The number of planets that orbit the sun depends on what you mean by “planet,” and that’s not so easy to define ...

While the composition of gas and dust in a molecular cloud is fairly uniform, everything changes once a star begins to form.

The solar system may hold a hidden surprise. Astronomers are investigating the possibility of a mysterious ninth planet, often referred to as Planet Nine. If found, this discovery could change the ...

An object eight times the mass of Jupiter may have swooped around the sun, coming superclose to Mars' present-day orbit ...