Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist between them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Dynamic characteristics

See also: Kepler's laws of planetary motion

Orbit

The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).

All planets revolve around stars. In the Solar System, all the planets orbit in the same direction as the Sun rotates. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its sidereal period or year.^[78] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.^[79]

Each planet's orbit is delineated by a set of elements:

• The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.^[78] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.^[80][81]
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  Illustration of the semi-major axis
  The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, as no planet's orbit has its star at its exact centre.^[78]
• The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth.^[82] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.^[83] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.^[78] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.^[78]

Axial tilt

Earth's axial tilt is about 23°.

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.^[84] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.^[85]

Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise^[86] (Uranus is generally said to be rotating clockwise as well^[87] though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction).^[88] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.^[89] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.^[90]

Orbital clearance

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.^[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.