Solar System

Solar System
Name	Solar System
Caption	Composite image showing the Sun, planets, and dwarf planet Pluto to scale.
Age	4.568 billion years
Location	Local Interstellar Cloud, Local Bubble, Orion–Cygnus Arm, Milky Way
Nearest star	Proxima Centauri (4.2465 ly)
Known planets	Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Known dwarf planets	Ceres, Pluto, Haumea, Makemake, Eris
Known natural satellites	700+ (285 planetary, 450+ minor planetary)
Known minor planets	1,300,000+
Known comets	4,600+

Solar System. The planetary system consists of a central star, the Sun, and all the celestial bodies gravitationally bound to it. This includes eight major planets, their moons, five officially recognized dwarf planets, and countless smaller objects like asteroids and comets. It formed approximately 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud and is located in the Orion–Cygnus Arm of the Milky Way galaxy.

Formation and evolution

The most widely accepted model for its origin is the solar nebula hypothesis, which posits it coalesced from a rotating cloud of gas and dust known as the pre-solar nebula. A nearby event, such as the shockwave from a supernova, may have triggered the nebula's collapse, leading to the formation of the protostar that became the Sun. Over tens of millions of years, dust grains within the protoplanetary disk accreted into planetesimals, which then collided and merged to form the planets, a period known as the Late Heavy Bombardment. Key evidence for this timeline comes from radiometric dating of ancient meteorite samples, such as those from the Allende meteorite, which contain calcium–aluminium-rich inclusions dated to 4.567 billion years old.

Structure and composition

The vast majority of the system's mass is contained within the Sun, with Jupiter holding most of the remaining mass. It is broadly divided into distinct regions: the inner system, the asteroid belt, the outer system, and the distant trans-Neptunian region. The four inner, terrestrial planets—Mercury, Venus, Earth, and Mars—are primarily composed of rock and metals. Beyond the asteroid belt, the four gas giants—Jupiter, Saturn, Uranus, and Neptune—are substantially larger and composed mainly of hydrogen and helium, with Uranus and Neptune often classified as ice giants. The theoretical Oort cloud, a sphere of icy planetesimals, is thought to extend nearly a light-year from the Sun.

Sun

The Sun is a G-type main-sequence star that comprises over 99.8% of the system's total mass. Its core is the site of continuous nuclear fusion, converting hydrogen into helium and releasing immense energy that radiates outward as electromagnetic radiation and the solar wind. This stream of charged particles creates the heliosphere, a vast bubble that shields the planets from interstellar material. Activity on the Sun, such as sunspots, solar flares, and coronal mass ejections, follows an approximately 11-year solar cycle and can significantly impact space weather, affecting satellites and power grids on Earth.

Planets and their moons

The eight planets orbit the Sun in roughly circular paths within a flat disc called the ecliptic. Each inner, rocky planet has a distinct character: Mercury is heavily cratered, Venus has a thick, toxic atmosphere, Earth hosts liquid water and life, and Mars features the largest volcano, Olympus Mons. The gas and ice giants host extensive systems of planetary rings and diverse moons. Jupiter's Galilean moons, such as volcanic Io and icy Europa, are worlds of great scientific interest. Saturn's moon Titan has a dense atmosphere and liquid hydrocarbon lakes, while Neptune's moon Triton orbits in a retrograde direction, suggesting it is a captured Kuiper belt object.

Small bodies

This category encompasses a vast population of objects that are not planets or dwarf planets. The asteroid belt, located between the orbits of Mars and Jupiter, contains millions of rocky and metallic bodies, with Ceres being the largest. Beyond Neptune, the Kuiper belt is a disc-shaped region populated with icy bodies, including Pluto, Haumea, and Makemake. Scattered disc objects like Eris have highly elliptical orbits. Icy bodies originating from the Kuiper belt and Oort cloud, such as Halley's Comet and Comet Hale–Bopp, become visible as comets when their orbits bring them close to the Sun, sublimating their ices to form spectacular tails.

Exploration and observation

For millennia, observations were limited to the naked eye, with early astronomers like Nicolaus Copernicus and Galileo Galilei revolutionizing our understanding of its structure. The launch of Sputnik 1 in 1957 by the Soviet Union inaugurated the space age, enabling robotic exploration. Landmark missions include NASA's Voyager probes, which conducted the Grand Tour of the outer planets and are now in interstellar space, the Cassini–Huygens mission that orbited Saturn, and rovers like Curiosity and Perseverance exploring the surface of Mars. Current telescopes, such as the Hubble Space Telescope and the James Webb Space Telescope, continue to provide detailed observations of distant objects and planetary atmospheres.

Category:Solar System Category:Planetary systems