The Solar System: Formation and Overview

• The Solar System is a gravitationally bound system that includes the Sun and all objects orbiting it, directly or indirectly.
• The largest objects orbiting the Sun directly are the eight planets, with dwarf planets and small Solar System bodies making up the remainder.
• Two moons that orbit planets indirectly are larger than Mercury, the smallest planet.

General Overview

• The Solar System's formation occurred 4.6 billion years ago via the gravitational collapse of a giant interstellar molecular cloud.
• Most of the system's mass is concentrated in the Sun, while Jupiter contains the majority of the remaining mass.
• Mercury, Venus, Earth, and Mars are the four inner, terrestrial planets, primarily composed of rock and metal.
• The four outer planets are giant planets, possessing significantly more mass than the terrestrial planets.
• Jupiter and Saturn, the two largest, are gas giants mainly composed of hydrogen and helium.
• Uranus and Neptune, the two outermost planets, are ice giants, composed mostly of substances that freeze at relatively high temperatures like water, ammonia, and methane.
• Nearly all planets orbit the Sun in a plane called the ecliptic, with their orbits nearly parallel to it.
• The Solar System includes smaller objects like asteroids and trans-Neptunian objects.
• The asteroid belt, located between Mars and Jupiter, mainly contains small Solar System bodies made of rock and metal.
• The Kuiper belt and scattered disc exist beyond Neptune's orbit and are populated by trans-Neptunian objects mostly made of ice.
• Several large objects within these populations, including Ceres, Pluto, Haumea, Makemake, and Eris, are classified as dwarf planets.
• Comets, centaurs, and interplanetary dust clouds are various populations of smaller bodies that travel between the regions.
• The Solar System orbits about 27,000 light-years from the Galactic Center within the Milky Way galaxy.

Structure and Composition

• The Sun, a G-type main-sequence star, is the Solar System's main component, containing 99.86% of its known mass and dominating it gravitationally.
• Jupiter, Saturn, Uranus, Neptune, and Earth account for 99% of the remaining mass after the Sun.
• All Solar System objects orbit the Sun.
• The Sun, composed of approximately 98% hydrogen and helium, accounts for 99.86% of the Solar System's mass.
• Jupiter and Saturn, which comprise nearly all the remaining mass, are also composed of roughly 98% hydrogen and helium.
• A compositional gradient exists in the Solar System due to heat and light pressure from the Sun, leading to objects closer to the Sun being composed of elements with higher melting points.
• Objects farther from the Sun consist largely of materials with lower melting points.
• The region within Jupiter's orbit is termed the inner Solar System.
• All objects in the inner Solar System are composed of rock.

Inner Solar System

• The four inner/terrestrial planets have high densities, primarily consist of rock, have few or no moons, and lack ring systems.
• They are largely made of refractory minerals like silicates (crusts and mantles) and metals like iron and nickel (cores).
• Venus, Earth, and Mars possess substantial atmospheres that generate weather, impact craters, and tectonic surface features like rift valleys and volcanoes.
• "Inner planet" shouldn't be confused with "inferior planet," which refers to Mercury and Venus, as they are closer to the Sun than Earth.

Inner Planets

• Mercury is the closest planet to the Sun and the smallest in the Solar System, with a mass of 0.055 Earth masses.
• Mercury has a heavily cratered surface due to its thin atmosphere, and no known natural satellites.
• Venus is similar in size to Earth (0.815 Earth masses), with a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity.
• Venus is much drier than Earth, and its atmosphere is ninety times as dense, and it lacks natural satellites.
• Earth is the largest and densest of the inner planets (one Earth mass), harbors life, and exhibits current geological activity.
• Earth's liquid hydrosphere and plate tectonics are unique among the terrestrial planets, and its atmosphere contains 21% free oxygen due to life.
• Earth has one natural satellite, the Moon, which is the only large satellite of a terrestrial planet in the Solar System.
• Mars (0.107 Earth masses) is smaller than Earth and Venus.
• Mars has a tenuous atmosphere of primarily carbon dioxide.
• Vast volcanoes like Olympus Mons and rift valleys such as Valles Marineris indicate geological activity that may have persisted until as recently as 2 million years ago.
• Mars has two tiny natural satellites, Phobos and Deimos, which are thought to be captured asteroids.

