager 2 conducted a fly-by of Neptune in 1989. It corrected the estimated mass of Neptune which evaporated the belief in Planet X. 

However, that wasn’t the end of Planet X. 

Starting in the early 90s, there was a string of discoveries of objects beyond the orbit of Pluto. These objects were really far away and to date, thousands of them have been discovered. 

Here, I’m going to take a brief digression here to explain the concept of an Astronomical Unit.

An Astronomical Unit, or AU, is simply the average distance from the Earth to the Sun. So, the Earth’s distance from the sun, on average, is defined to be 1 AU.

The AU is the standard measurement for expressing distances within the Solar System, where a lightyear wouldn’t really make sense, and kilometers would be too small. 

Mars is about 1.5 AU from the sun. Jupiter is about 5.2 AU, Neptune is 30 AU, and Pluto averages about 40 AU. 

In 2003, a trans-Neptunian object named Sedna was discovered. Sedna has a highly elliptical orbit that varies from 76 AU to 937 AU.  At its closest, it is twice as far from the sun as Pluto, and at its furthest point, it is 5 light days away. 

The time it takes to orbit the sun is 11,390 years. So, when it was last where it was now, the most recent ice age on Earth was ending. 

It was the discovery of Sedna that revived the Planet X hypothesis because something wasn’t right with its orbit. Something was influencing it, and that something had to be reasonably big. It was so far away, that it couldn’t be any of the known planets, and none of the discovered trans-Neptunian objects were big enough to affect its orbit.

It wasn’t just Sedna. Many of the other trans-Neptunian objects that were found seemed to have orbits that were influenced by something big, very far away. 

There was a lot more evidence piling up. 

In 2016, California Institute of Technology astronomers Mike Brown and Konstantin Batygin announced that many newly discovered trans-Neptunian objects had eerily similar orbits on the same side of the solar system. They also noted that the odds of such a thing happing by chance was only 1 in 14,000 and that the odds were good that something very massive and distant was nudging these bodies into these orbits. 

They theorized that this was the result of a large rocky planet five to ten times the mass of the Earth which orbited at a distance of 400 to 800 AU, or ten to twenty times the distance from Pluto from the Sun. 

This hypothetical planet would meet the new definition of a planet and would be the 9th planet.

Some astronomers think that there might be two major planets that are influencing the orbits of these objects.  This theory holds that there would be a 10th planet located about 1500 AU away which would be approximately the mass of Neptune. 

The existence of such a large planet or planets so far away would explain much of what we know about the orbits of trans-Neptunian objects. 

Over the last decade, the evidence supporting Planet Nine has grown along with the number of astronomers who think that such a planet is plausible. 

So, if we are seeing the evidence of it, where is it? 

Well, that is a very difficult thing to prove. Just because we might be seeing evidence of it in the orbits of other objects, that doesn’t mean we know where it is right now. 

Even if we did know where it was, observing it would be very difficult. The further away you get from the sun, the less light there is to be reflected back. Once you get to hundreds of astronomical units away, the amount of light hitting the planet would be incredibly small, and the amount reflected back to Earth would be only a tiny fraction of that. 

Moreover, if it is a rocky body like Earth or Mars, it wouldn’t reflect as much light as ice-covered bodies, which is what most of the trans-Neptunian objects seem to be. 

Finally, even if we did see it, we wouldn’t necessarily know that we are looking at it. At that distance, the planets move very slow, so the only way you can really tell if it is a planet is by looking at images taken over significant periods of time. 

For example, it is entirely possible that some ancient astronomers actually saw Uranus, and Gallielo might have seen Neptune through his telescope. However, they had no clue they were planets because they just looked like stars and moved so slow.

It is more difficult to find such a body in the distant reaches of our own solar system than it is to detect large planets orbiting other stars. The search for Planet Nine is pushing the limits of our ground-based astronomy.

If there is a Planet Nine, how did it get there? Speculation on the origin of a planet that hasn’t been discovered is difficult, but one theory is that a rocky mass interacted with Jupiter during the formation of the solar system which threw it very far away.

Another theory is that it might be a rogue planet that entered from outside the solar system and got caught into the sun’s orbit….but the odds of this are very slim.

I should note, that while there is growing evidence of Planet Nine, acceptance of this hypothesis is far from universal. 

Either way, the only way to resolve the issue is to actually find it. This is a pretty big priority in the world of astronomy right now. One new tool which might help in the search is the Vera Rubin Observatory, which is being built in Chile.

This will be a wide-angle telescope that will photograph the entire sky every few days. Its mission is to look for anything which is moving amongst the stars. It will collect an enormous amount of data over 10 years which should make huge strides in cataloging everything which is in our solar system. 

Until astronomers can actually find it and photograph it, we will have to keep looking for the shadows and echoes of a planet that may or may not even exist. 


The associate producers of Everything Everywhere Daily are Thor Thomsen and Peter Bennett. 

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