Almost 10 years ago, a NASA spacecraft was able to take several photos of Pluto, known for decades as a planet before it was ...

A powerful new observatory is the best hope yet for finding the elusive Planet 9, a large planet that some scientists say is ...

Moons of Jupiter and Saturn are wider than Mercury, and Pluto is smaller than Earth's moon. For years, astronomers tried to define a planet by shape, orbit around a star and its influence on other ...

Pluto was once called the ninth planet, but in 2006, scientists changed its status to a dwarf planet. A planet must clear its orbit of other objects, but Pluto shares its space with many small icy ...

The relative quantities of volatile gases like methane and ethane can reveal key details about distant Kuiper Belt objects.