What is a "planet"? Historical study revisits debate on Pluto and other dwarves

Twelve years after the International Astronomical Union had voted a definition of planned intelligence that reclassified Pluto, the debate continues.

A recently published study uses history to address the most controversial clause in the definition: the idea that a planet in the solar system must "clear the neighborhood of its orbit", so that no other world is in a similar orbital distance.

This requirement was used in 2006 to exclude Pluto, whose orbit periodically crosses the trajectories of Neptune's orbit. More precisely, other smaller worlds have been detected in the same region of the solar system frequented by Pluto.

What complicates the debate is the fact that a growing number of icy celestial objects are being discovered in the Kuiper Belt, a broad ring of matter scattered beyond Neptune. An object discovered in 2005, called Eris, was more massive than Pluto. This triggered IAU's effort to resolve the issue and ultimately led to the orbit compensation clause.

The clause excludes not only Pluto and Eris, but also Ceres in the main asteroid belt and other worlds discovered beyond Pluto. These worlds, which are still massive enough to break into a round form, have been designated as "dwarf planets".

In the new study, published in the journal Icarus, four planetary specialists claim that the orbit compensation clause does not have many historical precedents and is not commonly used in current research. Their study of the scientific literature revealed that only one publication used the orbit compensation requirement – in 1802, just after the discovery of Ceres – and this requirement was based on assumptions that have since been refuted.

"The definition of AIU would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that no one uses in its research", Lead author Philip Metzger, a global scientist at the University of Central Florida, said in a press release. "We now have a list of more than 100 recent examples of planetary scientists using the word planet in a way that violates the definition of IAU, but they do so because it is useful on the planet. functional plan. "

Metzger and his colleagues are in favor of a definition based on a geophysical standard: is the celestial object wide enough so that its gravity allows it to be spherical in shape? This criterion has historically been used to distinguish irregular asteroids from larger celestial objects. And this is already part of the definition given by the AIU for the planets as well as its distinct category of dwarf planets.

"It's not just an arbitrary definition," Metzger said. "It turns out that it's an important step in the evolution of a planetary body, because apparently, when that happens, it initiates an active geology into the body."

As an example, Metzger drew attention to the observations made by NASA's New Horizons spacecraft when it flew over Pluto in 2015. These observations proved that Pluto has mountains of ice, an underground ocean, traces of ancient lakes and a multilayered atmosphere. clouds.

"It's more vibrant and alive than Mars," Metzger said. "The only planet that has a more complex geology is the Earth."

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NASA's Dawn spacecraft, arriving at the end of its scientific mission to Ceres, has found similar evidence of groundwater that helps drive an active geology on this dwarf planet.

Metzger and the other authors of the newspaper have long criticized the definition of IAU and are part of the scientific team of New Horizons. This led Caltech astronomer, Mike Brown, who discovered Eris and wrote a book titled "How did I kill Pluto and why did he do it" to reject the article? d & # 39; Icarus.

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