It's A Jungle Out There

Neptune may have eaten a planet and stolen its moon

"NEPTUNE may have polished off a super-Earth that once roamed the outer solar system and stolen its moon to boot. The brutal deed could explain mysterious heat radiating from the icy planet and the odd orbit of its moon Triton.
Neptune's own existence was a puzzle until recently. The dusty cloud that gave birth to the planets probably thinned out further from the sun. With building material so scarce, it is hard to understand how Uranus and Neptune, the two outermost planets, managed to get so big.
But what if they formed closer in? In 2005, a team of scientists proposed that the giant planets shifted positions in an early upheaval (New Scientist, 25 November 2006, p 40). In this scenario, Uranus and Neptune formed much closer to the sun and migrated outwards, possibly swapping places in the process.
That would have left behind enough material just beyond their birthplace to form a planet with twice the Earth's mass, according to calculations published in 2008 by Steven Desch of Arizona State University in Tempe.
Neptune's peculiar moon Triton may once have been paired with this hypothetical super-Earth, Desch and colleague Simon Porter now say. Triton is larger than Pluto, and it moves through its orbit in the opposite direction to Neptune's rotation, suggesting that it did not form there but was captured instead."
"Neptune may have engulfed the super-Earth. Heat left over from the impact could explain why the planet radiates much more heat than its cousin Uranus, which is similar in mass and composition, Desch says."