Astronomers propose a new definition of a planetnearly two decades after the previous such change led to Plutodegradation. The new definition would add measurable criteria, including consideration of the planet’s mass — and still leave Pluto with dwarf planet status.
Currently the International Astronomical Union (IAU) defines a planet as a celestial body with three characteristics: It orbits the Sun, is large enough for gravity to shape it into a round shape, and removes any smaller objects other than moons and other satellites that surround its orbit.
“The problem in the past was that you had the word ‘planet’ but you didn’t have a quantitative definition of it,” Brett Gladman, an astronomer at the University of British Columbia who proposed the new definition with colleagues, told Live Science. The proposal was outlined in a document posted on July 10 to the preprint server arXiv and its presentation is scheduled for IAU General Assembly in August.
Without a quantitative reference point, the current definition runs into many problems, Gladman said. Indeed, the criteria are vague; it is not clear how large these bodies must be or “how bright” their orbits should be to be considered planets. For example, Earth and Jupiter have asteroids that their paths regularly cross, which raises the question of whether the planets actually have clear orbital paths. The current definition also does not account for objects that orbit stars other than the Sun, such as more than 5,500 exoplanets that have been discovered outside our solar system.
But “by far the most problematic is the roundness criterion,” Jean-Luc Margot, a UCLA astronomer and lead author of the paper, told Live Science. “Roundness is simply not observable. We don’t have the technology and we won’t have it anytime soon.”
Related: A giant planet discovered around a small star could disrupt our understanding of the formation of the solar system
Instead of focusing on the roundness and orbits of the planets, the new definition emphasizes a measurable quantity: the mass of the object. This new definition describes a planet as a celestial object that meets the following criteria:
To ensure that their classification framework was logical and unbiased, the researchers used a method called unsupervised clustering, an algorithm that groups similar objects together. This technique also successfully grouped eight planets in Solar System.
“No matter how many criticisms you can make of the current IAU definition, you can at least be satisfied that the result, eight planets, is a reasonable classification,” he said.
Astronomers claimed that the ability to clear orbital paths, known as dynamic dominance, can be determined by the mass of the planet. For example, the mass of each of the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) is at least three orders of magnitude more massive than that of the non-dynamically dominant dwarf planets.
Dynamic dominance also sets a lower weight limit on what qualifies as a planet. However, Pluto, which has a mass of only 2.88 x 10^22 pounds (1.31 x 10^22 kg), still would not qualify as a planet under the new definition.
Considering roundness, the mass of the celestial object is greater than 10^21 kg typically spherical due to gravitational force. That way, the new definition doesn’t really contradict the current definition, but rather adds specificity to it, Margot said.
“Humans are very tied to language and names and classification because that’s how we think about the world,” Gladman added. “That’s how we deal with the complexity of the world. We want to name things and put them into classes. And scientists want to do that too. We want to do it precisely.”
There is always the possibility of backtracking. Margot will present the proposal at the IAU General Assembly in August, but does not expect a consensus to be reached then. Instead, he hopes that through the presentation, the team will identify individuals who are interested in the idea and continue the discussion.
The proposal was written out of a “belief that we can be better as a community,” Margot said. “We owe it to ourselves and the public to come up with better definitions of these important astrophysical terms.”