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

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 ...

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

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