What Makes Planet a Planet? What Is a Planet And Who Defines It?

The question of what makes a planet a planet has occupied the minds of astronomers for centuries. From the times of the ancient Greeks, who called planets the “wandering stars,” to modern scientists trying to classify the diversity of celestial bodies, the concept of a “planet” has undergone significant changes. In this article, you will find a modern planet definition and learn who established the criteria for identifying planets, as well as how they have changed over time. 

What Is a Planet? 

A planet is an astronomical object that orbits a star or remnants of a star, is massive and compact enough to be spherical, and is not a satellite of another planet. Such celestial bodies must also have sufficient gravity to “clear” their orbit from other large objects.

Who Came up With the Name “Planet”?

The term comes from the ancient Greek words πλάνητες αστέρες, which means “wandering stars.” The Greeks gave this name to objects that moved across the sky, as opposed to the “fixed stars.” Ancient astronomers, particularly Ptolemy, who created the first star catalogue in 138 AD, used this term to describe the five planets they knew (Mars, Jupiter, Saturn, Venus, and Mercury), as well as the Sun and Moon, which they also considered planets.

Who Established the Criteria And How They Changed Over Time?

In 1781, William Herschel discovered Uranus, and in 1846, Johann G. Galle and Heinrich L. d’Arrest, based on the calculations of Urbain J. Le Verrier, rendered a verdict on the discovery of Neptune. These discoveries expanded the understanding of planetary bodies; however, their definition was still vague.

In 1919, the International Astronomical Union (IAU), an organisation uniting astronomical communities around the world, was created and it took on the responsibility of classifying astronomical objects. Specifically, solar system planets were defined as objects orbiting the Sun and having sufficient mass to be spherical. This planet meaning in science did not change until 2006, when the IAU was forced to radically revise its planet definition criteria. And the main culprit was Pluto.

What Changed Between 1930 And 2006?

The discovery of Pluto in 1930 raised numerous doubts and led to new debates on what is a planet. The IAU declared it the ninth planet in the solar system, but not all astronomers and physicists agreed. With the discovery of other trans-Neptunian objects in the Kuiper Belt, similar to Pluto, it became obvious that Pluto does not meet some of the conditions characteristic for “classical” planets. This led to the IAU revising its classification standards, and as a result, Pluto was demoted to a dwarf.

What Qualifies a Planet to Be a Planet?

New definition of planet by IAU from 2006 states that an object claiming this title must meet three criteria:

• revolve around its parent star (for solar system objects, this is the Sun),
• have sufficient mass so that its gravity could overcome the forces of a rigid body, and it took on a hydrostatic equilibrium (almost round) shape.
• “clear” the surrounding space of other objects.

These criteria mean that the desired object must be the dominant gravitational body in its orbit, form itself into a sphere, and clear the space around it of smaller bodies.

In the solar system, we currently know about eight such objects: terrestrial planets – Mercury, Venus, Earth and Mars. Outer Planets: gas giants Jupiter, Saturn, and ice giants Uranus and Neptune. Let’s find out what criteria the IAU classifies them by.

What Makes a Planet a Terrestrial Planet?

A terrestrial planet, also known as a telluric or rocky one, is defined by the following key characteristics:

1. A hard surface consisting of stones and/or metals.
2. Molten metal core that is mostly iron, giving it a high density, and is surrounded by a silicate mantle.
3. Few or no moons. For example, Mercury and Venus have no moons, while Earth has one and Mars has two small moons. 
4. Topological features: The surface contains various geological formations such as mountains, valleys and craters, resulting from geological processes such as tectonic activity, volcanism, and impact craters.
5. A thin atmosphere with varying composition, formed as a result of volcanic activity or collisions with other celestial bodies.

Terrestrial planets seem to offer hope for their habitability; however, the proximity of Venus and Mercury to the Sun makes them so hot that life on them is impossible. Mars is another matter. Intentions to colonise it confirm its viability. However, we will have to work hard. Read whether we can survive on Mars in our separate article.

What Makes a Planet a Gas/Ice Giant?

Gas and ice giants have greater mass than Earth and a complex system of satellites and rings. However, their main distinguishing feature is the absence of a hard surface in the traditional sense. Based on this, their viability is excluded, if only because not a single spacecraft that currently exists can land on them.

Gas and ice giants have solid cores made of rocks, silicates and metals, but they are surrounded by a liquid or gaseous mantle of water, hydrogen, helium, methane and ammonia, which gradually develops into deep, complex atmospheres dominated by hydrogen and helium. However, despite their severity, giants provide great benefits. They create a strong gravitational field, which helps maintain the orbital stability of other objects in the solar system and, like a shield, they absorb the impacts of comets and asteroids.

Why Is Pluto No Longer a Planet?

Pluto is neither a terrestrial planet nor a gas or ice giant, although it combines the characteristics of both. It consists of approximately 50% – 70% rock, and the rest is ice.

However, the main reason why Pluto was demoted was its failure to meet the third criterion of the present planetary definition. It has not “cleared” its orbit of other objects in the Kuiper Belt. In other words, its mass is too small to attract or repel other cosmic bodies. This key circumstance that makes a planet not a planet, allowed scientists to reclassify Pluto into the category of “dwarfs” along with other similar objects in the Kuiper belt, such as Eris and Ceres.

However, debates in scientific circles continue, and many are advocating for Pluto’s restoration to its status. If you want to learn more about Pluto’s difficult fate and his chances of returning to the highest planetary society, read our separate article Is Pluto a planet again?

What About Planets Outside the Solar System?

Planets outside the solar system are commonly called exoplanets. The prefix “exo-” comes from the ancient Greek word “ἔξω”, which means “beyond, outside,” and comes from the Proto-Indo-European root “*eghs,” meaning “out.” Thus, “exoplanet” literally means “outside” or “orbiting a star that is not the Sun.”

The first exoplanets were discovered in the late 1980s, but the first confirmed exoplanet was found in 1992 orbiting the pulsar PSR 1257+12. This discovery was an important milestone in astronomy because, before that, the existence of exoplanets was only speculation.

According to the latest data, the existence of 5660 exoplanets has been confirmed in 4167 planetary systems, which are classified into four types: terrestrial, Neptune-like, gas giant, and super-Earth.

Scientists believe that some of the discovered exoplanets may be habitable because they are Earth-like and located in the so-called “habitable zone,” where conditions may be suitable to support life. It is impossible to verify this for sure yet, but we are closer to the goal than ever before. The Pioneer 10, 11, and Voyager probes have recently left the boundaries of the solar system and may be able to reach some exoplanet in our immediate environment.

Discovering New Worlds 

Humanity, with its inherent curiosity, strives for the unknown, and this process of acquiring knowledge is as endless as the Universe itself. Each new discovery brings additional data that helps refine our astronomical classifications and better understand the mechanisms of planetary systems’ formation and evolution. This means that soon we might again expand our understanding of what makes a planet a planet and learn some new names.

 References and Additional Information:

• What is a planet? https://science.nasa.gov/solar-system/planets/what-is-a-planet/
• Exoplanets https://science.nasa.gov/exoplanets/
• Classification of the planets https://www.pas.rochester.edu/~blackman/ast104/classification4.html
• Pluto and dwarf planets https://science.nasa.gov/dwarf-planets/
• dwarf planet astronomy https://www.britannica.com/science/dwarf-planet
• Planets of other stars https://www.britannica.com/science/planet#ref256234
• IAU history https://www.iau-100.org/iau-history
• Ancient Astronomy, Science And The Ancient Greeks  https://explorable.com/greek-astronomy