Defining a Planet: A Journey Through Space and Time

Outer space, often perceived as a vast and empty void, teems with celestial objects. From stars and nebulae to asteroids, comets, and planets, the universe is far from barren. But what exactly defines a planet? Why was Pluto reclassified as a dwarf planet, and what distinguishes a planet from other space objects? Understanding the definition of a planet requires exploring its historical context and the criteria established by the International Astronomical Union (IAU).

Celestial Objects and Bodies: Setting the Stage

Before delving into the specifics of planetary definitions, it's essential to clarify some fundamental terminology. A celestial object, also known as an astronomical object, is any natural object located outside Earth's atmosphere. This broad definition encompasses star clusters, galaxies, and nebulae. Nebulae, for instance, are large masses of gas and dust without clearly defined boundaries.

A celestial body, or astronomical body, is a type of celestial object with a cohesive structure, not part of a larger group. Examples include the Sun, the Moon, and planets. The term "celestial object" is more general, potentially including objects with substructures, while a celestial body has definite boundaries. A galaxy is a celestial object, while individual planets and stars within the galaxy are celestial bodies.

What is a Planet?

A planet is a large, rounded astronomical body that orbits a star, stellar remnant, or brown dwarf, but is not itself a star. Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies.

The word "planet" comes from the Greek πλανήται (planḗtai) 'wanderers'. In antiquity, this word referred to the Sun, Moon, and five points of light visible to the naked eye that moved across the background of the stars-namely, Mercury, Venus, Mars, Jupiter, and Saturn.

The IAU Definition

The official definition of a planet comes from the International Astronomical Union (IAU), an organization of over 13,000 professional astronomers worldwide. In 2006, the IAU established the following criteria for a celestial body to be considered a planet:

It must orbit the Sun.
It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
It must have "cleared the neighborhood" around its orbit. This means it has become gravitationally dominant, and there are no other bodies of comparable size in its orbital path.

A celestial body must meet all three criteria to be classified as a planet. It’s the third point that got Pluto re-classified as not a true planet. Because Pluto shares its orbit with Charon and with many Kuiper Belt Objects, it has cannot be said to have cleared its neighborhood.

Dwarf Planets

The IAU created a new category, dwarf planets, to include celestial bodies like Pluto that meet the first two criteria but not the third. Dwarf planets:

Are not satellites of other planets.
Orbit the Sun.
Have achieved hydrostatic equilibrium (a nearly round shape).
Have not cleared their orbit of other celestial bodies.

Examples of dwarf planets include Pluto, Ceres, Eris, Haumea, and Makemake.

In June 2008 the IAU created a new category, plutoids, within the dwarf planet category. Plutoids are dwarf planets that are farther from the Sun than Neptune; that is, they are the largest objects in the Kuiper belt. Two of the dwarf planets, Pluto and Eris, are plutoids; Ceres, because of its location in the asteroid belt, is not.

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Moons

A moon, also known as a natural satellite, is a celestial body that orbits a planet or asteroid. There are over 200 moons in our solar system. Earth has one moon, while Jupiter has 79 known moons, and Saturn has 82.

Exoplanets

An exoplanet is a planet outside our solar system, orbiting a star other than our Sun. As of the writing of this article, over 4,000 confirmed exoplanets exist, with an additional 5,000 candidates.

A Historical Perspective: How Our Understanding Evolved

The definition of a planet has evolved significantly over time, reflecting advancements in observational capabilities and scientific understanding.

Ancient Observations

Ancient civilizations worldwide observed the night sky, developing explanations for the celestial objects they saw. Some objects appeared fixed, while others seemed to revolve around Earth.

In ancient Babylonia, astronomers recognized the periodic nature of astronomical phenomena and applied mathematics to their observations as early as three thousand years ago. In the first century BCE, Greek mathematicians developed theories to explain the movement of planets and other celestial bodies.

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Ptolemy, a Greek scientist, wrote the Almagest in the second century CE, which became widely accepted for centuries. Ptolemy's geocentric model placed Earth at the center of the universe, with all other planets revolving around it. He accounted for Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon.

The Heliocentric Revolution

While some scientists and philosophers proposed heliocentric models (with the Sun at the center) before Ptolemy, especially in India and the Islamic world, geocentrism remained dominant in Europe for centuries.

In 1543, Nicolaus Copernicus published "On the Revolutions of the Heavenly Spheres," presenting a new heliocentric model. Although initially slow to gain acceptance, the work of Johannes Kepler, Galileo Galilei, and Isaac Newton in the 17th and 18th centuries eventually led to the widespread understanding that planets orbit the Sun.

