Pluto, the erstwhile ninth planet of the solar system, was demoted to the status of a “dwarf planet” in August 2006 by International Astronomical Union (IAU). Ever since there has been a constant call from the public to “make Pluto a planet again”, but now, even scientists are beginning to say the same. According to a recent study, IAU’s definition of a ‘planet’ has only been used once in the last 200 years.
According to the University of Central Florida’s planetary scientist Philip Metzger,
“They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”
The IAU specifies that for an astronomical body to be called a ‘planet’, it should be spherical in shape, should be in orbit around the Sun and must have cleared the neighbourhood around its orbit. According to Metzger, the definition is sloppy as there have been more than 100 recent cases of scientists using the word ‘planet’ not as per IAU’s definition. Thus, according to him, the reasoning applies to Pluto as well.
Metzger instead recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.
“And that’s not just an arbitrary definition. It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”
With that definition, Pluto would be classified as a planet as it has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes as well as multiple moons.