A planet is an astronomical body orbiting a star or a stellar remnant that is massive enough to be rounded by its own gravity, but is not massive enough to cause thermonuclear fusion. It also has cleared its neighbouring region of planetesimals (a group of bodies come together by gravity to form a planet). The term planet means 'wandering star'.
In 2006, the International Astronomical Union (IAU) started defining planets, as objects of planetary mass based on where or what they orbit, as well as size. This is when Pluto was officially named a Dwarf planet rather than an actual planet. In the 17th Century, they realised planets orbit the Sun, and an astronomer named Kepler found planets' orbits to be elliptical not circular.
It has now been discovered that there are several thousand planets around other stars (extrasolar planets or exoplanets), which range from sizes just larger than our moon to gas giants twice the size of Jupiter. Some of these are at relative distances from their stars, as Earth is from the Sun, making them in the habitable/Goldilocks zone. About 1 in 5 sun-like stars are thought to have an Earth-sized planet in the habitable zone, meaning there could be life on any of them.
Planetary Characteristics
Orbit - according to all current definitions, all planets must revolve around stars.
In our solar system, all planets orbit the sun in the same direction as the sun rotates (anticlockwise as seen from above the sun's north pole). The period of one revolution of a planet's orbit is known as its sidereal period or year. Planet's years depend on their distance from their star - the further it is, the longer it has to travel and the slower it travels, because it is less affected by the star's gravity. No planet's orbit is circular; they all have elliptical orbits. Planets have varying approaches to their orbits: the closest approach of a planet to its' sun is called the periastron (perihelion in our solar system), and its furthest separation is called its apastron (aphelion). As a planet approaches its periastron, its speed increases, because it trades gravitational potential energy for kinetic energy. As a planet reaches apastron its speed decreases. The eccentricity of a planet's orbit describes how elongated it is. Low eccentricities have more circular orbits whilst high eccentricities have more elliptical orbits. Planets in our solar system have very low eccentricities, meaning they have nearly circular orbits.
Inclination of a planet - this describes how far above or below a plane its orbit lies. In our solar system, the reference plane is the ecliptic and the 8 planets lie close to the ecliptic. (Points at which a planet crosses above and below its reference plane are called its ascending and descending nodes).
Axial tilt - this is the angle to the plane of their stars' equator, and it varies for each planet - Earth's is 23.4°.
Rotation - planets rotate around invisible axes through their centres. A stellar day is the name of the planets' rotation period. All the planets in our solar system rotate anti-clockwise except Venus & Uranus (Uranus has an extreme axial tilt, meaning there is debate which is its north pole, and therefore whether it rotates anticlockwise or clockwise).
Physical characteristics of the planets in our solar system
Every planet began in a fluid state, so the denser, heavier materials sunk to the centre, leaving the lighter materials near the surface. The denser core is surrounded by mantle that either is or was fluid.
Terrestrial planets are sealed within hard crusts, with cores of elements eg. iron and nickel and mantles of silicates. Jupiter and Saturn have cores of rock & metal surrounded by mantles of metallic hydrogen, whilst Uranus and Neptune have rocky cores surrounded by mantles of water, ammonia, methane and other ices.
The mantle of the giant gas planets (Jupiter, Saturn, Uranus and Neptune) blends into its upper cloud layers.
All the planets in our solar system except Mercury have substantial atmospheres, because gravity is strong enough to keep the gases close to their surface.
The presence of a magnetic field indicates that a planet is still geologically alive. Magnetised planets have flows of electrically conducting material in their interiors. The magnetic field of a planet can be larger than the planet itself. Non-magnetised planets only have a small magnetosphere, which cannot effectively protect the planet from things like solar flares. In our solar system, only Venus and Mars lack a magnetic field, so they have had to adopt other methods to protect themselves. Mercury has the weakest magnetic field of the rest.
All the planets in our solar system except Venus and Mercury have moons.
The 4 giant gas planets in our solar system (Jupiter, Saturn, Uranus and Neptune) are orbited by planetary rings, which are primarily dust. They are believed to have originated from natural satellites which were torn apart by tidal forces.
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