There may possibly be at least two planets belonging to our solar system that are hidden beyond Pluto.

The analysis is based on calculations of the orbits of cosmic bodies found in the regions well beyond Neptune, which include the Kuiper Belt, the Oort cloud and the scattered disk.

These bodies, also known as extreme trans-Neptunian objects (ETNOs), may include two planets that are bigger than the Earth, and are only waiting to be discovered. The existence of these planets could likewise help explain the orbital behavior of ETNOs.

The most widely accepted theory posits that the orbits of these objects should be randomly distributed and their paths need to have a series of characteristics, including having a semi-major axis with value near 150 astronomical units (AU) and an inclination of nearly 0° relative to the plane of the solar system.

There are, however, discrepancies in what was observed in 13 ETNOs that were studied. The values of the semi-major axes, for instance, range from 150 to 525 and the average inclination is about 20 degrees.

Carlos de la Fuente Marcos, who is from the Complutense University of Madrid and author of the study published in two articles in the Monthly Notices of the Royal Astronomical Society, said that the excess of objects having unexpected orbital parameters suggests that some unseen forces change the distribution of the ETNOs' orbital element. The most feasible explanation for this is that other planets that have not yet been discovered exist beyond Neptune and Pluto.

"The exact number is uncertain, given that the data that we have is limited, but our calculations suggest that there are at least two planets, and probably more, within the confines of our solar system," the astrophysicist said.

The researchers said that the possibly existent but undiscovered world would be located at least 200 AU from the sun and this distance is so far away that it would be very difficult or even impossible to detect these using currently available instruments.

In March 2014, two scientists discovered a dwarf planet in the inner Oort Cloud, a very far and largely unexplored region of space that lies beyond the Kuiper Belt. Scientists considered that the orbit of this ETNO dubbed 2012 VP113 is influenced by the possible presence of an icy and dark super-Earth that is up to 10 times bigger than our planet.