Planets X and Pluto

I grew up learning about the nine planets. My son is growing up learning about eight planets. In 2006 the International Astronomical Union demoted beloved Pluto from its planetary status.

The demotion of Pluto was my impetus to read Planets X and Pluto by William Graves Hoyt. The book was written in 1981 and is since outdated. But I thought it would interesting to read a story about the discovery of Pluto that was written before the demotion. The book was on my shelf because it was a college textbook of Mrs. Doug. It is very dense, as you would expect from a college textbook. (and terrible to read)

Six of the planets can be seen with the human eye: Mercury, Venus, Earth (since you’re standing on it), Mars, Jupiter and Saturn. Ancient civilizations were able to see these “wandering stars.” Eventually math, science, technology and faith evolved so that the planets were seen for what they are and that Earth was not the center of the universe.


Planets X and Pluto opens with the 18th century discovery of Uranus. It was the first planet discovered with the use of technology, since its not otherwise visible. That discovery opened the door to the idea of there being other planets in our solar system.


After mapping out the distance between the seven planets known at the start of the 19th century, astronomers noticed a big gap between Mars and Jupiter. The theory was that there may be a yet unseen planet in that orbital space. That lead to the discovery of the Ceres and a short time later Pallas. After some time figuring out the size, they realized these two objects were much smaller than the other planets.

The Orbit of Uranus

Using a collection of current observations and a set of much older observations, 19th century astronomers were trying to create a model of the orbit of Uranus. They were having problems getting a model to work. Uranus seemed to be going slower than it should be.

A group of astronomers came up with the theory that there was another planet out there that was pulling on Uranus. At this point the math and technology had evolved to the point that they could calculate where this yet unseen planet should be and where they should look for it. The hunt was on. (At least for the few astronomers who believed the theory.)

Discovery of Neptune

Neptune became the first planet discovered by mathematical prediction. Johan Gottfried Galle used the position data calculated by Urbain LeVerrier. After observing the region of the sky, he saw a “star” moving retrograde. A sure sign that it was not a star, but something else. It turns out to be the eighth planet: Neptune.

Beyond Neptune

Following the discovery of Neptune in 1846, there was speculation that another planet might exist beyond its orbit. The math was not as definitive as that used with Neptune. Percival Lowell was convinced that there was a Planet X beyond Neptune. (Of course he also believed there were Martian-made canals on Mars.) Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have disturbed Uranus enough to account for the irregularities. He died before Pluto was identified.

Discovery of Pluto

Clyde Tombaugh was hired by the Lowell Observatory, funded by Percival’s estate. He discovered Pluto in 1930, apparently validating Lowell’s hypothesis. But the discovery was more because of Tomabugh’s persistence and the meticulous nature of his search. After the announcement, astronomers looked back at their old photographs and at least a half-dozen had overlooked Pluto.

From the beginning, there were disputes about Pluto. Some thought it was a comet. Others thought it was merely an asteroid. Pluto’s orbit was more eccentric than the other planets. There were many attempts to link Lowell’s formulas to the discovery of Pluto lasting for decades after the initial discovery. Any of hope of that was crushed in 1978 when Charon, one of Pluto’s moons, was discovered. Some of the reflectivity and measurements came from more than one body, meaning that Pluto was too small to be having much of difference on the orbit of Uranus or Neptune. Lowell predicted Planet X would have a mass that was six times larger than Earth’s mass.

Foreshadowing of Pluto’s Demotion

Even back in 1980 when Planets X and Pluto was written, Pluto was considered odd. It’s orbit was much more elliptical than the other planets. Enough that Pluto passes within the orbit of Neptune. That lead to speculation that Pluto was a lost moon of Neptune. It’s clear that in 1980, little was known about Pluto.

Planets X and Pluto ends with some unexplained disturbances in the orbit of Uranus and Neptune, leaving the reader with the possibility of there being a Planet X at the far reaches of our solar system.

Beyond the Book

As we now know, Pluto is just one of many Kuiper belt objects orbiting the sun beyond Neptune.

It also turns out that there is another big object out there. Eris was first identified in January 2005 as a trans-Neptunian object in a region of space beyond the Kuiper belt known as the scattered disc. Eris is actually bigger than Pluto. This was one of the new discoveries that led to the demotion of Pluto.

Planets X and Pluto is not very good, even with the understanding that it is dated. There is some interesting material and there is the possibility for an interesting story. It’s just not in this book.

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