Discovering New Planets

The "classical planets"-Mercury, Venus, Mars, Jupiter, and Saturn-were known for millennia because they are visible to the naked eye. Discovering Uranus and Neptune required technological advancements. William Herschel found Uranus in 1781 using a telescope, and Johann Galle located Neptune in 1846 based on mathematical predictions by Urbain Le Verrier.

The Pluto Debate and the IAU Decision

By the 1990s, advanced telescopes detected numerous icy bodies in the Kuiper Belt, beyond Neptune's orbit, where Pluto resides. This raised questions about Pluto's planetary status, as it appeared more similar to Kuiper Belt Objects (KBOs) than the other planets.

Pluto is much smaller than Mercury and even smaller than some planetary moons. Its largest moon, Charon, is close in size, and they share an orbit around a point in space between them, resembling a binary star system.

In 2005, astronomers discovered Eris, a KBO similar in size to Pluto, further fueling the debate about what defines a planet. This prompted the IAU to establish the official definition of a planet in 2006.

The Ongoing Discussion

The IAU's definitions have sparked controversy. Some astronomers find the criteria too vague, while others believe location within the solar system should be considered. Some feel the restrictions are too stringent, excluding objects that should be considered planets.

One idea is to simply define a planet as a natural object in space that is massive enough for gravity to make it approximately spherical. But some scientists object that this simple definition does not take into account what degree of measurable roundness is needed for an object to be considered round. In fact, it is often difficult to accurately determine the shapes of some distant objects. Others argue that where an object is located or what it is made of do matter and there should not be a concern with dynamics; that is, whether or not an object sweeps up or scatters away its immediate neighbors, or holds them in stable orbits.

Planets of Other Stars

The planets and other objects that circle the Sun are thought to have formed when part of an interstellar cloud of gas and dust collapsed under its own gravitational attraction and formed a disk-shaped nebula. Further compression of the disk’s central region formed the Sun, while the gas and dust left behind in the midplane of the surrounding disk eventually coalesced to form ever-larger objects and, ultimately, the planets.

Multiple exoplanets have been found to orbit in the habitable zones of their stars (where liquid water can potentially exist on a planetary surface), but Earth remains the only planet known to support life. Such a habitable zone is complicated by many factors, such as the atmospheric composition (if there is an atmosphere) of the planet; Mars, for example, has the temperature during the day to form liquid water on its surface on the equator, if only the pressure were higher.

With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account.

Exoplanet Types

There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which have masses between that of Earth and Neptune. Exoplanets have been found that are much closer to their parent star than any planet in the Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are hot Jupiters, such as 51 Pegasi b,[42] that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit (e.g.

General Planetary Characteristics

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them.

Orbit

In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates: counter-clockwise as seen from above the Sun's north pole. No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its periastron, or perihelion in the Solar System, whereas 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.

The eccentricity of an orbit describes the elongation of a planet's elliptical (oval) orbit. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The semi-major axis gives the size of the orbit. It is the distance from the midpoint to the longest diameter of its elliptical orbit.

The inclination of a planet tells how far above or below an established reference plane its orbit is tilted. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For exoplanets, the plane, known as the sky plane or plane of the sky, is the plane perpendicular to the observer's line of sight from Earth.[66] The orbits of the eight major planets of the Solar System all lie very close to the ecliptic; however, some smaller objects like Pallas, Pluto, and Eris orbit at far more extreme angles to it, as do comets.[67] The large moons are generally not very inclined to their parent planets' equators, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbits retrograde, i.e.

The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[62] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node.

Axial Tilt and Rotation

Earth's axial tilt is about 23.4°. Planets have varying degrees of axial tilt; they spin 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 has seasons, resulting in 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 with its day being the 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.

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole. The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[81] There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours.[82] The rotational periods of exoplanets are not known, but for hot Jupiters, their proximity to their stars means that they are tidally locked (that is, their orbits are in sync with their rotations).

Clearing the Neighborhood

The defining dynamic characteristic of a planet, according to the IAU definition, 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.

Shape, Mass, and Composition

Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, Earth radius or Jupiter radius). However, planets are not perfectly spherical; for example, the Earth's rotation causes it to be slightly flattened at the poles with a bulge around the equator.[95] Therefore, a better approximation of Earth's shape is an oblate spheroid, whose equatorial diameter is 43 kilometers (27 mi) larger than the pole-to-pole diameter.[96] Generally, a planet's shape may be described by giving polar and equatorial radii of a spheroid or specifying a reference ellipsoid.

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal.

Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower stellar mass limit is estimated to be around 75 to 80 times that of Jupiter (MJ). The smallest known exoplanet with an accurately known mass is PSR B1257+12A, one of the first exoplanets discovered, which was found in 1992 in orbit around a pulsar.

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets' mantles are sealed within hard crusts,[111] but in the giant planets the mantle simply transitions into a denser atmosphere.

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