WHAT MAKES A PLANET?
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| Pluto |
Astronomers have always been fascinated by planets and eagerly search for them across the universe. However, what may come as a surprise is that defining a planet is not as simple or straightforward as it seems. Even today, scientists continue to debate what truly qualifies an object to be called a planet.
Origin of the Word ‘Planet’: The word planet comes from the Greek term planētes asteres, which means “wandering stars.” Ancient astronomers noticed that some bright objects in the night sky did not remain fixed like stars but moved slowly across the sky. These wandering lights were later identified as planets.
Early Attempts to Define a Planet: Over the years, astronomers have suggested various definitions of a planet based on different characteristics, such as:
● Size and mass
● Internal activity
● Shape and roundness
● Whether it orbits a star
As scientific knowledge advanced and new celestial bodies were discovered, the need for a clear and official definition became more important.
The IAU Definition (2006): In 2006, the International Astronomical Union (IAU) introduced an official definition of a planet. Although widely used today, it is not universally accepted by all astronomers.
According to the IAU, for an object in our Solar System to be called a planet, it must meet three conditions:
1. It must orbit the Sun.
2. It must be massive enough for its own gravity to make it nearly spherical in shape.
3. It must have cleared its orbital neighbourhood, meaning it must gravitationally dominate its region of space and remove or control other objects of comparable size.
Why Pluto Lost Its Planet Status: Pluto satisfied the first two conditions—it orbits the Sun and is nearly spherical. However, it failed the third condition. Pluto shares its orbit with many other objects in the Kuiper Belt and does not dominate its region of space.
As a result, Pluto was reclassified as a dwarf planet, a decision that sparked widespread debate and public emotion.
Why the Definition Is Controversial: Despite its official status, the IAU definition has several problems and critics.
Orbital neighbourhoods are not always clear.
Earth and Jupiter both share their orbits with asteroids and smaller rocky bodies. By strict interpretation, they may not have fully cleared their zones either.
Neptune’s influence raises questions.
Some scientists argue that if Neptune had completely cleared its orbit, Pluto would not be found where it is today.
Distance from the Sun matters.
Planets farther from the Sun move more slowly and need to be much more massive to clear their orbital paths. This means a planet like Earth might not qualify as a planet if it were located in the outer Solar System.
Different stars, different outcomes.
Two identical planets orbiting two different stars could be classified differently due to variations in gravitational influence.
Intrinsic Properties vs Orbital Dynamics: Many astronomers believe the definition relies too heavily on orbital behaviour and not enough on a planet’s intrinsic properties, such as:
● Composition
● Internal structure
● Geological activity
They argue that a planet should be defined by what it is, not just where and how it moves.
Conclusion: The question “What makes a planet?” remains one of the most intriguing debates in modern astronomy. While the IAU definition provides a useful framework, it is not perfect and continues to be questioned. Pluto’s reclassification reminds us that science evolves with new discoveries, and definitions may change as our understanding of the universe deepens.
In astronomy, as in all sciences, asking questions is just as important as finding answers.
Do You Know?
● Pluto was considered the ninth planet for over 75 years before being reclassified.
● There are thousands of exoplanets discovered outside our Solar System, many of which challenge traditional definitions.
