Planet

The term “planet” originally comes from the Greek word for “wanderer” since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAUThe International Astronomical Union (IAU) was founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also works to promote research, education and public outreach activities in astronomy for the public. last defined the term planet in 2006, however the new definition has remained controversial. The IAU’s definition applies only to our Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. and so the definition below has been edited for clarity and generalized to apply to extrasolar planets (exoplanets) as well.

1. A celestial body that orbits a starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. or stellar remnant.
2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibriumBalance between gravity and gas pressure. In the case of a star, gravity originates in mutual gravitational attraction of the entire mass of the star. Gas pressure is produced by nuclear reactions that heat the star's gas. Any change in the internal temperature of a star results in a change (nearly round) shape, but not massive enough to become a star.
3. Has cleared the gravitational neighborhood around its orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around.

For an object to assume a round or nearly round shape, the object should generally have a mass above 5 x 10²⁰ kg and diameter greater than 800 km. However, borderline cases would have to be established by observation.

The original draft of the 2006 IAU resolution redefined hydrostatic equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static shape as applying “to objects with mass above 5 × 10²⁰ kg and diameter greater than 800 km”, but this was not retained in the final draft since the composition and mass of an object determine the minimum diameter to achieve a rounded shape. Even though bodies made of denser materials become spherical at smaller radii, an objects composition and associated compressibility tend to drive the minimum diameter. Therefore, for bodies made mainly of rock, the minimum diameter to assume a round or nearly round shape is about 600 km diameter. For bodies made mainly of ice, the minimum diameter drops to about 400 km diameter¹.

The IAU accepted planets of our Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with closest to farthest from the SunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A stellar memory device (mnemonic) can be seen on the page Remember the Planets!.

The Case For and Against Pluto

Since, according to the IAU’s new definition, Pluto has not cleared its orbital space and shares it with many other objects in the Kuiper BeltRegion in the outer solar system beyond Neptune's orbit that contains billions of small, icy planetesimals from the original protoplanetary disc that failed to coalesce into planets. The Kuiper Belt extends from Neptune's orbit at 30 AU to ~55 AU. It is ~20x wider and 20-200x more massive than the, it was reclassified as a dwarf planet. However, not all scientists agree. Dr. Alan Stern argues that the requirement for clearing the neighborhood or zone is “sloppy” based on the following reasons summarized from his interview Fighting for Pluto’s Planet Title and from this author’s additional research:

1. No planets in our Solar System orbit in a fully cleared zone. All have some form of asteroids, comets or Kuiper Belt objectsGenerally, small bodies found in the Kuiper Belt. The largest Kuiper Belt Object (KBO), Eris, is <3000 km in diameter. Image source: http://en.wikipedia.org/wiki/File:EightTNOs.png passing through their orbital zone. These objects typically have orbits that intersect Pluto’s orbit, but do not necessarily share its same orbit. However, even if we assume a stricter definition of clearing the neighborhood of objects that share the same orbit, even this requirement cannot be met by other planets in our Solar System. For example, the Trojan asteroids are comprised of two large groups of asteroids that orbit ahead and behind Jupiter in the same orbit around the Sun as Jupiter.
2. At increasingly farther distances from the sun, an object’s orbit increases and it’s orbital speed decreases, therefore zone clearing takes longer and is more difficult. This situation imposes a distance dependence where identical objects don’t classify identically at different distances from the Sun. As Alan Stern puts it, “when people say Pluto can’t clear its orbital zone, they should be fair and also point out that planet Earth also couldn’t clear a zone this far out, so the IAU definition would exclude an Earth – and a Mercury,a Venus or a Mars – at Pluto’s distance.”

Geophysical Definition of a Planet

This continued controversy led to the following geophysical definition for planet being proposed by Runyon et al., #1448 (2017), and emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties. This definition currently describes ~110 known planets in the solar system.

“A planet is a sub-stellar mass body that has never undergone nuclear fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”