There are two elements to theories about a Tenth Planet in our solar systems:

1) There is the historical data produced primarily by Zacharias Sitchin who using historical texts, artifacts, drawings and other data propounded the theory that a planet called Nibiru is the 10th planet of the solar system, with an elliptical orbit that only brings it home every 3600 years.

2) There are scientific observations of a large Kuiper Belt object that has been perturbing the orbits of Neptune and Uranus. The conclusion is that Pluto is too small to do this and a search ensued from about the 1980's to discover this object that could be the 10th Planet (Planet X). This search was led by NASA and some papers have been published and others have been witheld. There is also some scientific and astronomic suggestions that Earth has a dark twin (Nemesis), a brown dwarf or failed star, whose movement might account for the long period comets that swirl into Earth from the Oort Cloud. If the dark star exists as a companion to our Sun then it could be conceivable that a planet may orbit in a ellipse around both of them.

The Sitchin Theory & Nibiru & The Annunaki

The idea that Earth has been visited by aliens at some point in the distant past is not a new one. Erik von Daniken first touted the theory in his book "The Chariots of the Gods" back in the 1950's. Many people's first introduction to this concept has been via von Daniken's ideas. While he was widely read, the problem always was that his interpretation of ancient texts and artifacts always lacked a certain key. Zacharias Sitchin in 1976 provided that key!

Sitchin had apparently got started on his research by questioning the Bible stories while still at school in his native Israel. He notes in parts that Genesis (6:4) reads "There were giants in the earth in those days, and also afterward, when the sons of God came into the daughters of men, and they bare children unto them, the same became mighty men which were of old, men of renown."

Sitchin went much further examining the old Hebrew texts. He realised that the Hebrew word "Nefilim" was indeeed the name for the giants talked of in the Bible
and the men of renown were really men of the "shem". More importantly Nefilim in its original Sumerian interpretation literally means "those who came down to earth from heaven".

Sitchin goes on to claim that the Earth was visited by aliens as far back as 450,000 years ago. These aliens came from another unknown planet of the solar system called Nibiru. He claims that these aliens were seeking to save the Nibiru atmosphere from serious degradation and for this they needed extremely finely powdered gold dust pumped into the atmosphere that would resolve the problems associated with heat retention when their planet was in the aphelion region of its long looping orbit around the sun. Gold was, of course, relatively plentiful on Earth!

Sitchin applied his understanding of Bible stories to studies of ancient writing from Sumeria and Mesapotamia generally and reached the conclusion that the Nefilim (Gods and Sons of God) used advanced genetic engineering to create human from a combination of Nefilim genes and those of a compatible pre-human evolved species. The result was humans, initially created to mine gold for the Nefilim but later with the capacity to understand commands, speak and procreate! Thus Eve really did follow Adam!

Sitchin studied many ancient civilisations and concluded the Gods worshipped by all of them were really ancient space travellers with a superb knowledge of advanced technology and a desire for gold! When viewed this way, much of the world history as told in the Bible and other manuscripts falls into place. Suddenly we can understand why there were gold mines 8-12,000 years old in South and Central America. And Sitchin concludes that humankind was developed to mine this gold to satisfy their needs.

Interestingly, other theories exist about the importance of the gold that the Nefilim so desired. These relate to the idea of White Powder of Gold, or the "Food of the Gods". Essentially this idea is that monatomic gold is a superconductor with many exotic properties, and indeed when the correct type is ingested it can bestow elongated lifespan and much enhanced mental faculties and produce ascension. Consider for a moment the twin objectives of the Alchemists, to transmute gold to lead and to find the elixir of life.

However, this is not the purpose of this discussion about Planet X and those wanting to do more research can follow these links:

It is a fascinating subject in itself and may be the area where Sitchin misinterpreted the real Annunaki/Nefilim desire for gold! Perhaps it was really the secret of their longevity and their "god-like" powers. It may also explain man's obsession with the yellow gold and indeed other precious metals.

