What Is a Planet? Definition and History of the Term

In astronomy, the definition of a planet seems straightforward at first but has changed dramatically over time, especially in the 21st century. From ancient wanderers in the night sky to a precise list of criteria developed by scientists, the concept of what qualifies as a planet continues to evolve. These changes have sparked both public debate and scientific controversy, most famously involving Pluto.

This article explores the modern planet definition, the historical context behind it, and the implications for the bodies we include in our planetary census.

Key Takeaways

• A planet is a celestial body that orbits a star, is spherical due to its own gravity, and has cleared its orbital neighborhood of other debris.
• The current official definition of a planet comes from the International Astronomical Union (IAU), adopted in 2006.
• This definition applies specifically to planets within our Solar System, not necessarily to extrasolar planets.
• Pluto is no longer classified as a planet because it does not dominate its orbit.
• The Moon and other satellites are not considered planets because they orbit planets, not the Sun.
• The meaning of the word “planet” has changed multiple times through history as our scientific understanding and observational tools have improved.

Planet Definition

According to the IAU’s 2006 resolution, a planet is defined as a celestial body that:

1. Orbits the Sun (or a star in broader contexts),
2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape (hydrostatic equilibrium),
3. Has cleared the neighborhood around its orbit.

Word Origin: The term planet comes from the Greek word planētēs (πλανήτης), meaning “wanderer,” referring to how planets appeared to move across the sky relative to the background stars.

Evolution of the Planet Definition

Ancient and Classical Definitions

Historically, a planet was any “wandering star” that moved against the fixed background of stars. Ancient Greeks recognized seven classical planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun. Earth was not considered a planet because it was believed to be stationary at the center of the universe.

With the Copernican Revolution in the 16th century, the heliocentric model placed the Sun at the center, making Earth a planet and removing the Sun and Moon from the list. This brought the count to six planets: Mercury through Saturn, with Earth added.

The 18th and 19th Centuries

The discovery of Uranus in 1781 and Neptune in 1846 expanded the planetary list. When astronomers discovered Ceres in 1801 between Mars and Jupiter, it was briefly considered a planet. However, as more objects in that region were found, Ceres and its neighbors were reclassified as asteroids (later as dwarf planets).

The Pluto Era

Pluto was discovered in 1930 and hailed as the ninth planet. For decades, textbooks listed it alongside the others. However, Pluto’s small size, irregular orbit, and the later discovery of other similar icy bodies (like Eris) led to reconsideration. These objects were collectively labeled Trans-Neptunian Objects (TNOs) or Kuiper Belt Objects.

The 2006 IAU Redefinition

In 2006, the IAU convened to clarify what counts as a planet. Their definition reduced the number of planets in the Solar System to eight and introduced the category of dwarf planets, which includes Pluto, Eris, Haumea, Makemake, and Ceres.

This redefinition remains controversial, particularly among planetary scientists, who argue that the third criterion (clearing the orbit) excludes many significant worlds. Alternative proposals define planets based solely on intrinsic characteristics, not orbital dominance.

Classification of Planets in the Solar System

The eight recognized planets in the Solar System fall into distinct categories based on their composition, size, and location. These are terrestrial planets, gas giants, and ice giants.

Terrestrial Planets

The four inner planets are called terrestrial or rocky planets because they have solid surfaces and are composed primarily of silicate rock and metal:

• Mercury – Smallest planet, lacks a significant atmosphere.
• Venus – Similar in size to Earth, but with a thick carbon dioxide atmosphere and extreme surface heat.
• Earth – The only known planet to support life, with liquid water and a nitrogen-oxygen atmosphere.
• Mars – Known as the Red Planet, with polar ice caps and a thin atmosphere.

Gas Giants

The next two planets are much larger and composed mostly of hydrogen and helium. They do not have solid surfaces.

• Jupiter – The largest planet, with dozens of moons and a prominent system of rings and storms (including the Great Red Spot).
• Saturn – Famous for its extensive and bright ring system, with over 140 known moons.

Ice Giants

The outermost planets, while similar to gas giants in size, contain more ices like water, ammonia, and methane.

• Uranus – Rotates on its side and has a bluish color from atmospheric methane.
• Neptune – Known for its deep blue hue and dynamic weather, including supersonic winds.

A summary of these groups can be presented in the following table:

What Is a Dwarf Planet?

A dwarf planet is a celestial body that resembles a planet but does not meet all of the criteria to be classified as a full-fledged planet. According to the IAU, a dwarf planet must:

1. Orbit the Sun,
2. Have sufficient mass to be nearly spherical in shape (hydrostatic equilibrium),
3. Not have cleared the neighborhood around its orbit,
4. Not be a satellite of another object.

This makes dwarf planets fundamentally different from both regular planets and small solar system bodies (like comets and asteroids).

