Planetary Core

Lore & Background

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars. As the pre-solar nebula collapsed, conservation of angular momentum caused it to rotate faster, flattening into a protoplanetary disc with a diameter of roughly 200 AU. The planets formed by accretion from this disc. In the inner Solar System, heat from the accretion process exceeded the boiling point of hydrocarbon molecules for the first million years, leading to low carbon content for the inner planets. The giant planets formed further out, beyond the frost line at about five astronomical units, where volatile icy compounds could remain solid. Within 50 million years, the protostar began thermonuclear fusion. Solar wind from the Sun created the heliosphere and swept away remaining gas and dust from the protoplanetary disc into interstellar space.

In Their Own Story

The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun. Although it has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years. The Sun's main-sequence phase will last about 10 billion years. Roughly 5 billion years from now, the hydrogen in the core of the Sun will be entirely converted to helium, marking the end of its main-sequence life. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable.

Reader's Guide

The Solar System includes the Sun and all objects that are bound to it by gravity and orbit it. The International Astronomical Union describes the Solar System as all objects that are bound by the gravity of the Sun, the Sun itself, its eight planets, and the other celestial bodies which orbit it. NASA describes the solar system as including the Sun and its planetary system. Capitalization of the name varies; when not used as a proper noun, 'solar system' may refer to either the Solar System itself or any planetary system reminiscent of it. The International Astronomical Union specifies capitalizing the names of all individual astronomical objects but uses mixed 'Solar System' and 'solar system' structures in their naming guidelines document.

Did You Know?

• The Sun accounts for 99.86% of the Solar System's total mass.
• Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.
• The largest two moons, Ganymede and Titan, are larger than the smallest planet Mercury.
• The Solar System formed at least 4.568 billion years ago.
• The Oort cloud extends from somewhere beyond 2,000 AU to the edge of the Sun's Hill sphere at up to 3.59 light-years.

Origins and Stellar Dominance

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars. As the pre-solar nebula collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings. As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the center. The planets formed by accretion from this disc. The Sun accounts for 99.86% of the Solar System's total mass. Inside the Sun's core, hydrogen is fused into helium, releasing energy that is emitted through the Sun's photosphere. This creates the heliosphere and a decreasing temperature gradient across the Solar System.

Planetary Architecture and Habitability

The eight planets dominate the orbits they occupy. Closest to the Sun in order of increasing distance are the terrestrial planets – Mercury, Venus, Earth and Mars. These four planets are part of the inner Solar System. Earth and Mars are the only planets that orbit within the Sun's habitable zone, in which sunlight can keep surface water liquid under atmospheric pressure. Beyond the frost line at about five astronomical units (AU), are the planets of the outer Solar System: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.

Minor Bodies and Dwarf Planets

Objects of planetary mass that do not dominate their orbit but directly orbit the Sun are called dwarf planets. The International Astronomical Union's Minor Planet Center lists Ceres, Pluto, Eris, Makemake, and Haumea as dwarf planets. Four other Solar System objects are generally identified as such: Orcus, Quaoar, Gonggong, and Sedna. Less massive than the dwarf planets are the vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. The dwarf planet Ceres and many of these smaller bodies are located in the asteroid belt (between Mars's and Jupiter's orbit), while all other dwarf planets are members of populations of trans-Neptunian objects, which may be found in the Kuiper belt just outside Neptune or in the further scattered disc.

Satellites and Outer Boundaries

Many objects in the Solar System do not orbit the Sun directly and are instead natural satellites, commonly called 'moons', of larger bodies. These can be found throughout the Solar System in sizes from planetary-mass moons at their largest to much less massive moonlets at their smallest. The largest two moons (Ganymede of Jupiter and Titan of Saturn) are larger (though less massive) than the smallest planet (Mercury), while the seven most massive, which includes Earth's Moon, are more massive and larger than any of the dwarf planets. Within the heliosphere, the Solar System is constantly flooded by the charged plasma particles of the solar wind, which, along with interplanetary dust, gas and cosmic rays, form an interplanetary medium between the bodies of the Solar System. At around 70–90 AU from the Sun, the solar wind is halted by the interstellar medium, resulting in the heliopause and the border of the interplanetary medium to interstellar space. The theorized Oort cloud – the source for long-period comets – extends from somewhere beyond 2,000 AU to the outermost region of the Solar System. The latter reaches to the edge of the Sun's Hill sphere, at 178,000–227,000 AU (2.81–3.59 ly), where its gravitational potential becomes equal to the galactic potential.