The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light, ultraviolet light, and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth. Its mass is about 330,000 times that of Earth, and it accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron. According to its spectral class, the Sun is a G-type main-sequence star (G2V). As such, it is informally, and not completely accurately, referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god Mercurius (Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (Ἑρμῆς). Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, and its apparent distance from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight. At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus. From Earth, the planet telescopically displays the complete range of phases, similar to Venus and the Moon, which recurs over its synodic period of approximately 116 days.
Venus is the second planet from the Sun and is named after the Roman goddess of love and beauty. As the brightest natural object in Earth's night sky after the Moon, Venus can cast shadows and can be visible to the naked eye in broad daylight. Venus's orbit is smaller than that of Earth, but its maximal elongation is 47°; thus, at latitudes with a day-night cycle, it is most readily visible for up to a few hours following the start of sunset or before sunrise. At times, it has been seen in a completely dark sky. Venus orbits the Sun every 224.7 Earth days. It has a synodic day length of 117 Earth days and a sidereal rotation period of 243 Earth days. Consequently, it takes longer to rotate about its axis than any other planet in the Solar System, and does so in the opposite direction to all but Uranus. This means that the Sun rises from its western horizon and sets in its east. Venus does not have any moons, a distinction it shares only with Mercury among the planets in the Solar System
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface is made up of the ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29% of Earth's surface is land, consisting of continents and islands. Earth's surface layer is formed of several slowly moving tectonic plates, interacting to produce mountain ranges, volcanoes, and earthquakes. Earth's liquid outer core generates the magnetic field that shapes Earth's magnetosphere, deflecting destructive solar winds. Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. Water vapor is widely present in the atmosphere and forms clouds that cover most of the planet. Greenhouse gases in the atmosphere like carbon dioxide (CO2) trap a part of the energy from the Sun close to the surface. A region's climate is governed by latitude, but also by elevation and proximity to moderating oceans. Severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs in most areas and greatly impacts life.
The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia), it is the fifth largest satellite in the Solar System, the largest satellite in the Solar System relative to its major planet, and larger than any known dwarf planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density. Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar.
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury. In English, Mars carries the name of the Roman god of war. Mars is a terrestrial planet with a thin atmosphere, and has a crust primarily composed of elements similar to Earth's crust, as well as a core made of iron and nickel. Mars has surface features such as impact craters, valleys, dunes, and polar ice caps. It also has two small and irregularly shaped moons, Phobos and Deimos. Some of the most notable surface features on mars include features such as Olympus Mons, the largest volcano and highest known mountain on any Solar System planet, and Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the Northern Hemisphere covers 40% of the planet and may be a giant impact feature. The days and seasons on Mars are comparable to those of Earth as the planets have a similar rotation period and tilt of the rotational axis relative to the ecliptic plane. Liquid water on the surface of Mars cannot exist due to low atmospheric pressure, which is less than 1% of the atmospheric pressure on Earth. Both of Mars's polar ice caps appear to be made largely of water.
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after the Roman god Jupiter, the king of the gods. Jupiter is primarily composed of hydrogen, but helium constitutes one-quarter of its mass and one-tenth of its volume. It probably has a rocky core of heavier elements, but, like the other giant planets in the Solar System, it lacks a well-defined solid surface. The ongoing contraction of Jupiter's interior generates more heat than it receives from the Sun. Because of its rapid rotation, the planet's shape is an oblate spheroid: it has a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm which has been observed since at least 1831.
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. Saturn's interior is most likely composed of a core of iron–nickel and rock (silicon and oxygen compounds). Its core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally, a gaseous outer layer. Saturn has a pale yellow hue due to ammonia crystals in its upper atmosphere. An electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's, but which has a magnetic moment 580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's. The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph; 500 m/s), higher than on Jupiter but not as high as on Neptune.
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of Cronus (Saturn). It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the other giant planets. As with gas giants, ice giants also lack a well defined "solid surface." Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C; −371 °F), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock
Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, and slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. It is referred to as one of the solar system's two ice giant planets (the other one being its near-twin Uranus). Being composed primarily of gases and liquids, it has no well-defined "solid surface." The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km; 2.8 billion mi). It is named after the Roman god of the sea and has the astronomical symbol ♆, representing Neptune's trident.