Did Neptune munch a planet and steal its moon? (Image: Jet Propulsion Laboratory / NASA)

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. …