Evolutionary:Example system
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The Sol system, also called the Solar System, is a single-star system consisting of the Sun and everything bound to it by gravity: eight planets, at least five dwarf planets, hundreds of moons, and countless smaller bodies. It lies in the Orion Arm of the Milky Way, roughly 26,000 light-years from the galactic centre.
Sol is the home system of humans, and its third planet, Earth, is the only world in it known to host life. Its planets divide neatly into two groups: four small, rocky inner worlds, and four giant outer worlds made mostly of gas and ice.
Overview
The Sun contains about 99.86 percent of the system's total mass. It is a G-type main-sequence star, about 4.6 billion years old and roughly halfway through its life on the main sequence. Its light reaches Earth in about 8 minutes and 20 seconds.
The system formed around 4.6 billion years ago from the collapse of part of a cold molecular cloud. Most of the material gathered at the centre to form the Sun, while the remainder flattened into a spinning disc in which the planets assembled. This is why all eight planets orbit in nearly the same plane, and in the same direction.
Beyond the planets lie three reservoirs of smaller bodies:
- The asteroid belt, between Mars and Jupiter, home to rocky and metallic asteroids and the dwarf planet Ceres.
- The Kuiper belt, a wide ring of icy bodies beyond Neptune, including the dwarf planet Pluto.
- The Oort cloud, a vast, thinly scattered shell of icy objects thought to extend more than a light-year from the Sun, and the source of long-period comets.
Planets
Mercury
Mercury is the smallest planet and the closest to the Sun. With almost no atmosphere to hold heat, its surface swings between some of the most extreme temperatures in the system. Its heavily cratered surface resembles that of Earth's moon.
Venus
Venus is almost the same size as Earth, but a runaway greenhouse effect has made it the hottest planet in the system, hot enough to melt lead. Thick clouds of sulphuric acid hide its surface completely.
Earth
Earth is the only planet in the system with liquid water on its surface and the only one known to support life. Its unusually large moon stabilises the tilt of its axis, keeping its seasons and climate relatively steady over long periods. It is the homeworld of humans.
Mars
Mars is a cold desert world whose red colour comes from iron oxide dust. Dry riverbeds and lake deposits show that liquid water once flowed on its surface. It is home to Olympus Mons, the tallest known volcano in the system.
Jupiter
Jupiter is the largest planet, more than twice as massive as all the other planets combined. Its Great Red Spot is a storm wider than Earth that has raged for centuries. Its moon Europa is thought to hide a liquid water ocean beneath its icy crust.
Saturn
Saturn is best known for its bright ring system, made of countless pieces of water ice and rock. It is the least dense planet, less dense than water. Its largest moon, Titan, has a thick atmosphere and lakes of liquid methane.
Uranus
Uranus is tilted almost completely on its side, with an axial tilt of about 98 degrees, so each pole spends decades in continuous sunlight followed by decades of darkness. Methane in its atmosphere gives it a pale cyan colour.
Neptune
Neptune is the outermost planet and the windiest, with storms reaching over 2,000 km/h. It was the first planet found by mathematical prediction rather than direct observation. Its largest moon, Triton, orbits backwards and is probably a captured Kuiper belt object.
Life
Earth is the only place in the system where life is known to exist, including its sole sapient species, humans. Several other worlds are considered possible homes for simple life, past or present:
- Mars, which had liquid water on its surface billions of years ago.
- Europa (Jupiter) and Enceladus (Saturn), which have liquid water oceans beneath their icy crusts.
- Titan (Saturn), with its thick atmosphere and complex organic chemistry.
History
Life appeared on Earth at least 3.5 billion years ago, less than a billion years after the planet formed. Humans began exploring the system directly in the 20th century: the first artificial satellite reached orbit in 1957, the first humans landed on Earth's moon in 1969, and robotic probes have since visited every planet. Voyager 1, launched in 1977, became the first spacecraft to reach interstellar space in 2012.
Behind the scenes
- Written as the example star system page for this wiki, using real-world facts about the Solar System.