The planet, which is too distant to be easily spotted by Earth-based telescopes, could be gravitationally tugging on small icy objects past Neptune, helping explain the mystery of those objects' peculiar orbits, the scientists said.
The claim comes from Rodney Gomes, a noted astronomer at the National Observatory of Brazil in Rio de Janeiro, who presented his computer models suggesting the existence of the distant planet at a meeting of the American Astronomical Society in Timberline Lodge recently.
Astronomers who attended the talk found Gomes' arguments compelling, but said much more evidence is needed before the hypothetical planet can be said as real, LiveScience reported.
For many years, scientists have observed that a handful of the small icy bodies that lie in the so-called "scattered disc" beyond the orbit of the planet Neptune, including the dwarf planet Sedna, deviate from the paths around the Sun that would be expected based on the gravitational pulls of all the known objects in the solar system.
Sedna, for example, swings around the Sun in an extremely elongated orbit -- tracing out a very long oval. However, when Gomes ran the calculations with the addition of gravitational pull of a massive planet at the outskirts of the solar system, Sedna and the other anomalous objects' expected orbits fell in line with observations.
The unseen planet would be too far away to perceptibly perturb the motions of Earth and the other inner planets, but close enough to the scattered disc objects to sway them.
Several planet types could fit the disturbances seen in Gomes' calculations. For example, a Neptune-size planet, about four times bigger than Earth, orbiting 225 billion kilometres away from the Sun would influence the anomalous objects in the observed manner.
Or, a Mars-size planet with a highly elongated orbit, but one that always keeps it well beyond the orbit of Pluto, could yield similar results, Gomes said.
As for how it got there, Gomes said the planet could have been born in and expelled from a distant star system and later captured by our sun's gravity. Or it could have formed near the Sun and gradually been thrust outward by gravitational interactions with other planets, he added. (More)
