A new formula for defining a planet still keeps Pluto out of the club

A mathematical formula to determine whether objects in space should be classed as a planet could make it easier to identify objects around other stars – but it is bad news for anyone hoping that Pluto could regain its planethood.

In 2006, the International Astronomical Union (IAU) infamously demoted Pluto from planet to dwarf planet status in a controversial decision that left our solar system with eight planets. The IAU cited the fact that Pluto hadn’t cleared its orbit of other debris as a critical factor in determining what counts as planet, a decision that irked many astronomers and the public alike.

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Jean-Luc Margot at the University of California, Los Angeles, and his colleagues now propose a much easier way to define a planet. They say a planet is any body that orbits a star or the remains of a star, and has a mass larger than 0.005 per cent that of Jupiter but smaller than 13 times the mass of Jupiter.

The lower bound, which is equal to 10²³ kilograms, corresponds to a midpoint between the mass of current dwarf planets and planets of our solar system, but could be adjusted slightly in future, says Margot. The upper mass end is the point at which deuterium fusion begins, moving into the realm of stars rather than planets.

“The current IAU definition is quite restrictive,” says Margot. “We’re proposing a simpler mass-based system.”

Margot’s new system would leave our solar system with its eight clearly defined planets, Pluto not included, and also applies neatly to all known 5700 exoplanets orbiting other stars. “All of them satisfy this criterion by a large margin,” says Margot. “So that’s sort of satisfying.”

One objection to the IAU’s planet definition is that it is difficult to observe if a body has cleared its orbit or not. To address this, Margot and his colleagues investigated if a planet can “dynamically dominate” its orbit, meaning it has the potential to do so.

The team used a mathematical formula linked to the planet’s mass and distance from its star that produced a clear divide between the planets of our solar system and smaller bodies, such as the dwarf planets Pluto, Eris and Ceres. Importantly, there is no need to actually observe objects in a planet’s vicinity to make the distinction, says Margot, which is impossible to do for exoplanets with our current technology.

The definition also does not require any direct measure of a planet’s roundness, known as hydrostatic equilibrium, which is a requirement of the IAU definition. Roundness is difficult to measure for exoplanets and for smaller bodies in our solar system. “Objects that are dynamically dominant typically will be round,” says Margot, and such objects will always be larger than 10²³ kilograms.

Mike Brown at the California Institute of Technology, who was instrumental in Pluto’s demotion with the discovery of Eris in 2003, says the work is a “fun way” to define a planet that “will spark some more debate”. However, he argues that is hard to have clear definitions of objects in astronomy. “There is no definition of what a star is, what a galaxy is, what a nebula is,” he says. “Planets could have been the same way, except that the public want to know what the planets in the solar system are.”

Margot will present a brief summary of his work at the IAU’s General Assembly in Cape Town, South Africa, in August, but hopes it might be picked up as a more formal proposal at the next assembly in 2027.

“In any branch of science there is a special place reserved for classification and taxonomy,” he says. “It helps people communicate with one another and the public, but also it guides our thinking. What causes nature to have this distribution? So I think it is important.”