Pluto, here’s how and why we reclassified it

The resolution of the Iau

Either a Solar System was accepted in which there were at least 12 planets, but in all probability destined to increase further, or a definition of a planet had to be finally built. In the 2006shortly after the launch of the probe New Horizons pointing to Pluto, theInternational Astronomical Union set up a commission to decide what were the characteristics that a celestial object must have to be defined as a planet. The commission then came to the historical one resolution B5, which defined that a planet, to be defined as such, had to respect three characteristics: orbit around the sun, have a mass sufficient to be more or less spherical, and have cleaned up the vicinity of its orbit. An object that does not respect this last characteristic is called a dwarf planet. An object that is not even spheroidal is called a minor body, be it an asteroid or a comet. Pluto, Eris, Makemake, Haumea, and even the asteroid Ceres, then became dwarf planetsand to Pluto, with the resolution B6, the historical role of the progenitor of this class of objects was recognized. So we often talk about derating of Pluto, but this is a historical error, because before 2006 a definition of a planet did not exist at all. It would be more correct to say that since the day of the publication of resolution B5, Pluto and all the objects of the Solar System have finally obtained a definition. In the meantime, however, we have discovered many other dwarf planets. In addition to those mentioned, there are Orcus, Quaoar, Gonggong, Salacia, 2002 Ms4 And Sedna, which are on the waiting list to be officially recognized as dwarf planets. And then there are another fifty or so that could be added to the list in the coming years if further studies confirm their characteristics.