Solar System

The Solar System is a gravitationally bound system of celestial bodies. It consists of the Sun and all the masses that orbit it. This includes the eight major planets and many other smaller objects. The Sun is the most important part of this system. It contains 99.86% of the total mass of the entire Solar System. Because the Sun is so massive, its gravity holds everything else in place.

Our system formed about 4.6 billion years ago. It began when a dense region of a molecular cloud collapsed. As this nebula contracted, it began to rotate faster due to the conservation of angular momentum. This spinning motion caused the material to flatten into a protoplanetary disc. At the center, a hot and dense protostar began to form. Within 50 million years, the pressure in the center became high enough for thermonuclear fusion. This is the process where hydrogen atoms fuse into helium. This reaction releases the massive amounts of energy that make the Sun shine.

The structure of the Solar System is divided into distinct regions. The inner Solar System contains the terrestrial planets. These are Mercury, Venus, Earth, and Mars. These planets are composed mostly of metals and silicates. They are relatively small because they formed inside the soot line. Heat from the early Sun prevented lighter materials from staying solid here. Beyond the frost line lies the outer Solar System. This region contains the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune. These massive planets formed where it was cold enough for volatile icy compounds to remain solid.

Many objects in the system do not qualify as major planets. Dwarf planets are objects with planetary mass that do not dominate their orbits. The International Astronomical Union recognizes Ceres, Pluto, Eris, Makemake, and Haumea as dwarf planets. Other objects like Orcus, Quaoar, Gonggong, and Sedna are also identified as dwarf planets.

Smaller bodies include asteroids, comets, centaurs, and meteoroids. Many of these are found in the asteroid belt between Mars and Jupiter. Others reside in the Kuiper belt or the distant, theorized Oort cloud. The Oort cloud is a vast region that may extend to 227,000 AU from the Sun.

Natural satellites, or moons, are also found throughout the system. Many objects do not orbit the Sun directly but orbit larger bodies instead. These moons vary greatly in size. Some are tiny moonlets, while others are massive. For example, Jupiter's moon Ganymede and Saturn's moon Titan are larger than the planet Mercury. However, they have less mass than Mercury. The seven most massive moons in the system are larger and more massive than any dwarf planet.

The Sun's influence creates a protective bubble called the heliosphere. The solar wind, which is a stream of charged plasma particles, flows outward from the Sun. This wind creates an interplanetary medium filled with dust, gas, and cosmic rays. At about 70 to 90 AU from the Sun, the solar wind hits the interstellar medium. This boundary is known as the heliopause. The entire Solar System is currently moving through a region called the Local Cloud. Our closest neighboring star is Proxima Centauri, located 269,000 AU away.

The Solar System is currently in a stable state, but it will change in the future. The Sun's main-sequence life will last about 10 billion years. In roughly 5 billion years, the Sun will run out of hydrogen in its core. It will then expand into a red giant, growing to 260 times its current diameter.

This expansion may vaporize Mercury and Venus. It could also make Earth and Mars uninhabitable. Eventually, the Sun will eject its outer layers and leave behind a white dwarf. This is a dense core about the size of Earth. The ejected material may form a planetary nebula, returning elements to space.