In 2006, Pluto ceased to be considered a planet in the Solar System after the International Astronomical Union (IAU) change the definition that scientifically explains what a planet is. From that moment on, Pluto was reclassified as a dwarf planet. Now, almost two decades later, a team of scientists is proposing new parameters to define whether a celestial body can be considered a planet.
According to an article published on the pre-print server arXiv and accepted for publication in the scientific journal Planetary Science Journal, there are some changes that could help refine the definition currently accepted by the scientific community. A pre-print is an article released by scientists before being published in a scientific journal, when the data is reviewed by other researchers.
Led by University of California (UCLA) professor of earth, planetary and space sciences Jean-Luc Margot, the researchers plan to present the proposal at the next IAU general assembly, scheduled to take place in August 2024.
One of the main changes advocated is the addition of an important feature: planets should not be limited to celestial bodies that orbit the Sun. Currently, all cosmic objects that have similarities with the planets in our system, but do not orbit the Sun, are called exoplanets. In other words, the authors believe that many of these exoplanets are, in fact, planets.
What is a planet?
Established in 2006 by the International Astronomical Union, the current definition of what a planet includes three specific features. It is important to emphasize that the object must present these three attributes simultaneously — it is not enough to have each one separately.
Three characteristics to define a planet:
- The object must orbit the Sun;
- The celestial body must be large enough to take on an almost spherical shape, as occurs on Earth and the other planets in the Solar System;
- Finally, the object must have enough mass for its gravitational field to “clean” its orbit of other large bodies.
The recent study explains that the “clearing the orbit” feature is too simplistic and does not describe exactly what it means. Thus, the new change proposes to add some criteria that are easier to mathematically quantify both “cleanliness” and other attributes defined by the IAU.
Three characteristics proposed for the new definition of what a planet is:
- The planet must orbit one or more stars, brown dwarfs, or stellar remnants — not just the Sun;
- It must be more massive than 10^23 kg (ten to the power of 23);
- The object must be less massive than 13 Jupiter masses (2.5 × 10^28 kg).
Many scientists believe the change could be considerably important, as current definitions are vague and could theoretically lead to errors. The idea is that the characteristics are more detailed to ensure that cosmic objects are correctly identified by astronomers.
“Having definitions anchored in the most easily measurable quantity — mass — eliminates arguments about whether or not a specific object meets the criteria. This is a weakness of the current definition,” said one of the paper’s co-authors and associate at the University of British Columbia, Brett Gladman, in a statement.
Could Pluto become a planet again?
In 2006, Pluto was reclassified as a dwarf planet because it does not present the last characteristic mentioned in the list of the current IAU definition: the celestial body did not “clean” its orbit enough.
If the new additions are actually accepted at the IAU general assembly, they will not help reclassify Pluto. The new proposal includes a minimum mass limit for planets to be able to clear their orbits and the dwarf planet does not fit this characteristic.
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