The International Astronomical Union (IAU) reclassified Pluto as a dwarf planet because it didn’t fulfil one of the three criteria they use to define a full-sized planet, namely, it hasn’t cleared its nearby region of other objects.
Pluto is not a planet because of 3 characteristics that 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 an ongoing scientific debate on categorising them. In 2006, the International Astronomical Union established the most recent planet definition, stating that a planet must:
- Orbit a star (a sun)
- Be large enough for gravity to mould it into a sphere
- Have strong enough gravity to clear its orbital path of other objects
Taking the above into account, we can see quite clearly that Pluto is not a planet.
Science is a dynamic process that involves asking questions, forming theories, making discoveries, and revising our thinking. 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 understanding of the universe and our place within it has evolved over time. New insights can prompt us to reconsider what we know and reclassify objects to gain a deeper understanding. Fresh viewpoints and ideas often emerge from questioning established theories or identifying the limits of current classifications.
How has our understanding of planets evolved?
The term “planet” is significant because it reflects our understanding of our solar system’s origin, structure, and development. Throughout history, the objects classified as planets have changed. In ancient times, the Greeks considered the Moon and the Sun planets. However, 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 was introduced by Aristarchus of Samos in the third century BCE but wasn’t widely accepted. It wasn’t until the 16th century that Nicolaus Copernicus revived this idea, which led to the heliocentric model of the solar system. This model placed the Sun at the centre of the solar system, with the planets orbiting around it.
By the 17th century, astronomers discovered that our sun was the centre of our solar system, and all planets, including Earth, orbit around it. The Moon was recognized as Earth’s satellite, not a planet. Uranus was discovered in 1781, and Neptune in 1846.
In 1801, Ceres was found between Mars and Jupiter and it 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 is not a planet whether we like it or not
We now know that Pluto is not a planet, but when was it discovered? Pluto was discovered in 1930 and was initially considered the ninth planet in our solar system. However, it was much smaller than Mercury and even some planetary moons, and it didn’t fit the characteristics of terrestrial planets, gas giants, or ice giants.
Additionally, its large satellite, Charon, shared its orbit. In the 1990s, new discoveries in the Kuiper Belt led to a reevaluation of Pluto’s status. As a result, in 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet.
In 2005, astronomers found another Kuiper Belt Object 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 KBO.
The IAU definitions of planets
Our understanding of planetary systems is evolving with modern observations, and our naming system must reflect this. This is particularly relevant when it comes to the term “planets.” Initially, “planets” referred to celestial wanderers or moving objects in the sky. With recent findings, we can now create a new definition based on current scientific data.
With this, the International Astronomical Union (IAU) decided that objects in our Solar System, excluding moons, should be classified into three clear groups as follows:
- 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.
- 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.
- All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
The future of planets
There was disagreement among astronomers and planetary scientists regarding the classification of planets. Some believed that the classification aimed to limit the number of planets, while others found it incomplete and the terms unclear. Additionally, some astronomers argued that context matters, especially in understanding solar system formation and evolution.
One idea to define a planet is to consider it 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, as 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 debate on what defines a planet continues
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. There is 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.
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