Astronomers call for new planet definition, but it’s not about Pluto

Astronomers want to broaden the definition of a “planet”.

The current definition, argue the US and Canadian researchers, is too vague and excludes exoplanets.

They’ve proposed a new set of criteria, which they will be presenting at the General Assembly of the International Astronomical Union (IAU) in August.

Pluto still won’t count as a planet under this new definition.

The IAU, which is responsible for naming things in space, last set the definition of a planet in 2006 – the famous, Pluto-excluding decision.

Since then, research in exoplanets has exploded. But according to the 2006 definition, none of these exoplanets are technically planets.

“The current definition specifically mentions orbiting our sun. We now know about the existence of thousands of planets, but the IAU definition applies only to the ones in our solar system,” says Professor Jean-Luc Margot, an astronomer at the University of California – Los Angeles,  and lead author on a pre-print that is soon to be published in The Planetary Science Journal.

“We propose a new definition that can be applied to celestial bodies that orbit any star, stellar remnant or brown dwarf.”

Margot and colleagues’ suggested definition of a planet is a celestial body that orbits one or more stars, brown dwarfs or stellar remnants, and is between 10²³kg and 2.5 × 10²⁸ kg in mass (or, between about a third of Mercury and 13 Jupiters in mass).

“Having definitions anchored to the most easily measurable quantity — mass — removes arguments about whether or not a specific object meets the criterion,” says co-author Professor Brett Gladman, an astronomer at the University of British Columbia, Canada.

“This is a weakness of the current definition.”

The researchers arrived at their definition by analysing the planets in our solar system with an algorithm, looking for qualities they shared.

One quality they found was “dynamical dominance”, which means that an object has enough gravity to clear a path by accumulating or ejecting other objects.

“All the planets in our solar system are dynamically dominant, but other objects — including dwarf planets like Pluto, which is not a true planet, and asteroids — are not,” says Margot.

“So this property can be included in the definition of planet.”

Dynamical dominance set the lower limit. The upper limit of mass in the definition relates to when an object can be considered a sub-star or brown dwarf.

Above a mass of 13 Jupiters, an object gets so big that it undergoes thermonuclear fusion. This is why the researchers set this mass as the maximum.

If, as they hope, the researchers’ new criteria sparks a conversation at the IAU, an official change to the planetary rules would still likely take several years.