The same old question returns: “Is there a ‘secret’ planet ‘hiding’ behind Neptune”?
Actually, it was very common back in the 80s, for NASA to speak of a tenth planet, also known as ‘Planet X’:
“On June 17, 1982, a NASA press release from Ames Research Center officially recognized the possibility of “some kind of mystery object” beyond the outermost planets.
Various press releases around this time confirmed that scientists were indeed looking for the infamous Planet X.
For instance, Astronomy magazine published an article in December of 1981 entitled ‘Search for the Tenth Planet’, and another article in October of 1982 entitled ‘Searching for a Tenth Planet.’
In addition, Newsweek covered the story of Planet X on June 28, 1982 in an article entitled ‘Does the Sun Have a Dark Companion?’ This article implied that the tenth planet actually orbits a two sun (binary star) system, but we cannot see the other sun because it is a ‘dark’ star.” – 
But, this theory was released to the public for the first time in 1846
“In 1846, researchers noticed that Uranus was wobbling in a way that confounded Newton’s Law of Motion. This meant they had two options: rewrite the most time-honored of the laws of physics, or find a new planet to account for the extra gravitational pull.” – 
At the beginning of 2011, NASA admitted to be tracking a giant planet, four times the size of Jupiter, lurking at the very edge of our solar system. No further news followed this amazing announcement, but the very famous and prolific telescope WISE was shoot down. – 
Read: NASA Mistakenly Confirms Nibiru at NEOWISE Conference;
The most recent article regarding this planet/brown dwarf, dates May 11, 2012, and was published by National Geographic. It is entitled “New Planet Found in Our Solar System?”.
Before proceeding, please allow me to remind you the Spanish astronomers from the “Star Viewer Team” which, in 2011, publicly released a document attesting the discovery of a Brown Dwarf Star, named “G1.0”:
“This newly discovered ‘brown dwarf’ is believed to have formed from the same condensed matter that gave birth to our Sun. It is believed that, after the large planets formed around the Sun, they pushed it to the edge of the Solar system where it formed a sphere about 1.9MJ – well below the mass needed to ignite it as a ‘sun’ (…) We are close to our Sun and within its gravitational influence. So as we are travel through space, it appears to us that the G1.9 is moving in an ellipse between our furthest planetoid, Pluto, and the edge of our Solar system, near the Oort Cloud.” – 
“Too far out to be easily spotted by telescopes, the potential unseen planet appears to be making its presence felt by disturbing the orbits of so-called Kuiper belt objects, said Rodney Gomes, an astronomer at the National Observatory of Brazil in Rio de Janeiro. (…)
‘But I think the easiest one is a planetary-mass solar companion’ — a planet that orbits very far out from the sun but that’s massive enough to be having gravitational effects on Kuiper belt objects. (…)
For the new work, Gomes analyzed the orbits of 92 Kuiper belt objects, then compared his results to computer models of how the bodies should be distributed, with and without an additional planet. If there’s no distant world, Gomes concludes, the models don’t produce the highly elongated orbits we see for six of the objects.
How big exactly the planetary body might be isn’t clear, but there are a lot of possibilities, Gomes added. Based on his calculations, Gomes thinks a Neptune-size world, about four times bigger than Earth, orbiting 140 billion miles (225 billion kilometers) away from the sun — about 1,500 times farther than Earth — would do the trick.
But so would a Mars-size object — roughly half Earth’s size — in a highly elongated orbit that would occasionally bring the body sweeping to within 5 billion miles (8 billion kilometers) of the sun.” – 
Feb 15, 2011: CNN Admits that a Brown Dwarf may reside at the edge of our solar system:
Year after year, new astronomers come forward and acknowledge that a massive object is lurking at the edge of our solar system. The size of this object varies. The smallest estimation is about the size of Jupiter, while the biggest is four times its size. Some say it’s a planet, others say it’s a brawn dwarf, but almost all reports agree on its elliptical orbit.
The scientists disagree on its name: G1.9, Tyche, Hercolubus, Tenth Planet or Planet X;
Some historians as well: Nemesis, Kachina or Nibiru.
But most of them agree on its existence, massive size, elliptic orbit and that it’s part of our solar system.
Yet, NASA, with the most advanced technology at hand, and its super high-tech & high-resolution telescopes, play dead. No videos, no pictures, no press conference to openly and undoubtedly admit its existence.
By Alexander Light, HumansAreFree.com;