Asteroid Belt

• The asteroid belt is located between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun.
• It contains numerous irregularly shaped bodies of varying sizes, known as asteroids or minor planets.
• The asteroid belt is thought to be remnants from the Solar System's formation that failed to coalesce due to Jupiter's gravitational interference.
• The asteroid belt contains millions of objects over one kilometer in diameter but their total mass is unlikely to exceed one-thousandth of Earth's mass.
• The asteroid belt isn't densely populated, allowing spacecraft to pass through it routinely.
• Asteroids are categorized by composition.
• Carbonaceous (C-type) asteroids are the most common, making up over 75% of the belt's asteroids and consisting of clay and silicate rocks.
• S-type asteroids (silicate-rich) are the next most common, dominating the inner belt and composed of silicate materials and nickel-iron.
• M-type asteroids (metallic) make up the remainder.
• Main-belt asteroids range in size from hundreds of kilometers across to microscopic dust grains.
• All asteroids except Ceres are classified as small Solar System bodies.
• Ceres, the only dwarf planet in the asteroid belt, has a diameter of about 975 km.

Outer Solar System

• The outer Solar System contains giant planets and their large moons.
• Many short-period comets, including Jupiter-family comets, also orbit in this region.
• Solid objects in the outer Solar System contain a higher proportion of volatiles like water, ammonia, and methane due to lower temperatures.

Outer Planets

• The four outer (giant/Jovian) planets make up 99% of the mass known to orbit the Sun.
• Jupiter and Saturn are gas giants, mainly composed of hydrogen and helium.
• Uranus and Neptune are ice giants, containing more water, ammonia, methane, and other heavier elements.
• All four giant planets have rings, but only Saturn's are easily visible from Earth.
• The term "outer planet" shouldn't be confused with "superior planet," which refers to any planet outside Earth's orbit.

Gas Giants

• Jupiter, with 318 Earth masses, is 2.5 times the mass of all the other planets combined.
• Jupiter radiates more heat than it gets from the Sun and is largely composed of hydrogen and helium.
• Jupiter has 79 known satellites.
• Ganymede, Callisto, Io, and Europa are the four largest satellites, and they exhibit similarities to terrestrial planets, like volcanism and internal heating.
• Ganymede is the largest satellite in the Solar System and is larger than Mercury.
• Saturn (95 Earth masses) has an extensive ring system and shares characteristics with Jupiter, like atmospheric composition and magnetosphere.
• Saturn has 62 confirmed satellites, 53 of which are officially named.
• Titan and Enceladus, two of Saturn's satellites, show geological activity.
• Titan is the second-largest moon in the Solar System, is larger than Mercury, and is the only satellite with a substantial atmosphere.

Ice Giants

• Uranus (14 Earth masses) is the lightest of the outer planets.
• Uranus orbits the Sun on its side, with an axial tilt of over ninety degrees to the ecliptic.
• Uranus has a much colder core than the other giant planets and radiates very little heat into space.
• Uranus has 27 known satellites, with the Titania, Oberon, Umbriel, Ariel, and Miranda being the largest.
• Neptune (17 Earth masses) is denser and slightly more massive than Uranus.
• Neptune radiates more internal heat but not as much as Jupiter or Saturn.
• Neptune has 14 known satellites.
• Triton, the largest, is retrograde and dynamically bound to Neptune; it is thought to have formed independently and was later captured.
• Triton is geologically active and has geysers of liquid nitrogen.

Trans-Neptunian Region

• The trans-Neptunian region or outer Solar System, beyond Neptune, remains largely unexplored.
• Small, icy worlds primarily populate it, with the largest exceeding a quarter of Earth's diameter.

Explore the Solar System, a gravitationally bound system including the Sun and orbiting objects. Formed 4.6 billion years ago, it consists of eight planets, dwarf planets, and moons. The system is dominated by the Sun's mass, with Jupiter holding most of the remaining mass.