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We were nine

The small and frozen Pluto is relegated in status and the solar system returns to having eight planets

ALAN STERN/MARC BUIE/NASA/ESAAbove, the new-old configuration of the solar system, with only eight membersALAN STERN/MARC BUIE/NASA/ESA

This was the public correction of a historical error, which for more than seven decades had upset the majority of astrophysics. Pluto lost its status as a planet and our system returned to having only eight members, in accordance with a resolution approved on the 24th of August last, after heated discussions, inclusive of semantic order, during the 26th Annual General Meeting (AGM) of the International Astronomical Union, which met in Prague. “There were no more scientific arguments to defend maintaining Pluto as a planet”, says the astrophysicist Enos Picazzio, from the University of São Paulo (USP). Frozen and distant, the diminutive Pluto was always a stranger in the solar family and should never have been allocated the condition of planet. It was not in the lineage of the terrestrial planets (Mercury, Venus, Earth and Mars), nor in the bloodline of the gigantic gaseous worlds (Jupiter, Saturn, Uranus and Neptune). And as well had an orbit very different from that exhibited by the other planets. The discomfort has now ended – school books and encyclopedias will have to rewrite their chapter about the solar system.

The meeting in the Czech capital, which brought together 2,500 astronomers from 75 countries, altered the concept of a planet and created two new categories for astro-objects of the solar system: that of the ‘dwarf planets’, where, for now, one can accommodate the relegated Pluto, the asteroid Ceres and Xena (the nickname of the frozen and thin object 2003 UB313, which until recently had figured as a strong candidate for the tenth solar planet); and the second, ‘small bodies of the solar system’, which covers all of the other objects, with the exception of satellites. By the new parameters, a celestial body has to fulfill three conditions in order to merit the title of planet: to be in orbit around the Sun, to present hydrostatic equilibrium (to practically have the form of a sphere) and to possess a dimension sufficient to dominate its orbit, having brushed from its pathway lesser objects. Only eight bodies fulfill this trio of requirements. Pluto was barred because of the third item, it having been seen that its trajectory crosses with that of the giant Neptune. For this reason it is now a dwarf planet.

With the exception of some dissident voices, especially from American researchers who alleged reasons more historical than scientific for the conservation of the ancient status of Pluto, the decision by the International Astronomical Union (IAU) to reclassify this small frozen object was well received. Even because the previous proposal, outvoted and reformed by the participants of the general assembly, threatened to trivialize the term planet and open up the door so that dozens of astro-objects would leap into this condition. If approved, it would then have brought about chaos in the school books. The new resolution accomplished the reduction of the classification of Pluto in the solar system, but it is not perfect. For some astrophysicists, the definition of a planet approved at the Prague meeting is flawed and does not make clear references to which physical parameters (mass, chemical elements etc.) describe this type of celestial body.

More polemic even would be the recently created categories of astro-objects, the ‘dwarf planets’, which appear to be a consolation prize  for the Pluto fans, and that of the ‘small bodies of the solar system’, an umbrella like expression. “The definition of what is a dwarf planet is going to give lots of things to talk about”, comments Picazzio. The astrophysicist Sylvio Ferraz Mello, also from IAG/USP, in an article published on his institute’s website shortly after the reclassification of Pluto, summarizes well the spirit with which decisions coming from Prague must be confronted. “The adoption of a definition doesn’t mean that the discussion has ended! The International Astronomical Union does not have any legal power to impose a definition”, wrote astrophysicist Mello. “But it’s very convenient to abide by the ruling (…) approved by the IAU so that a common standard can be adopted for didactic books and transmitted to the young.”