Mercury is the innermost and smallest of the eight major planets in our Solar System, orbiting closest to the Sun. Though only slightly larger than Earth’s Moon, Mercury endures some of the most ...

Over the past 250 years, the number of "planets" in our solar system has ranged from six to nine — and, briefly, even 11 — depending on what astronomers knew at the time and how they defined a planet.

The solar system's planet count has changed seven times in 250 years — from 6 to 11, back down, then up to 9, then 8. Planet Nine could change it again. Full history explained.

Made up of billions of objects, the Solar System has been studied for generations - but there is still so much to learn about it. Full of celestial bodies orbiting the Sun, the Solar System is roughly ...