Two other planets believed to be bigger than the earth in size, have been discovered in the solar syatem.
The official list of planets in our star system runs to eight, with gas giant Neptune the outermost.
However, a study published in the latest issue of the British journal Monthly Notices of the Royal Astronomical Society, researchers said, at least two planets lie beyond Pluto.
According to them, the discovery is based on the unusual orbital behaviour of very distant space rocks called extreme trans-Neptunian objects, or ETNOs.? ETNOs should be dispersed in a band some 150 Astronomical Units (AU) from the Sun.
ETNOs should also be more or less on the same orbital plane as the Solar System planets.
Nonetheless, about a dozen ETNOs have suggested a quite different picture.
By the study, ETNOs are scattered much more widely, at between 150 and 525 AU, and with an orbital inclination of about 20 degrees.
The ?study also suggested that some very large objects, planets, must be in the neighbourhood and their gravitational force is bossing the much smaller ETNOs around.
Beyond Neptune, Pluto was relegated to the status of dwarf planet by the International Astronomical Union in 2006, although it is still championed by some as the most distant planet from the Sun.