Pluto was demoted to dwarf planet status in 2006, but some researchers say the International Astronomical Union made the wrong call.
It has been more than a decade since the International Astronomical Union (IAU) established a definition of a planet. This meant a demotion for Pluto as, under the 2006 rules, it did not meet the criteria.
How do we define a planet?
The IAU defines a planet as “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighbourhood around its orbit.”
The third point is where Pluto runs into trouble. Neptune influences Pluto’s gravity and it also shares its orbit with objects and gases in the Kuiper belt.
Since Pluto’s demotion to dwarf planet, some astronomers have been making arguments for its return to the planet club. The latest research from the University of Central Florida argues that the IAU made the wrong call.
Dr Philip Metzger, a planetary scientist, argues that there is no support in the research literature requiring a planet to clear its orbit. Metzger combed through more than 200 years worth of research and publications, and found only a single publication from 1802 using that particular requirement to classify a planet.
According to Metzger, Pluto is “more dynamic and alive than Mars”. He described it as the “second-most complex, interesting planet in our solar system”.
The expert planetary scientist added that the 2006 definition from the IAU was “sloppy”. He said that planets should be classified based on being large enough that their gravity allows them to be spherical. He said that his proposed definition was important in a planet’s lifespan. “It turns out this is an important milestone in the evolution of a planetary body because, apparently, when it [reaches a certain size] … it initiates active geology in the body.”
A problematic definition
Metzger added that moons such as Saturn’s Titan and Jupiter’s Europa have often been called planets since the Galileo era. “The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” Metzger said.
He said that the real delineation between planets and other celestial bodies begin in the early 1950s with the publication of a paper by Gerard Kuiper.
The classification of Pluto has been a talking point for years since its demotion. In 2017, NASA’s New Horizons group proposed a major reworking of how planets are defined, which would return Pluto to planet status as well as qualify certain moons as planets.