Recognized Dwarf Planets

As of now, five bodies are officially recognized as dwarf planets:

• Pluto – Once the ninth planet, reclassified in 2006. It has five moons and a complex surface with nitrogen ice and cryovolcanoes.
• Ceres – The only dwarf planet in the asteroid belt and the first object of its kind discovered.
• Eris – Slightly smaller than Pluto but more massive; located in the scattered disc beyond Neptune.
• Makemake – A bright Kuiper Belt object discovered in 2005.
• Haumea – An ellipsoidal dwarf planet with a rapid rotation and faint ring system.

There may be hundreds of dwarf planets in the outer Solar System yet to be officially recognized.

Why Pluto Is No Longer Considered a Planet

Pluto was reclassified because it does not meet the third criterion of the IAU definition: it has not cleared its orbital neighborhood. Pluto shares its space with other Kuiper Belt objects and crosses Neptune’s orbit. While it is massive enough to be spherical and orbits the Sun, its inability to gravitationally dominate its region makes it a dwarf planet rather than a full-fledged planet.

This reclassification was not due to any change in Pluto itself, but rather a change in how scientists categorize celestial objects. Some scientists and many members of the public continue to advocate for Pluto’s reinstatement, emphasizing its geological complexity and active surface features.

Why the Moon Is Not a Planet

Although Earth’s Moon is large and spherical, it is not classified as a planet because it does not orbit the Sun directly. Instead, it orbits Earth. By IAU standards, a planet must orbit the Sun (or a star), not another planet.

Even large moons like Ganymede (Jupiter’s moon) and Titan (Saturn’s moon), which are larger than Mercury, are considered natural satellites, not planets. The distinction lies in the orbital relationship, not size or shape.

Does the IAU Definition Apply to Extrasolar Planets?

The IAU’s 2006 definition applies only to objects within the Solar System. For exoplanets (planets outside our Solar System) there is no universally adopted, detailed definition. Instead, scientists use working criteria based on mass, formation history, and orbital dynamics.

Generally, exoplanets are objects that:

• Orbit stars (or sometimes brown dwarfs),
• Are below the mass threshold for deuterium fusion (around 13 Jupiter masses),
• Formed through core accretion or disk instability, not stellar fusion.

Some definitions also exclude objects that orbit other planets (exomoons), although such terminology is still evolving as more is discovered about planetary systems beyond our own.

Proposed Alternate Definitions of a Planet

While the IAU’s definition is the official standard, many planetary scientists argue that it is too restrictive or flawed. Several alternate definitions have been proposed.

The Geophysical Planet Definition

Led by scientists like Alan Stern, this approach defines a planet based on intrinsic physical properties, not orbital dynamics. According to this view, a planet is any:

“Substellar object that has never undergone nuclear fusion and is massive enough to be rounded by its own gravity.”

This definition would include:

• Pluto and all dwarf planets,
• Large moons like Ganymede, Titan, and the Moon,
• Free-floating planetary-mass objects not bound to a star.

Why It Matters

This broader view emphasizes geological complexity and active processes—many of which Pluto and other dwarf planets display. It reflects how planetary scientists study worlds as diverse and dynamic, rather than filtering them by orbital behavior.

However, it would vastly increase the number of planets in the Solar System, possibly to over 100, which many educators and institutions view as impractical.

Frequently Asked Questions (FAQs)

Q: Can Pluto become a planet again?
A: Possibly. If the IAU reconsiders its definition or adopts a geophysical model, Pluto could regain planet status. However, such a change would require international consensus.

Q: Is Earth’s Moon a planet?
A: No. The Moon is not a planet because it orbits Earth, not the Sun. It is classified as a natural satellite, even though it is larger than some planets and dwarf planets.

Q: How many dwarf planets are there?
A: Five are officially recognized by the IAU, but astronomers estimate there could be hundreds in the Kuiper Belt and beyond.

Q: What is the difference between a planet and a dwarf planet?
A: Both are spherical and orbit the Sun, but a planet clears its orbit of debris, while a dwarf planet does not.

Q: Are exoplanets subject to the same definition?
A: Not officially. The IAU’s planet definition applies only to the Solar System. Exoplanets are generally defined by mass and formation process, but a standardized global definition is still evolving.

References

• Lecavelier des Etangs, A.; Lissauer, Jack J. (2022). “The IAU working definition of an exoplanet”. New Astronomy Reviews. 94 101641. doi:10.1016/j.newar.2022.101641
• Moses, Julianne (2014). “Extrasolar planets: Cloudy with a chance of dustballs”. Nature. 505 (7481): 31–32. doi:10.1038/505031a
• Stern, S.A.; Levison, H.F. (2002) Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes. In: Rickman, H., Ed., Highlights of Astronomy 12: 205-213, Astronomical Society of the Pacific, San Francisco. doi:10.1017/S1539299600013289
• Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. ISBN 978-0-03-006228-5.