DWARF PLANETS
At least two unknown planets could exist in our solar system beyond Pluto.NASA/JPL-Caltech

The strange orbital behaviour and distribution of objects beyond Neptune can only be explained by the presence of at least two unknown planets hidden beyond Pluto, suggests a new study based on numerical calculations.

If confirmed, this could extend our solar system beyond present boundaries.

Researchers at the Complutense University of Madrid and the University of Cambridge analysed the effects of the so-called "Kozai mechanism", related to the gravitational perturbation that a large body exerts on the orbit of another much smaller and further away object.

Unlike the figures expected by the accepted theory for the semi-major axis and orbit inclination values of the trans-Neptune objects, they observed diverse figures for the semi-major axis with a wide range between 150 AU and 525 AU, while orbit inclination was around 20 degrees instead of the almost 0 expected.

Current models on the formation of the solar system state that there are no other planets moving in circular orbits beyond Neptune.

However, the recent discovery by the ALMA radio telescope of a planet-forming disk at 100 AUs from the parent star HL Tauri suggests that planets can form several hundred astronomical units away from the centre of the system.

Last year a dwarf planet called 2012 VP113 was found in the Oort cloud, just beyond our solar system.

Its orbit is believed to be influenced by the possible presence of a dark and icy super-Earth, up to ten times larger than our planet.

The discovery prompted astronomers to rethink accepted theories of solar system formation that cannot explain these dwarf planets.

Another dwarf planet Sedna that takes 11,400 years to go once around the sun was discovered a decade ago.

As a handful more dwarf planets have been detected, it will take years of observation of the movement of these dwarf planets to confirm their nature.

The International Astronomical Union defines a planet as an object that circles the sun and is large enough to be rounded by its own gravity (but not big enough to turn into a star) and has "cleared its neighbourhood" of most other orbiting bodies.

Pluto was the first Kuiper belt object to be seen in the region beyond Neptune filled with icy bodies.

The Kuiper Belt, spanning 4.5 to 7.4 billion kilometres, holds trillions of objects which are remnants of the early solar system. Even beyond lies the spherical Oort cloud from where many comets originate.