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Episode 13: The True Story of Planet X

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Recap: As the first of my four episodes of Intro to 2012, I take a look at the true, historical story behind Planet X and delve a little into why a big planet won't swing by Earth any time soon.

Puzzler: Rather than the normal form of the puzzler, I want to ask a broad question, and I'll read some of your responses on December 16th's episode. The question is, what do you think a planet is, and why? And yes, I do realize that the international astronomical union has a definition of a planet that's out there, but almost no one actually likes it.

Solution to Episode 11's Puzzler: Events creating the moon dust differ across the the moon. Different areas are affected by different events which create, move or destroy the regolith. For example, the various moon 'seas' were created by molten rock that flowed into low lying areas, and any dust there would have to settle after the event. On the other hand, another adjacent area which did not have this happen to it would have older deposits. Also, there would be deeper areas of dust surrounding an impact crater than areas further away. Finally, since there is effectively no weather on the moon, then there is very little action to distribute the dust once it settled.

Q&A: There were no questions submitted for Q&A.

Additional Materials:

Transcript of the Main Material:

What's in a Name?

Today, the term "Planet X" is often used to represent something dark, mysterious, and potentially capable of jumping out at us in the night and raining destruction down upon our pale blue dot. Many familiar with the skeptical community will often dismiss such ideas as rubbish, and with it the notion of a Planet X.

In the astronomical community, the term "Planet X" has had a sordid past and most at all. But, others still use it as a term to describe any as-yet undiscovered planet-sized object that is "somewhere out there" in the solar system, waiting to be discovered.

Is there a Planet X still out there awaiting discovery? It's possible. Some may even say probable, and others are using it as a method to explain some of the far-away solar system structure (structure well beyond the orbit of Pluto). Is there a Planet X that is going to suddenly show up at Earth and do bad things to our planet in 2012? No. But before we get into that, we're going to take a short stroll through history.

Finding Uranus

Until 1781, the solar system was known to consist of Earth, Venus, Mercury, Sun, Mars, Jupiter, and Saturn, along with the moon, some other moons, and some unexplainable and unpredictable comets. That was it, and it wasn't until William Herschel observed a ball-like object that he described as non-star-like and moving among the fixed background stars:

"I know that the diameters of the fixed stars are not ... magnified with higher powers, as planets are; therefore I now put [higher powers on] and found that the diameter of the comet increased in proportion to the power, as it ought to ..."

In writing to the British Astronomer Royal at the time, Nevil Maskelyne, he received a response:

"I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis."

Discovering a planet at the time was unheard of -- the last planet that had been discovered was known as long as there had been written history, and William Herschel would have become the first person to discover one in modernity. It took two years for Herschel to admit that he had really discovered the first planet in recorded history.

English scientist Isaac Newton had codified calculus over a century earlier, and Dutch astronomer and mathematician Johannas Kepler had developed his three Laws of Planetary Motion in the last century, as well. By using these, astronomers and mathematicians were able to use observations of the location of Herschel's new planet, named Uranus, and predict its orbit based on its distance from the sun and the gravitational interactions with other planets.

The First Planet X

One of these people was Alexis Bouvard, who published tables of dates and coordinates that predicted where Uranus should be at a given time. These were based on the known laws of physics. But, Uranus refused to follow Bouvard's tables and behave.

In 1843, Englishman John Couch Adams calculated the orbit of a hypothesized eighth planet that could account for Uranus' odd orbit. But, unfortunately for Adams, no one really seemed to care about this undiscovered Planet X.

Two years later, a Frenchman by the name of Urbain Le Verrier did the same thing, but more precisely than Adams. Again, no one seemed to care. That was until Le Verrier sent his calculations to the Berlin Observatory's astronomer Johann Gottfried Galle. A then-student at the observatory, Heinrich d'Arrest, convinced Galle to look for it.

That evening, September 23, 1846, Galle looked for this mysterious planet, potentially responsible for Uranus' weird orbit, and he found the planet within 1° of where Le Verrier had calculated it should be (for reference, the moon on the sky is 0.5°). This was within 12° of where Adams thought it should be.

