The Definition of a Planet: Time for a Change

For over a century, the International Astronomical Union (IAU) has defined a planet as a celestial body that orbits the Sun, is massive enough to be spherical in shape, and has “cleared its orbit” of other objects. This definition, however, is now considered outdated, as we know that there are many celestial bodies that orbit stars other than the Sun.

In a recent article in the Planetary Science Journal, scientists propose a new definition of a planet that is not limited to the Solar System. The lead author of the article, Jean-Luc Margot, a professor at UCLA, will present the proposed new definition at the IAU General Assembly in August 2024.

Under the current definition, a planet is a celestial body that orbits the Sun and is massive enough to be spherical in shape and has “cleared its orbit” of other objects.

“The current definition specifically mentions orbiting our Sun. We now know of thousands of planets, but the IAU definition only applies to those in our solar system,” Margot said. “We propose a new definition that could apply equally to objects orbiting any star, stellar remnant, or brown dwarf.”

According to the researchers, while the requirement to orbit the Sun is very specific, other criteria in the IAU definition are quite vague. For example, a planet is said to have “cleared its orbit,” but what this means is not well-defined. The proposed new definition includes quantitative criteria that can be applied to planets both inside and outside the Solar System.

Under the proposed new definition, a planet is a celestial body that:

-Orbits one or more stars, brown dwarfs, or stellar remnants

-Has a mass greater than 10^23 kilograms

-Is not massive enough to undergo nuclear fusion

-Is not a moon

The proposed new definition is more inclusive than the current definition, as it would apply to planets that orbit stars other than the Sun. It is also more precise, as it includes quantitative criteria for what it means to “clear an orbit.” The researchers believe that the new definition will be more useful for astronomers as they continue to discover and study planets outside the Solar System.

Implications of the New Definition

The proposed new definition of a planet has a number of implications. First, it would mean that there are many more planets in the universe than we currently know about. This is because the new definition would include planets that orbit stars other than the Sun, which are not currently counted in the total number of known planets.

Second, the new definition would change the way we think about the Solar System. Under the current definition, the Solar System is unique in having eight planets that orbit the Sun. However, under the new definition, the Solar System would be just one of many planetary systems in the universe.

Third, the new definition would make it easier to identify and study planets outside the Solar System. This is because the new definition would provide a clear set of criteria for what constitutes a planet. This would make it easier for astronomers to search for and identify planets, and to study their properties.

Conclusion

The proposed new definition of a planet is a significant change from the current definition. It is more inclusive, more precise, and more useful for astronomers. The new definition will help us to better understand the universe and our place in it.