The mystery of Planet X

Urbain Le Verrier

In 1845, some 60 years after the discovery of planet Uranus, the French astronomer Urbain Le Verrier tried to solve the problem posed by the discrepancies of a few minutes of arc between the observed orbit of this planet and the predictions made by applying Newton’s theory. Le Verrier thought the problem could be solved if there were another unknown planet beyond Uranus. On September 23, 1846, the German astronomer Galle discovered this new planet, which received the name of Neptune. The success of the prediction became top-notch scientific news and boosted Newton's theory of gravitation.

For decades, some unexplained irregularities in the orbits of Uranus and Neptune were attributed to the existence of another possible planet, located even further from the sun. In 1906, Percival Lowell undertook, at his private observatory in Falstaff, Arizona, a search program for the so-called Planet X, not because of its number in the list of planets (which would be 9), but because the letter X traditionally represents the unknown in a mathematical expression. In 1930, after Lowell's death, Clyde Tombaugh discovered Pluto, which came to be considered planet number 9, but as its small mass was insufficient to explain the discrepancies, the X in the name of planet X automatically came also to mean number 10, in accordance with the meaning of that letter in the Roman numeral system.

In 1987, the search for planet X was still unsuccessful. From the analysis of the orbits of Pioneer X and XI, it was deduced that none of those space capsules had been subject to the influence of the mysterious planet X, so if this planet existed, it must be in a very elliptical orbit, inclined at least by 30º with respect to the plane of the trajectory of the two space capsules.

In 1992, it was proposed that planet X might not be a planet, but a swarm of bodies the size of Pluto (1,000 at least). In 1999, the analysis of the orbits of some comets gave rise to the proposal of the existence of a planet the size of Jupiter (or even larger) 25,000 astronomical units from the sun. An astronomical unit (1 AU) is the distance from Earth to the sun. This possibility was later rejected when NASA's WISE satellite ruled out the existence of an unknown body the size of Saturn less than half a light-year from the sun (about 30,000 AU). Nor can there be any Jupiter-sized body less than 1.5 light-years away (90,000 AU).

In 2001, the analysis of the orbit of Comet 2000 CR105 gave rise to the proposal of the existence of an unknown planet, intermediate in size between Mars and Earth, at the distance of the Kuiper Belt, an accumulation of objects beyond Neptune, between 30 and 55 astronomical units of the sun.

Pluto

On August 24, 2006, the International Astronomical Union decided that Pluto would no longer be considered a planet, moving it to the category of dwarf planet or plutoid, together with other bodies, such as Ceres, the largest of the asteroids, and Eris, more massive than Pluto and located farther away. With this change, automatically, planet X became known again with the alternative name of planet 9, and the double meaning of the letter X was no longer applicable.

Starting in 2016, the analysis of the orbits of several recently discovered bodies in the Kuiper belt and some simulations, suggested the idea that there could be an unknown planet up to 10 times larger than Earth, or another body of equivalent mass, at 500 or 600 AU from the sun. In comparison, Neptune's distance from the sun is less than 30 astronomical units, and its mass about 17 times that of Earth.

From Investigación y Ciencia, April 2016

The latest theory suggests that planet 9 might not be a planet, but a primordial black hole about 10 times more massive than Earth, located about 500 AU from the sun, which would have been formed shortly after the Big Bang, and which would be much more difficult to detect than a planet. However, there are proposals to locate its exact position, such as one, made in 2020, which would consist of sending a fleet of spacecraft weighing about 100 grams each, accelerated by laser beams to a speed of 300 km/second, with which they could reach that distance in about 10 years. They would send radio pulses that would make it possible to detect if the small probes had been subjected, somewhere in their trajectory, to the attraction of an unknown body. Edward Witten, one of the authors of the proposal, is skeptical and says that it is far from clear that this approach is practical, while other researchers, such as Mike Brown, assert that there is zero reason to think that Planet Nine is a black hole.

Meanwhile, there are other options. A theory dating back to 1984 argues that the sun could be part of a binary star system, and that its companion (nicknamed Nemesis, the Greek goddess of revenge) would have to move through an extremely elongated orbit that would take it to a maximum distance of the sun of 88,000 astronomical units (more than a light-year). When the star, which could be a brown dwarf (which would make it very difficult to locate), approached the sun once every 26 million years, its influence on the Oort comet cloud would cause many of them to rush over the inner solar system. Their impacts with the Earth would cause massive extinctions, similar to that at the end of the Cretaceous period that put an end to dinosaurs. This theory lost weight when it was found that the 26 million-year period of mass extinctions could, after all, be a statistical artifact.

The same post in Spanish

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Manuel Alfonseca