Even though its believed to be 10 times the size of Earth, it has likely escaped the gaze of telescopes because it is so far away from the Sun.
This week the pair of scientists, along with fellow astronomer Elizabeth Bailey, stoked the flames of speculation by presenting a study at the annual meeting of planetary scientists of the American Astronomical Society claiming the mysterious planet is responsible for the perceived tilt of the sun.
We've known that the sun's rotational axis is tilted for about 200 years, but scientists have never understood exactly why.
Bailey, Batygin and Brown believe a large planet lurking in the depths of the solar system is responsible and the latest addition to their theory was published this week by the Astrophysical Journal.
"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," lead author Elizabeth Bailey said in a statement.
The sun rotates on a different axis to all the other planets. Earth, Venus, Mars, Mercury, Uranus, Neptune, Saturn and Jupiter all share a common plane but for some reason it is tilted by about six degrees in relation to the sun - causing our perception that the sun is tilted.
It's a quirk that has long troubled scientists.
"It's such a deep-rooted mystery and so difficult to explain that people just don't talk about it," said Mr Brown. "If you ask yourself where the sun is tilted in real life there's where we predict it should be," he added
He believes it's a matter of time until we have visual confirmation of Planet Nine saying he is "pretty sure" we'll track it down within about 16 months.
"I think that there'll be enough people looking for it that ... somebody's actually going to track this down," he said.
At the "next one of these (conferences) we'll be talking about finding Planet Nine instead of just looking for it," he added.
For now, researchers are left to speculating over the mind boggling measurements associated with the elusive space body.
Thought to be roughly 10 times the mass of planet Earth, scientists believe it lies hundreds of astronomical units from the sun. One astronomical unit is the mean distance between the earth and the sun which equates to 149.6 million kilometres.
They also think it would take the planet between 10,000 and 20,000 years to make just one full orbit around the sun.
It's important to remember that at this point, the planet is still very much hypothetical. But the theory is underpinned by the measurement of unusual outcomes in the solar system which are otherwise tough to explain.
It is the same technique astronomers used to find Neptune.
NASA has previously warned that it is too early to confirm the existence of Planet Nine for certain but pledged "if it's there, we'll find it."
Some science buffs have long believed in the existence of a distant ninth planet, but there was never any real evidence to support such a claim.
However that has certainly changed this year with a number of papers published supporting the idea.
When proposed, the theory was well received by the scientific community and it has been gaining momentum ever since.
Astronomer Alessandro Morbidelli who works at an observatory in Nice, France believes Batygin and Brown's work provides a compelling answer to the state of the solar system.
"I don't see any alternative explanation to that offered by Batygin and Brown," he told The Washington Post at the time.
"We will find it one day," he added. "The question is when."
But for now, the hunt is on.
