Is an unseen planet skirting edge of our solar system?

By Natalie Wolchover

A planet four times the size of Earth may be skirting the edges of the solar system beyond Pluto, according to new research. Too distant to be easily spotted by Earth-based telescopes, the unseen planet could be gravitationally tugging on small icy objects past Neptune, helping explain the mystery of those objects’ peculiar orbits.

The claim comes from Rodney Gomes, a noted astronomer at the National Observatory of Brazil in Rio de Janeiro. Gomes presented his recently completed computer models suggesting the existence of the distant planet at a meeting of the American Astronomical Society in Timberline Lodge, Ore., earlier this month.
Astronomers who attended the talk find Gomes’ arguments compelling, but they say much more evidence is needed before the hypothetical planet can be crowned as real.

For several years, astronomers 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. “Sedna’s orbit is truly peculiar,” said Mike Brown, an astronomer at Caltech who led the team that discovered Sedna in 2003.

However, when Gomes ran the same calculations with the addition of the 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.

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