Why Is Pluto Not A Planet?

What 3 Characteristics Make A Planet?

The question of what makes a planet isn't as simple as it may seem. While Earth, Mars, and Jupiter are widely accepted as planets, Pluto and Ceres were once classified as such until new discoveries led to a ongoing scientific debate on how to categorize them. In 2006, the International Astronomical Union established the most recent planet definition, stating that a planet must:

1. Orbit a star, like the Sun in our case.
2. Be large enough for gravity to shape it into a sphere.
3. Have enough gravity to clear its orbital path of other objects.



Why Did The Classification Of Planets Change?

Science is an ever-evolving process of asking questions, forming hypotheses, making discoveries, and revising our thinking as we learn more. Scientific concepts are crafted through logic and confirmed by observations. Scientists rigorously evaluate and question each other's work through a critical process called peer review.



Our comprehension of the universe and our role within it has evolved with time. Fresh insights can prompt us to reconsider what we know and reclassify objects to gain a deeper understanding. New viewpoints and ideas often emerge from questioning established theories or identifying the limits of current classifications.

How Has Our Understanding Of Planets Evolved Over Time?

Defining the term "planet" is significant because it reflects our understanding of the origin, structure, and development of our solar system. Throughout history, the objects classified as planets have changed. In ancient times, the Greeks considered the Moon and the Sun as planets, along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not regarded as a planet; instead, it was seen as the central celestial body around which everything else moved.

The concept of the Sun at the centre of the universe with Earth-orbiting it was introduced by Aristarchus of Samos in the third century BCE but wasn't widely accepted. It wasn't until the 16th century when Nicolaus Copernicus revived this idea.

By the 17th century, aided by the telescope's invention, astronomers realized that the Sun was the centre around which all planets, including Earth, orbited. The Moon was recognized as Earth's satellite, not a planet. Uranus became a planet in 1781, and Neptune was discovered in 1846.

In 1801, Ceres was found between Mars and Jupiter and was initially considered a planet. However, as more similar objects were discovered in that region, Ceres and its kind were reclassified as asteroids or minor planets.

Pluto, discovered in 1930, was designated as the ninth planet, but it was much smaller than Mercury and even some planetary moons. It didn't fit the characteristics of terrestrial planets, gas giants, or ice giants. Furthermore, its large satellite, Charon, shared its orbit. In the 1990s, new discoveries in the Kuiper Belt, including objects similar in size to Pluto, led to a reevaluation of Pluto's status.

In 2005, astronomers found another Kuiper Belt Object (KBO) similar in size to Pluto, raising questions about what defines a planet. This sparked a debate about the nature of planets. In 2006, the International Astronomical Union (IAU) addressed this challenge by defining "planet" and introducing a new category, "dwarf planet." Eris, Ceres, Pluto, Haumea, and Makemake were recognized as dwarf planets by the IAU. There may be around 100 more dwarf planets in the solar system, with hundreds more in and near the Kuiper Belt.

The IAU Definitions Of Planets

Modern observations are reshaping how we see planetary systems, and it's crucial that our naming system reflects our up-to-date knowledge. This is especially relevant when it comes to the term "planets." Initially, "planets" referred to celestial wanderers, simply moving lights in the sky. With recent findings, we can now create a new definition based on current scientific data.

So, the International Astronomical Union (IAU) decided that objects in our Solar System, excluding moons, should be categorized into three clear groups as follows:

1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
2. A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
3. All other objects,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

The Future Of Planets

Astronomers and planetary scientists didn't all agree on these definitions. Some felt that the classification aimed to limit the number of planets, while others found it incomplete and the terms unclear. Certain astronomers argued that context matters, especially in understanding solar system formation and evolution.

One suggestion was to define a planet as a natural object in space with enough mass for gravity to shape it into a roughly spherical form. However, some scientists raised concerns about how round an object needs to be to qualify. Determining the shapes of distant objects can be challenging. Others emphasized the importance of an object's location and composition, dismissing concerns about its dynamics, like whether it clears its orbit or holds neighbouring objects in stable paths. The lively debate on what defines a planet continued.

As our knowledge of the universe deepens and expands, it becomes more complex and fascinating. Researchers have discovered hundreds of exoplanets, planets beyond our solar system, with the possibility of billions more in the Milky Way Galaxy alone, some of which might be habitable. Whether our planet's definitions can be applied to these newly found objects remains uncertain.