Over a thousand objects are now known to orbit the Sun beyond Neptune, and once again we can't imagine torturing school children by adding even a dozen of the biggest ones to the list of planets.

Mars has two moons, Phobos and Deimos. Scientists still debate whether these came from asteroids that passed close into Mars' ...

Earth-like exoplanets (planets orbiting stars other than our sun) with oceans, lakes and rivers are thought to be unlikely, but the work reveals that many stars will harbor geological conditions ...