Now, Sitchin postulated the theory that Nibiru, the Planet X, took an eliptical orbit around the Sun (and perhaps a dead star further out). His hypotheses are mapped out as a time and distance ephemeris orbital modeal at this site:

In his view, many catastrophic earth events can be explained by the 3600 year cycle of this planet which brought it back into our solar system at those
periods and depending on the closeness this caused biblical disasters such as the Great Flood! This occured as the immense gravitational pull of this planet
impacted on others within the solar system.

Sitchin based many of his observations of Nibiru and its behaviour on historical texts, the Bible and other such material but also the Sumerian artifacts that
described the Annunaki (the Nefilim/Gods) and their planet and chariots. There is a large collection of pictures of the artifacts that better explains Sitchin's interpretations at this site:

This page - pulls together a lot of the evidence about Sumerian culture, the Annunaki and Nibiru (Planet X).

You can read more about Sitchin's theories and Planet X theory in general at:

Scientific Observations about possible Planet X and a "dark star" companion

There is much speculation that there are many objects residing in the Kuiper belt that is full of remnants from the planet formation era of the solar system.

Here are some examples of discoveries:

New Planet-Shaped Body Found in Our Solar System

Hindu God Points Way To Planet X

Another Candidate For Planet X Found Beyond Pluto

Large Object Discovered Orbiting Sun

A New Class of Solar System Objects

Why, though, have so many people been searching for this Planet X. Well, several astronomers detected perturbations of the orbits of Neptune and Uranus. In fact, in 1846 Neptune was discovered after its existence was predicted because of discrepencies between calculations and data for the planet Uranus.

An interesting article is available here from John Anderson for the "Encyclopedia of Planetary Sciences":

He discusses how the Robert S. Harrinton US Naval Observatory study started as a result of unexplained motions in the orbits of both Uranus and Neptune. The Pluto/Charon System was ruled out as the cause of these perturbations and this led to the IRAS/Pioneer searches for Planext X.

Interestingly, he says:

"On the other hand, in 1986, A.S. Guliev in Azerbaidzhan used orbits of long-period comets to derive the orbital elements of a single Planet X at a distance of 36.2 AU,and more recently the orbital planesówith inclination of about 30 arc degrees to the ecliptic for two Planets X at distances of 48.5-56.6 AU and 102-112 AU respectively."

He then questions why IRAS failed to find it. Or did it?

Controversy surrounds the whole Harrington studies and question exist to this day about what Harrington of the US Naval Observatory did or did not discover since an initial statement that Planet X had been located was later denied and later Harrington studies from 1991 were never made available to the public.

A quick search will show a number of papers from 1988 to 1991 about the Harrington search for Planet X but many of the papers are not available, only the tantalising abstracts:
NASA themselves tend to point people searching for Planet X at this page:

Interestingly this makes little mention of the work of Harrington but does include the conclusions of John Anderson in his 1987 study of the motions of Pioneer 10 and 11 which concludes that a tenth planet with a highly elliptical orbit does exist!

So the question has always remained about what NASA really know and what they are telling us!

On the question of Nemesis, the dark twin of our sun, there is a tantalisingly interesting story from the Daily Telegraph:

Does the Sun have a doomsday twin?

Interesting article here too, covering the Telegraph article but taking it further by examining the Oort Cloud in some detail:

And, of course, does this article from News Science and Invention Encyclopedia in 1987 really show what our solar system actually looks like:

To finish with a tantalising abstract from an unrealeased Harrington paper from 1988 entitled "Planet X - The Current Status":

"The history of the discoveries of Neptune and Pluto on the basis of discrepancies in the orbit of Uranus is recalled in detail, and the persistence of such discrepancies in 9-planet computations is considered. Particular attention is given to ongoing efforts to compute the current position of planet X, and to ground-based and space-based (IRAS and Pioneer) searches. Diagrams and graphs are provided"

Now wouldn't we all like to see THOSE diagrams and graphs, or perhaps that's what appeared in that Encyclopedia!