Planet X, also known as Nibiru, Marduk, Draco or Plutinos, was introduced to the world over 30 years ago by Zecharia Sitchin. He claimed that this planet has an orbit that takes it to the outer reaches of our solar system returning to the inner planets, including earth, every 3,600 years. He further claimed, based on his interpretation of Sumerian texts, that a superior civilisation developed on Planet X and that on one of their periodic visits near Earth they landed here and created Man becoming his gods. This outlandish idea was taken up by others and embellished further, sometimes changing the periodicity of the planet, claiming that various catastrophic events such as the Flood of Noah, the Plagues of Egypt and the destruction of Atlantis were caused by a close encounter with Planet X and promising(a) further devastation when it returns in 2012. Books purporting to be guides for surviving this event were widely promoted. It would seem that the use of fear is an effective way of selling books.
Feeble attempts were made to give this daft idea a scientific basis, such as when it was suggested that anomalies in the orbits of Uranus and Neptune could only be explained by the gravitational pull of Planet X. However, in 1993 it was found that these ‘anomalies’ were the result of orbital computation errors. Furthermore, the idea that this ‘12th planet’ harboured life similar to our own is even greater nonsense. Life on earth would cease if we were deprived of the sun’s heat and light for a short period, so to suggest that a planet could wander off into the outer reaches of the solar system for thousands of years and sustain life as we know it, is as stupid an idea as that of a flat earth.
The idea of an extraterrestrial body returning periodically to threaten our planet did not begin with Sitchin. Two physicists, Daniel Whitmire and Albert Jackson proposed the existence of a ‘Planet X’ in the 1980s to explain what they perceived to be evidence for recurring mass extinctions every 26 million years. The development of this idea is told in Nemesis  by another physicist, Dr Richard Muller.
Nevertheless, early January 2016 saw a claim(c) that a large planet probably exists in the outer reaches of our solar system, as it is the most likely explanation for unexpected movements by objects in the Kuiper Belt. The search for Planet Nine seems to have intensified as astronomers suggested(g) that the unusual ’tilt’ of our sun may be caused by this very large planet! New Scientist of 30th January 2016, explores the possible existence of this ‘Planet Nine’(f). The difficulty in locating such a planet may be explained by a snippet (p.7) in the same magazine which refers to an exoplanet with an orbit one trillion kilometres from its star.
In the same month, Daniel Whitmire, a retired astrophysics professor, “published findings in the January issue of Monthly Notices of the Royal Astronomical Society that the as yet undiscovered “Planet X” triggers comet showers linked to mass extinctions on Earth at intervals of approximately 27 million years.”(d) This was not a new claim from him as he “and his colleague, John Matese, first published research on the connection between Planet X and mass extinctions in the journal Nature in 1985 while working as astrophysicists at the University of Louisiana at Lafayette. Their work was featured in a 1985 Time magazine cover story titled, “Did Comets Kill the Dinosaurs? A Bold New Theory About Mass Extinctions.”
In 1982, J. J. Sepkoski & David Raup published a paper in which they claimed that the extinction of dinosaurs 66 mya was part of a cycle of mass extinctions that may have occurred every 26 million years. However, they did not offer a possible cause for these periodic events. Clark R. Chapman & David Morrison published a book, Cosmic Catastrophes  with a comparable theme. It was Richard Muller’s Nemesis, which contained the idea that a possible companion star to our sun. a brown dwarf, was the culprit. His theory held good until 2014, when an infrared survey found no brown dwarf up to 10,000 astronomical units (0.16 ly) from Sun.
Another writer has added to Sitchin’s Anunaki invention with embellishments that leave you wondering whether to laugh or cry(j). The author also managed to link the Anunnaki silliness with Atlantis with the following addition;
“In the heyday of Atlantis, there was space travel for the privileged classes. Teleportation was also common in those days for the privileged classes. As the Atlantean “slaves” became more and more advanced, they began to disobey their “gods”, the Anunnaki Elite. This concerned their masters. Thus a decision was made by them to destroy Atlantis.”
An extensive review(e) of Sitchin’s theory and astronomical data that seems to support the existence of another planet in the outer reaches of our solar system, concluded that “Quite simply, Zecharia Sitchin’s Nibiru, does not exist…”
“Caltech researchers have found mathematical evidence suggesting there may be a “Planet X” deep in the solar system. This hypothetical Neptune-sized planet orbits our Sun in a highly elongated orbit far beyond Pluto. The object, which the researchers have nicknamed “Planet Nine,” could have a mass about 10 times that of Earth and orbit about 20 times farther from the Sun on average than Neptune. It may take between 10,000 and 20,000 Earth years to make one full orbit around the Sun.” This speculation is to be found on a NASA website(r).
Nevertheless, in 2016, two astronomers independently proposed that Planet 9 (should that be Planet IX?) was possibly responsible for changing the axial tilt of the other planets relative to the axis of our sun(h).
More recently an object with the exciting name of 2015 TG387, probably a dwarf planet, about 300 km in diameter, had its 40,000-year orbit determined. However, aberrations in that orbit are claimed by some to be evidence for a large distant body; the hypothesised Planet X(i)!
Another variation of the Planet X idea is offered by Stuart L. Harris whose visiting planet he named after the Babylonian god, Marduk. He claims that its close encounter in 9577 BC resulted in the destruction of Atlantis, which he places in the vicinity of Rockall in the North Atlantic. His ideas are expressed in a series of four papers on the Academia.edu website(m-p).
In 2000, Anthony Austin & Brian Crowley published The Dragon’s Tail  in which they also proposed a tenth planet, they called Draco. According to them it returns every 892 years to cause catastrophes on Earth, such as the biblical Deluge, the plagues of Egypt and the demise of Atlantis. According to Austin(k), following the death of the original publisher, it was republished in 2003, with some small changes, as Draco: The Tenth Planet by Black Rabbit Press(l).
In 2020, it was reported that evidence for the existence of Planet X continues to build(q) and with a new telescope at the Rubin Observatory in Chile due to be operational in the near future, final confirmation is hoped to be found.
(f) New Scientist No. 3058, Jan. 30 2016 (p.8)