Scientists plan to revisit definition of planet but Pluto still remains out of list

A new definition of what a planet is, has been proposed by planetary scientists to replace the one which already exists, and is seen as outdated and sun-centric by many experts.

The International Astronomical Union (IAU) had established the current definition in 2006 which says to qualify for a planet a celestial body needs to orbit the sun in our solar system.

The International Astronomical Union (IAU) is the organisation which officially names objects present in space.

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In an article, which is to be published in the Planetary Science Journal, the scientists have argued that there is a new definition needed for a planet, which includes celestial bodies unrestrained by the bounds of our solar system.

The proposal further speaks about adding quantitative criteria to give clarity to the definition of a planet. Currently, the paper is availableon thearXivpreprint server.

UCLA professor of Earth, planetary and space sciences and of physics and astronomy and the article's lead author Jean-Luc Margot will be presenting the proposed new definition of the planet at the IAU General Assembly in August 2024.

According to the current definition, a planet refers to a celestial body which orbits the sun and is big enough for gravity to force it into a spherical shape and clear away other objects which are close to its orbit around the sun.

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"The current definition specifically mentions orbiting our sun. We now know about the existence of thousands ofplanets, but the IAU definition applies only to the ones in our solar system," said Margot.

"We propose a new definition that can be applied to celestial bodies that orbit any star, stellar remnant or brown dwarf," he added.

Why do authors want to redefine the planet?

The authors have argued although the requirement to orbit the Sun is very specific, the other criteria in the definition sanctioned by the IAU are vague.

The definition says that the planet has "cleared its orbit" without clearly saying what it means.

In the new definition, there is a quantifiable criterion which can be applied for defining planets which are mentioned inside and outside our solar system.

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Margot and co-authors Brett Gladman of the University of British Columbia and Tony Yang worked on a mathematical algorithm to understand how objects cluster together in our solar system.

In the analysis, it was found that the planets share distinct qualities in our solar system which can be used as a starting point to build a taxonomy for the planets.

"All the planets in our solar system are dynamically dominant, but other objects—including dwarf planets like Pluto and asteroids—are not. So this property can be included in the definition of planet," Margot said.

(With inputs from agencies)