Is Pluto a planet? This astronomical debate does not seem to have an end. In 2006, the International Astronomical Union (IAU) proposed a definition of a planet that left out Pluto, causing it to lose status and relegate it as a dwarf planet. But now, 12 years later, a new study published in Icarus, seems to have the power to return the reclassification to the precious “planet nine”.

Definition of planet

For the IAU, a planet must meet three requirements. The first one is that the celestial body must be in orbit around the sun. In addition, the object must have enough mass so that its own gravity makes it take an almost spherical shape. And finally, a planet must clear its orbit or, in other words, be the object with the greatest gravitational influence in its journey.

Pluto complies with the first two requirements, but not with the third. This, for now, a dwarf planet is under the influence of Neptune gravitational and, in addition, shares its orbit with gases and frozen objects of the Kuiper Belt.

The controversy

For Philip Metzger, a planetary scientist who works for the Florida Space Institute at the University of Central Florida in Orlando (USA) and the study’s lead author, the latter requirement is not valid.

Metzger says there is no support in the research literature for requiring a planet to clear its orbit. The scientist reviewed the publications made in the last 200 years and found that only one, from 1802, used this requirement to classify a planet. In addition, this single publication was based on a reasoning that has since been refuted.

For this man, the definition of the IAU is neglected. “They did not say what they meant by clearing their orbit if you take that literally, then there are no planets because no planet clears its orbit.”

A solution

Instead, Metzger proposes classifying a planet according to whether it is large enough so that its gravity allows it to have a (roughly) spherical shape.

This recommended definition is not arbitrary. “It turns out that this is an important milestone in the evolution of a planetary body, because apparently when it happens, active geology starts in the body,” explains the scientist.

Metzger points out that Pluto has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons. “It’s more dynamic and alive than Mars,” he said. “The only planet that has a more complex geology is Earth.”

This is not the first time that the definition of the planet enters into the debate. Previously the team of New Horizons of NASA tried to propose a new definition of planet that would return the status to Pluto. However, it was ruled out because some moons and other objects in the solar system also qualified as planets.