On August 24, 2006, 16 years ago, the International Astronomical Union (IAU) made a decision that would change the Solar System forever. IAU members voted on a longstanding proposal to define exactly what the term “planet” means. And so Pluto was reclassified as a dwarf planet and the number of planets in the Solar System was reduced to eight.
But despite the intervening 16 years, this decision remains as controversial as ever.
The IAU has gone through many possible iterations of definitions to include planets around other stars (exoplanets) and planets removed from star systems. But problems arose with the inability to place rogue planets emanating from star systems.
On the last day of the 10-day conference in Prague, the current definition was laid down, voted on and approved. According to this definition, a planet must meet three conditions to become a planet.
- The Sun is supposed to orbit, so all exoplanets and rogue planets fall outside the definition.
- It needs to have reached hydrostatic equilibrium, meaning it needs to be roughly spherical.
- As the main gravitational object, it should have cleared its orbit.
Pluto fails to satisfy the third of these conditions. This distant planet has not cleared its orbit, which it partially shares with Neptune. But proponents of Pluto’s planetary status argue that Earth, Mars, Jupiter and Neptune don’t clear their orbits either. Excluding the Moon, Earth is 1.7 million times heavier than other asteroids that cross its orbit. Pluto, on the other hand, is only 0.07 heavier than those in its orbit.
Discussions within and outside of scientific circles have called it a reduction rather than a classification, but it could also be argued that Pluto was elevated as the first of two new object classes: dwarf planets and plutoids.
People all over the debate are emphasizing how far the definition is from being perfect and how often planets are described with extra adjectives and not just planets: rocky, gas giant, ice giants… So maybe someday the term dwarf is “planet” rather than something different. can be considered as a sub-category of The term was first suggested by New Horizons principal investigator Alan Stern, who visited Pluto in 2015 with this in mind. Stern also says he wants Pluto to become a planet again.
If Pluto is considered a planet again…
There is another detail that is certain among astronomers, but unfortunately not much talked about by the public. We can no longer have nine planets in the Solar System. This is because Eris, a dwarf planet named after the goddess of strife and discord, was discovered.
It is more massive and only slightly smaller than Pluto. So if Pluto is considered a planet again, it would probably make sense to consider Eris as a planet as well. It was also named the 10th planet at the time of the announcement of its discovery by Mike Brown, Chad Trujillo, and David Rabinowitz.
The existence of this planet compelled the IAU, which adopted the current resolution the following year, to take a decision. So we either have eight planets or add the dwarf planets Ceres, Pluto, Eris, Haumea, and Makemake to have at least 13 planets. There are also many other celestial bodies that are considered dwarf planets by the astronomy community but not yet recognized by the IAU.
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