This post was updated Oct. 1 at 10:01 p.m.

There are eight recognized planets, according to the International Astronomical Union’s current definition of the term – but a new proposal could change that.

The proposal – co-written by Jean-Luc Margot, a professor in the Departments of Earth, Planetary and Space Sciences as well as Physics and Astronomy – was submitted to the IAU to be considered at its 2024 General Assembly, which occurred in August. If approved, it would replace the IAU’s definition of a planet, which was adopted in 2006. The new paper found the 2006 definition to be overly vague in some respects and overly specific in others, Margot said.

For instance, the current definition states two of the stipulations for an object to be considered a planet are that it must be round and able to “clear its neighborhood” or display gravitational dominance over nearby objects. However, Margot said it is impossible to accurately evaluate these characteristics in distant objects, particularly those outside the solar system.

Instead, the new proposal suggests estimating these characteristics using more easily measured factors, such as the time a planet takes to orbit its star and the masses of both objects, Margot said. Erik Petigura, an associate professor of physics and astronomy, said the proposal aims to replace the qualitative criteria of the 2006 definition with quantitative measures.

The current definition also requires that planets orbit the sun, which technically excludes exoplanets – planets beyond the solar system – from being labeled as such, Margot said.

“On that basis, there are eight planets right now in the universe, whereas we know that there are billions of planets in the galaxy,” Margot said.

Margot added that his proposed definition would include planets orbiting one or more stars or similar objects such as brown dwarfs – celestial objects more massive than planets but less massive than stars.

In addition to submitting the proposal to the IAU, Margot gave a lecture at the general assembly about his paper Aug. 9. He said the IAU invited him to hold the presentation because his proposal was not given time for consideration at the 2024 meeting, adding that the earliest it could be voted on is the organization’s 2027 general assembly.

Despite this, Margot said the overall reception to his talk was positive.

“It’s clear that the community agrees with our proposal,” he said. “Maybe not with all the details of the proposal, but it agrees that we need a better definition of a planet.”

Members of the astronomy community at UCLA also responded favorably to the proposal. Petigura said while he was surprised to see someone tackle the issue after the controversy of the 2006 definition, which excluded Pluto from planetary status, he agreed with the points raised in Margot’s paper.

David Paige, a professor of planetary science, added that the proposal could gain traction within the wider scientific community, as it represents a legitimate effort at improving the definition of a planet.

Petigura also said the changes proposed in the paper represent a tangible improvement over the 2006 definition. The delay in consideration until 2027 could give the scientific community more time to refine the proposal than it had in 2006, he said.

“It’s a little bit more complex, but I think that complexity stems from being more thorough,” Petigura said. “The initial definition is shorter because it’s not very precise.”

Paige said he believes the new definition would be an improvement, as it is more inclusive of bodies such as exoplanets. However, he said it does not fully resolve the question of what should be considered a planet, adding that as a planetary scientist, he cares more about the physical and chemical properties of an object than its orbit.

The issue of what qualifies as a planet opens up a broader discussion about the importance of nomenclature in science, Paige said.

“Who has the real right to define anything?” Paige said. “People will use their own definitions as our understanding of the science, and the scope of the thing that’s being defined expands.”

Ultimately, precise definitions are helpful for scientists when communicating both with the public and fellow scientists, Margot said. He added that if other fields of science can agree on clear definitions, planetary science should be able to do the same.

“As astronomers, we can do a better job,” Margot said. “Scientists are precise, and we’re in a situation where planetary scientists and astronomers have not managed to define a planet precisely.”

Beyond communication, he said having a precise classification system also encourages a more nuanced understanding of the processes involved in shaping celestial objects. For instance, although the proposed definition could extend planetary status to numerous objects currently excluded from it, Pluto would still not be one of them, as there are significant differences between it and the eight planets in terms of their gravitational influence over other nearby bodies, he said.

However, Margot added that this label should not serve to limit scientific inquiry but rather raise questions about why those differences exist.

“The classification of a body does not convey scientific importance. Just because a body is not a planet does not make it any less exciting,” he said. “Our classification that clearly shows eight planets does not mean that Pluto is any less interesting or worthy of exploration. It’s just a way of organizing our thoughts and a way of communicating.”