Eris is a dwarf planet in the outer Solar System that is slightly smaller than Pluto by diameter but 27% more massive. Its 2005 discovery directly triggered the debate that demoted Pluto the following year.

Europa is smaller than Earth's Moon, but the ocean hidden beneath its ice holds two to three times as much water as every ocean on Earth combined. Here is what makes this moon one of the best bets for life beyond Earth.

Ganymede, Jupiter's largest moon, is bigger than the planet Mercury and is the only moon in the Solar System known to generate its own magnetic field. Its hidden ocean may hold more water than every ocean on Earth.

Halley's Comet is the only short-period comet consistently visible to the naked eye from Earth. Recorded since at least 240 BC, it returns roughly every 75 to 76 years and last appeared in 1986.

Haumea is a dwarf planet beyond Neptune that spins faster than any other known body its size, distorting it into an elongated, egg-like shape. It has two moons, a ring, and a discovery story clouded by controversy.

Io, the innermost of Jupiter's four Galilean moons, has more than 400 active volcanoes and almost no water at all. Squeezed by Jupiter's gravity, it is the most geologically active object in the Solar System.

Jupiter is the only planet massive enough to pull its shared center of gravity with the Sun outside the Sun itself. Here is what else makes it the Solar System's giant.

The Kuiper belt is a vast, icy region beyond Neptune that dwarfs the asteroid belt in size, yet it holds barely a sliver of its original mass. Here is what its discovery, structure, and one snowman-shaped visitor reveal.

Makemake is a bright, highly reflective dwarf planet in the Kuiper belt whose 2005 discovery helped push astronomers to reclassify Pluto. It has just one known moon, fittingly nicknamed "MK 2."

On Mercury, a single day from sunrise to sunrise lasts 176 Earth days — twice as long as its entire year. Here are the most surprising facts about the Solar System's smallest and strangest planet.

A meteor is not a falling star but a streak of light created when a speck of space debris burns up 76 to 100 kilometers above Earth's surface. Here is what is actually happening over your head.

A meteorite is what's left after a piece of space rock survives the fall to the ground. A handful of famous ones have rewritten what we know about the Solar System's age, its chemistry, and its neighboring planets.