Planet

planetsFormer classification of planetsplanetary-mass objectplanemoplanetary massplanetary bodiesplanetary mass objectworldinterplanetaryearth
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.wikipedia
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Orbit

orbitsorbital motionplanetary motion
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.

Astronomical object

celestial bodiescelestial bodycelestial object
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
Examples of astronomical objects include planetary systems, star clusters, nebulae, and galaxies, while asteroids, moons, planets, and stars are astronomical bodies.

Gravity

gravitationgravitationalgravitational force
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy— including planets, stars, galaxies, and even light —are brought toward (or gravitate toward) one another.

Clearing the neighbourhood

cleared the neighborhoodcleared its neighborhoodcleared their neighbourhoods
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
"Clearing the neighbourhood around its orbit" is a criterion for a celestial body to be considered a planet in the Solar System.

Earth

terrestrialworldGlobal
The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions.
Earth is the third planet from the Sun and the only astronomical object known to harbor life.

International Astronomical Union

IAUInternational Astronomical Union (IAU)I.A.U.
In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System.
Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies (stars, planets, asteroids, etc.) and any surface features on them.

Planetesimal

planetesimalsasteroid impactsproto
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies.

Pluto

134340 Pluto(134340) Plutomass of Pluto
Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.
After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt.

Deferent and epicycle

epicyclesdeferentdeferents and epicycles
The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions.
In the Hipparchian and Ptolemaic systems of astronomy, the epicycle (from, literally upon the circle, meaning circle moving on another circle ) was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets.

Volcano

volcanicvolcanoesextinct volcano
Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Terrestrial planet

terrestrial planetsrockyrocky planet
Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials.
A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals.

Mercury (planet)

Mercuryplanet MercuryMercurio
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Mercury is the smallest and innermost planet in the Solar System.

Giant planet

jovian planetgiant planetsJovian
Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials.
A giant planet is any massive planet.

Venus

Morning Starevening starCytherocentric
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days.

Jupiter

Jovianplanet JupiterGiove
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Jupiter is the fifth planet from the Sun and the largest in the Solar System.

Uranus

Uranian34 TauriGeorgium Sidus
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Uranus (from the Latin name "Ūranus" for the Greek god Οὐρανός) is the seventh planet from the Sun.

Neptune

NeptunianNeptune-masseighth planet
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Neptune is the eighth and farthest known planet from the Sun in the Solar System.

3 Juno

Junoasteroid
Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.
It was the third asteroid found, but was initially considered to be a planet; it was reclassified as an asteroid and minor planet during the 1850s.

Natural satellite

moonmoonssatellite
Six of the planets are orbited by one or more natural satellites.
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body).

Orbital pole

ecliptic poleeclipticecliptic north pole
As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons.
An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit (of a revolving body like a planet) and is perpendicular to the orbital plane.

2 Pallas

Pallasasteroid Pallas
Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.
When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was counted as a planet, as were other asteroids in the early 19th century.

Hydrostatic equilibrium

hydrostatic balanceequilibriumhydrostatic
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
There are 32 observationally confirmed such objects (apart from the Sun), sometimes called planemos, in the Solar System, seven more that are virtually certain, and a hundred or so more that are likely.

Astronomer

astronomersastrophysicistprofessional astronomers
As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons.
They observe astronomical objects such as stars, planets, moons, comets, and galaxies – in either observational (by analyzing the data) or theoretical astronomy.

Milky Way

galaxyMilky Way Galaxyour galaxy
Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way.
It is estimated to contain 100–400 billion stars and more than 100 billion planets.

Astronomy

astronomicalastronomerastronomers
The five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy.
Objects of interest include planets, moons, stars, nebulae, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation.