The Solar System contains eight celestial bodies officially classified as planets by the International Astronomical Union (IAU). This designation was formalized in 2006. For an object to be considered a planet, it must orbit the Sun, possess sufficient mass to achieve a nearly round shape, and have gravitationally cleared its orbital path. This framework groups the eight planets into two distinct categories based on their composition and location.
The Terrestrial Planets
The four innermost planets—Mercury, Venus, Earth, and Mars—are collectively known as the terrestrial planets. These worlds are defined by their composition, consisting primarily of dense material like rock and metal. They share a fundamental internal structure, featuring a central metallic core surrounded by a silicate mantle and crust.
Mercury is the smallest of the group and possesses a massive iron core. Due to its very thin atmosphere, the planet experiences the most extreme temperature swings, with surface temperatures ranging hundreds of degrees between day and night. Earth is the largest and densest terrestrial planet and is unique for sustaining extensive regions of liquid water on its surface.
Venus is shrouded by a dense atmosphere of carbon dioxide, which creates a runaway greenhouse effect, making it the hottest planet. This atmosphere is so thick that it exerts a pressure ninety times greater than Earth’s at sea level. Mars, often called the “Red Planet,” has a surface rich in iron oxide dust, giving it its reddish-brown color. Though smaller than Earth, Mars features massive extinct volcanoes and the largest canyon system.
The Gas and Ice Giants
Beyond the asteroid belt lie the four outer planets, which are substantially larger and less dense than their inner counterparts. These giant planets are divided into two sub-groups: the Gas Giants (Jupiter and Saturn) and the Ice Giants (Uranus and Neptune). All four are characterized by their low mean density, massive size, and the presence of ring systems and numerous natural satellites.
Jupiter and Saturn are the Gas Giants, composed overwhelmingly of hydrogen and helium, mirroring the composition of the Sun. Jupiter is the most massive planet, with a volume large enough to contain all the others combined. Deep within the immense pressure, hydrogen is compressed into liquid metallic hydrogen, which acts as a powerful electrical conductor.
Uranus and Neptune are the Ice Giants, which are significantly smaller and denser than the Gas Giants. While they also contain hydrogen and helium, their composition is dominated by heavier volatile materials, such as water, methane, and ammonia, often referred to as “ices.” Methane in their upper atmospheres absorbs red light and reflects blue light, giving both planets their pale blue-green color. They have smaller, rock and ice cores surrounded by a thick mantle layer of these volatile compounds.