A new Proposal defining a planet, which has significant implications for classifying objects in our solar system and star systems beyond has been put forward to the International Astronomical Union. Professor Margot and his colleagues argue that while the requirement to orbit our Sun is too specific, other criteria in the IAU definition are too vague. Let’s break down the old definition, the background behind the changes, the importance of these rules, and the specific case of Pluto.
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The need for a more precise definition arose with the discovery of numerous new celestial objects in the outer solar system, many of which may be similar in size to Pluto. This led to confusion and debate about what should be classified as a planet.
Earlier in 2006, the IAU formalized a definition of “planet” to resolve these issues and to distinguish between different types of objects in our solar system. This definition aimed to clarify the classification of objects and address the increasing number of discoveries. Since the old definition (2006), seems vague in defining certain criteria, a new definition with more precise criteria was required. The new proposed definition (2024) is based upon the quantitative criteria to leave no presumptuous confusion.
Old Definition
According to the IAU’s 2006 definition, for a celestial body to be considered a planet, it must meet three criteria:
- Orbit the Sun: The object must orbit only the Sun in its independent orbit.
- Sufficient Mass for a Nearly Round Shape: The object must have mass sufficient enough to achieve a spherical shape due to its self-gravity.
- Cleared the Neighbourhood Around Its Orbit: The object must be gravitationally dominant, clear the neighbourhood around its orbit, and remove smaller bodies near its orbit.
New Definition
A new definition says that to classify the planets we need more precise parameters. Astronomers have argued that the current definition to classify objects as planets is vague. For instance, the old definition states that a planet shall have a “clear path” without a clear understanding of what that means exactly. The new definition focuses more on the quantifiable criteria that can be applied to the objects to classify planets in and outside of our solar system.
In the new definition, a planet is a celestial body that:
- orbits one or more stars, brown dwarfs, or stellar remnants
- is more massive than 1023 kg, and is less massive than 13 Jupiter masses (2.5 X 1028 kg).
Professor Margot and his colleagues have suggested based on some mathematical algorithm that if an object has enough gravity to accumulate or eject some objects in its orbit that is to be said as the dynamically dominant. The planets of our solar system are dynamically dominant whereas the fate of objects, including Pluto, is going to remain the same as it cannot be considered as dynamically dominant. The lower limit of mass for a dynamically dominant object is quantified in the new definition. On the contrary, the potential planets beyond our solar system can be too big to fit in the new definition of planets. Some gas giants, large enough that thermonuclear fusion of deuterium occurs, and the object becomes brown dwarf and therefore not a planet. The limit is quantified as equal to or more than the 13 Jupiter.
The definition anchored based on the quantifiable parameter – mass, makes it more rational and quashes the arguments whether or not any object fulfils the criterion. In contrast to the solar system objects, all the objects having mass larger than the 1021 Kg appears to be spherical and the object having the mass equal to the proposed limit of 1023 Kg are expected to have the spherical shape.
Summary:
In summary, any official change in the IUA definition of planets is likely to be seen in the coming months or years. The new definition might introduce on the quantitative criteria for more clarity in classifying the planets and non-planet objects. The definition provides a more rigorous framework for understanding and categorizing celestial bodies, thereby enhancing the clarity and consistency of astronomical classifications. If you want to read more on the topic do check the research paper published: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3
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