In the early 1970s, mathematicians in the Computation Division of the Lawrence Livermore National Laboratory posited the existence of a massive planet far beyond the orbit of Pluto, then considered to be the outermost known planet in the solar system. After years of research and computational calculations aimed at retracing and predicting the next appearance of Halley’s Comet, Livermore’s Joseph Brady and Edna Vienop theorized that the gravitational pull of a 10th planet, dubbed Planet X, three times as massive as Saturn and orbiting the Sun at twice the distance from Neptune, as the causal factor for the perturbations, or deviations, in the comet’s orbit that had eluded explanation over the years. While Joseph Brady and his team published their research in 1972, the story and the search for Planet X are ones that began over a century ago.
In the 1800s, perturbations in the orbit of the planet Uranus led many in the astronomical community to theorize that another outer planet must be exerting its gravitational influence. For many, the discovery of Neptune in 1846 provided a resolution to the mystery, but others argued that one planet alone could not possibly account for all the irregularities in Uranus’s orbit and pointed to the possible existence of an unseen ninth planet. Among the most vocal proponents for a trans-Neptunian planet was that of Percival Lowell — a wealthy Harvard graduate, mathematician and astronomer.
In the 1890s, Percival Lowell was popularly, and infamously, known as an advocate for the idea that Mars was crisscrossed with an elaborate network of canals constructed by an intelligent alien race. In 1894, to prove his belief, he founded and began construction on the Lowell Observatory in Flagstaff, Arizona. For some 15 years, Lowell extensively studied and published on his observations of Mars, stirring excitement in the public, though derision and ostracization in the academic community.
In 1906, Lowell shifted focus to a more scientifically credible theory and began an extensive search for a proposed trans-Neptunian planet that he called Planet X. The “X” in this case was not the Roman numeral ten but rather the letter, representing an unknown. He believed that perturbations in the orbits of both Uranus and Neptune were caused by the gravity of this unseen planet. Lowell, however, died suddenly in 1916, and the search for Planet X was temporarily halted. However, in 1929, a young, high school-educated amateur astronomer named Clyde Tombaugh came to the observatory and after just over a year of observations, managed to succeed where others more erudite than he had failed.
On March 13, 1930, the Lowell Observatory announced Tombaugh’s find: the discovery of Pluto. The date of the announcement was specifically chosen, marking not only the discovery of Uranus in 1781 but Percival Lowell’s birthday as well. For those at the observatory, the discovery of Pluto was a vindication of Lowell’s work. The newly found planet’s position was so closely aligned with Lowell’s calculations that Tombaugh’s discovery was clear: Lowell’s Planet X had been revealed.
While the announcement was greeted with enthusiasm by both professionals and the public, many remained skeptical, and questions about Pluto’s status as a planet were raised almost immediately. The main issue was that Pluto showed no visible disk. It appeared as a point, six times dimmer than Lowell’s predictions, meaning it was either very small or very dark. For nearly 50 years, without a way to measure the mass of Pluto, there was no way to prove or disprove it as Lowell’s Planet X. However, in 1978, a moon orbiting Pluto was discovered, providing an unambiguous measurement of its mass: Pluto was not Planet X.
Meanwhile, some 30 years after Tombaugh’s discovery, mathematicians at Livermore, working a seemingly unrelated celestial mystery, were attempting to develop a code that could retrace and predict the next appearance of Halley’s Comet. The famous comet, named for Edmund Halley, who in 1705 applied Newton’s new gravitational theory to predict its appearance, was due to return in 1986 after its periodic 76-year absence. However, perturbations in the comet’s orbit had caused prior astronomical predictions in the comet’s appearance to be off as much as four days.
The comet project, conducted by Joseph Brady and Edna Vienop, was one of several studies in basic science authorized by the Atomic Energy Commission for “nominal use” of Livermore’s computers. The Lab’s facilities were uniquely suited to this kind of complex calculation, and the Livermore-developed code being used in the study, the THEMIS code, had been utilized somewhat successfully a few years prior to track the world’s first artificial satellite, Sputnik I.
To refine the code to compute the precise return of the comet, that is when the comet was closest to the Sun, Livermore mathematicians began by collecting massive amounts of data, compiling some 5,000 celestial observations for the years 1682-1910, from the Naval Observatory, the Library of Congress and the Harvard College Library. However, in running the code with the new data, Brady and Vienop could only generate a reasonable fit for just two of the comet’s appearances, with a third appearance always off by approximately three days.
To get a better fit for the prediction, Brady and Vienop decided to employ an empirical term in their equation of motion for the comet to make their equation agree with the prior observations. According to Brady, “when I integrated Halley’s Comet again using this empirical term, I was able to fit not only the last four apparitions, but all of them reasonably well.” Using their equation, Brady and Vienop were able to calculate forward in detail where the comet would be in the sky from 1982 to 1990, concluding that Halley’s Comet would be closest to the Sun on Feb. 9, 1986. Their prediction, years later, turned out to be correct within a few seconds of arc.
As to what the empirical term was, Brady and Vienop later posited that it might represent the gravitational influence of an undiscovered planet: the so called “Planet X.” Their name was a nod to Lowell’s work; but, in this case, the “X” did not represent an unknown, but rather the Roman numeral designation for a possible 10th planet. Brady believed that his calculations for a 10th planet located on the edge of the Milky Way resolved the perturbations in the orbit of Halley’s Comet, as well as those of two other periodic comets, Olbers and Pons-Brooks. According to Brady, his Planet X was predicted to be three times as massive as Saturn, inclined at an improbably large angle, and orbiting the Sun some 5.5 billion miles away and backwards with respect to the other planets.
Ironically, the then-executive director of the Lowell Observatory, Henry L. Giclas, along with P.K. Seidelmann of the U.S. Naval Observatory, immediately dismissed Brady’s and Vienop’s work as a “myth.” According to Giclas, “we have concluded there is no planet having the mass, magnitude, mean distance and orbital inclination hypothesized by Brady. If such a planet did indeed exist, it would have showed up on photographic plates….”
According to an interview with Brady in 1994, subsequent experimental work was done at the Lick Observatory and the Royal Observatory at Herstmonceux but proved fruitless. “I knew there were two possible positions for Planet X…. One of these positions had been searched for Pluto, very carefully, back in the ‘30s when Pluto was discovered. But the other position hadn’t been searched.” Both observatories “searched down to the 13th magnitude, and couldn’t find it. Planet X was predicted to be more massive than Saturn, so it should have been quite bright and easy to find. It's massive. It’s between Jupiter and Saturn in mass. So, they searched down to the 13th magnitude. Well, that’s plenty.”
While the subsequent consensus among most astronomers was against the existence of Lowell’s Planet X, some have recently revived the idea of a large unseen planet influencing observable gravitational effects in the outer Solar System. In 2016, two astronomers, Michael Brown and Konstantin Batygin, proposed the idea of a super-Earth (Planet Nine) as a possible explanation for the peculiar orbits of extreme trans-Neptunian objects (ETNOs) in the Kuiper Belt. Their models suggest a body with a mass 10 times that of Earth and a predicted orbit 20 times farther from our Sun than Neptune.
With the Vera C. Rubin Observatory in Chile set to go online in 2025, a definitive answer to the existence of Planet X or Planet Nine may be finally resolved. For now, the debate over and the search for the hypothetical planet continues …
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