Creating our solar system

26.1 Current model of the solar system Today, we define the solar system as the sun and all objects that are gravitationally bound to the sun. The solar system is roughly divided into the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune) The dwarf planet Pluto is the oldest known member of a smaller group of frozen worlds orbiting beyond Neptune.

26.1 Comparing size and distance The Sun is by far the largest object in the solar system. One astronomical unit (AU) is equal to 150 million km, or the distance from Earth to the Sun.

26.3 The planets The planets are commonly classified in two groups. The terrestrial planets include Mercury, Venus, Earth, and Mars. The gas giants include Jupiter, Saturn, Uranus, and Neptune.

Comparing properties of the planets

26.3 Mercury Mercury, the closest planet to the sun, is the second smallest (after Pluto) in both size and mass.

26.3 Venus Venus appears as the brightest planet in the evening sky and is the third brightest observable object (after the sun and moon).

26.3 Earth Earth is a small, rocky planet with an atmosphere that is made of mostly nitrogen (78 percent N2) and oxygen (21 percent O2).

26.3 Mars The fourth planet out from the sun, Mars appears as a reddish point of light in the night sky.

26.3 Jupiter The fifth planet out from the sun, Jupiter is by far the largest. Jupiter’s mass is greater than the combined mass of all of the other planets. With 63 known moons, Jupiter is like a mini solar system.

26.3 Saturn Saturn, at almost 10 times the size of Earth, is the second largest planet. The most striking feature of Saturn is its system of rings and like Jupiter, has many natural satellites.

26.3 Uranus The seventh planet from the sun, Uranus can barely be seen without a good telescope and was not discovered until 1781.

26.3 Neptune Neptune, the eighth planet from the sun, is the outermost of the gas planets. It was discovered in 1846 and its discovery almost doubled the diameter of the known solar system because of its great distance from the sun.

26.3 Pluto Pluto is a dwarf planet. Most of the time Pluto is the farthest from the sun. Discovered in 1930, Pluto was named for the Roman god of the underworld.

26.3 Pluto and the Kuiper Belt What is the Kuiper belt? A vast cloud of small rocky and icy objects orbiting the Sun at great distances, beyond Neptune and Pluto. Pluto is grouped along with Sedna, Xena, and similar distant bodies in the Kuiper Belt Objects (or KBOs). It extends to a distance of about 50 astronomical units