Paris (AFP) - The Solar System has at least two more planets waiting to be discovered beyond the orbit of Pluto, Spanish and British astronomers say.

The official list of planets in our star system runs to eight, with gas giant Neptune the outermost.

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.

In a study published in the latest issue of the British journal Monthly Notices of the Royal Astronomical Society, researchers propose that "at least two" planets lie beyond Pluto.

 Their calculations are based on the unusual orbital behaviour of very distant space rocks called extreme trans-Neptunian objects, or ETNOs.