A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune.
The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. The largest of these objects are the eight planets, which in order from the Sun are four terrestrial planets ; two gas giants ; and two ice giants. The Solar System developed 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc.
In astronomy, a conjunction occurs when two astronomical objects or spacecraft appear to be close to each other in the sky. This means they have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.
In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet's position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since an inferior planet is well within the area of Earth's orbit around the Sun, observation of its elongation should not pose that much a challenge. When a planet is at its greatest elongation, it appears farthest from the Sun as viewed from Earth, so its apparition is also best at that point.
A classical planet is an astronomical object that is visible to the naked eye and moves across the sky and its backdrop of fixed stars. Visible to humans on Earth there are seven classical planets. They are from brightest to dimmest: the Sun, the Moon, Venus, Jupiter, Mars, Mercury and Saturn.
A triple conjunction is an astronomical event when two planets or a planet and a star appear to meet each other three times during a brief period, either in opposition or at the time of inferior conjunction, if an inferior planet is involved. The visible movement of the planet or the planets in the sky appears therefore normally prograde at the first conjunction, retrograde at the second conjunction, and again prograde at the third conjunction.
There is evidence that the formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.
The semi-analytic planetary theory VSOP is a mathematical model describing long-term changes in the orbits of the planets Mercury to Neptune. The earliest modern scientific model considered only the gravitational attraction between the Sun and each planet, with the resulting orbits being unvarying Keplerian ellipses. In reality, all the planets exert slight forces on each other, causing slow changes in the shape and orientation of these ellipses. Increasingly complex analytical models have been made of these deviations, as well as efficient and accurate numerical approximation methods.
A planet symbol or planetary symbol is a graphical symbol used in astrology and astronomy to represent a classical planet or one of the modern planets. The symbols were also used in alchemy to represent the metals associated with the planets, and in calendars for their associated days. The use of these symbols derives from classical Greco-Roman astronomy, although their current altered shapes were developed in the 16th century.
Planet Heroes was a line of toys from Fisher-Price depicting various heroic characters each identified with a specific planet.
Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.
In astrology, planets have a meaning different from the astronomical understanding of what a planet is. Before the age of telescopes, the night sky was thought to consist of two similar components: fixed stars, which remained motionless in relation to each other, and moving objects/"wandering stars", which moved relative to the fixed stars over the course of the year(s).
The Planetary Science Decadal Survey is a serial publication of the United States National Research Council produced for NASA and other United States Government Agencies such as the National Science Foundation. The documents identify key questions facing planetary science and outlines recommendations for space and ground-based exploration ten years into the future. Missions to gather data to answer these big questions are described and prioritized, where appropriate. Similar decadal surveys cover astronomy and astrophysics, earth science, and heliophysics.
The following outline is provided as an overview of and topical guide to the Solar System:
The word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.
Historical models of the Solar System began during prehistoric periods and are updated to this day. The models of the Solar System throughout history were first represented in the early form of cave markings and drawings, calendars and astronomical symbols. Then books and written records became the main source of information that expressed the way the people of the time thought of the Solar System.
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