The Solar System
The Solar System consists of the Sun and the celestial objects gravitationally bound to it, including eight planets, their moons, and countless asteroids and comets.
For most of the 20th century, textbooks counted nine planets; a single 2006 vote changed that number, demoting a longtime member of the family to a new category entirely.
Born from Cosmic Dust
The Solar System formed roughly 4.6 billion years ago from the gravitational collapse of a giant molecular cloud of gas and dust, with the vast majority of the resulting mass — over 99.8% — concentrated into the Sun itself.
Two Families of Planets
The eight planets divide naturally into two distinct groups: the smaller, rocky inner planets — Mercury, Venus, Earth, and Mars — and the much larger outer gas and ice giants — Jupiter, Saturn, Uranus, and Neptune, which are composed primarily of hydrogen, helium, and various ices.
Pluto's Demotion
For most of the 20th century, Pluto was considered the ninth planet, but in 2006 the International Astronomical Union formally reclassified it as a "dwarf planet" after the discovery of similarly sized objects in the outer Solar System raised questions about what truly defines a planet.
A Vast Neighborhood of Small Bodies
Beyond the planets, the Solar System contains countless smaller objects, including the rocky asteroid belt between Mars and Jupiter, and the distant, icy Kuiper Belt beyond Neptune, home to Pluto and numerous comets that occasionally travel into the inner Solar System.
Continued Exploration
Ongoing space exploration missions have dramatically expanded human understanding of the Solar System, with robotic spacecraft having now visited every planet and many of their major moons, along with several asteroids and comets, transforming abstract points of light into richly detailed worlds.
