Take that, other scientists!
Back in 2006, our understanding of the little patch of the universe we call our solar system was turned upside down when it was revealed that Pluto – the smallest and furthest-from-the-sun planet in our celestial neighbourhood – was not actually a planet.
No, turns out the little tacker was, in fact, a dwarf planet.
While it was a process that took over a decade to nail down, the issue stemmed from the fact there were other objects orbiting our sun that were of a similar size to Pluto. So if Pluto was a planet, then so were myriad other objects located in the Kuiper Belt (which NASA call “a doughnut-shaped ring of icy objects around the Sun, extending just beyond the orbit of Neptune”).
As a result, on August 24 2006, the International Astronomical Union (IAU) gave the following definition of a planet:
A “planet” is 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.
However, planetary scientist Philip Metzger of the University of Central Florida is now arguing that Pluto totally is a planet, because this new definition is flawed.
“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that nobody uses in their research,” he says.
Metzger claims Pluto is “more dynamic and alive than Mars” and is the “second-most complex, interesting planet in our solar system” behind Earth. What’s more, he says that while we have been happy to exclude Pluto, countless other bodies are being accepted as planets yet have similar constraints.
“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful,” Metzger says.
“It’s a sloppy definition. 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.”
Instead of focusing on orbit, Metzger proposes that a planet be defined by whether the body is large enough that its gravity becomes spherical.
“And that’s not just an arbitrary definition,” Metzger says.
“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.”