At the time, there was no real debate that this object was a "planet," as they had been looking for it and thought it was massive enough to account for Uranus' orbit.

The Second Planet X

However, there were still some unexplained perturbations of Uranus' orbit. These persisted for 70 years, to the time that wealthy Bostonian Percival Lowell became interested in the problem and wanted to search for a now possible ninth planet at the observatory he had built in Flagstaff, Arizona. I think that he was the one who really first coined the term, "Planet X," in describing this mysterious and unseen object that, nonetheless, was thought to account for the perturbations on Uranus. Lowell searched for 12 years, 1905-1916, until he died, without finding it.

The search resumed in 1929 when the then-director of the observatory assigned the task to a young, 23-year-old Clyde Tombaugh. After a year of fruitless searching, Toubaugh found an object moving against the background of stars from two photographs he had taken in January of 1930. Pluto was discovered, Planet X, that was supposed to solve all the orbital problems.

The Third Planet X? -- No, Just Fixing Neptune's Mass

When Pluto was initially discovered, it was assumed to weigh in at several times Earth's mass. However, estimates over subsequent decades were refined down, not up, and it was realized that Pluto could not account for Uranus' orbit. The present-day mass estimate is about 20% Earth's, and this will be no doubt revised again when the New Horizons spacecraft passes by Pluto in 2015.

The search for a Planet X to explain Uranus half-heartedly was put to rest at the time.

In 1989, the space probe Voyager 2 flew by Neptune. Calculations based on the orbital changes from that gravitational interaction were published in 1993 by Myles Standish, and they revised Neptune's mass downward by 0.5%. This revised mass, when put into the calculations for the orbits of the outer planets, was then able to precisely account for Uranus' orbit. No mystery object was needed, nor found, and as a result, nearly all astronomers today discount its existence.

Modern-Day "Planet Xs" (The "Real" Ones)

To be sure, I do not mean to imply that there are no more large objects out in the solar system. But "large" is always a relative term that needs to be quantified. The proton is gigantic relative to an electron. A sequoia tree is large relative to an oak. And Neptune is large relative to Pluto.

What I mean by "large" in this context is 100s to possibly 1000s of kilometers in diameter, icy bodies much like Pluto. These are the Kuiper Belt Objects, or at least the large members of the Kuiper Belt Objects. To-date, (November 2011), four are large enough such that the International Astronomical Union has termed them "Dwarf Planets" (Pluto, Eris, Makemake, Haumeamea). These objects are "large," but they are smaller than our moon (our moon is 1740 km in diameter). And, since density is related to volume which is the cube of a linear measurement, the actual mass of these objects is much smaller than that of a planet.

For there to still be these "large" objects that are undiscovered, they would have to be very far away from the 8 planets and inner Kuiper Belt Objects. Remember, even with the technology over 160 years ago, astronomers were able to calculate that Uranus, an object 19 times farther away from the sun than Earth was being very slightly perturbed by an object 30 times farther away from the sun than Earth. And these were both objects that weigh about a dozen times more than Earth – fairly small compared with what modern-day Planet Xers are claiming.

Nothing in science is locked in stone, so-to-speak, and it is fairly impossible to prove a negative. However, keep in mind now that we can explain all the orbits of the planets with known, observed solar system objects. For there to be another object out there, it either has to be very small, or it has to be very far away. And when things are very far away, they take a very long time to move. Even a comet out by Jupiter heading towards us would take at least a year to get to Earth. And we could see it. The idea that there is a massive, planet-sized object that will hit or pass by Earth in just 1 year is ridiculous, unless you invoke the supernatural or physics that we don't know about that can somehow shield even gravity.

Moving Forward

This isn't the final word on this podcast about Planet X. I'll also talk about the supposed 3600-year-orbit and other things, like the dynamical stability of the asteroid belt. But those are topics for future episodes over the next year.

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