A solar system is defined as the collection of planets and other heavenly bodies — such as asteroids, comets, and meteors — that orbit a central star. Most of the time when we speak of a solar system, we are speaking of the one of which Earth is a part: the group of planets and other heavenly bodies that orbit the star we call the Sun.
In our solar system, the heavenly bodies that surround the sun and are held by its gravity include dwarf planets and natural satellites, as well as asteroids, comets, and meteors. It is now considered that there are eight planets, which in order from the sun outwards are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Though considered a planet for many years, Pluto was reclassified in 2006, and is now called a dwarf planet.
Planets are now defined by the International Astronomical Union by several criteria. To be considered a planet, a heavenly body must:
- not be a star
- orbit a star
- have sufficient mass to be nearly spherical
- have cleared its orbit of other objects
The planets in our solar system have much in common: they all rotate on their axis as they revolve around the sun in the same direction. Nevertheless, the planets in our system have different physical properties, and based on these properties and their arrangement in space, they are commonly grouped into the inner planets and the Jovian planets. Jove is an alternative name for the god Jupiter.
The inner planets of our solar system — those nearest the sun—are Mercury, Venus, Mars, and Earth. These planets are smaller and denser, with solid crusts and molten interiors. Three of the four — all except Mercury — have a gaseous atmosphere. The Jovian planets are larger and less dense, with thick atmospheres.
The sun’s gravitational pull is responsible for the elliptical orbits of the planets in our solar system. Johannes Kepler first described planetary motion accurately in the early 1600’s. It was Isaac Newton who demonstrated that Kepler’s laws of planetary motion applied to other heavenly bodies in the solar system and were the result of the sun’s gravitational force.