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| IAU 2006 definition of planet | |
|---|---|
| Name | IAU 2006 definition of planet |
| Adopted | 2006 |
| Organization | International Astronomical Union |
| Location | Prague |
| Key figures | Mike Brown; Alan Stern; Cecilia Payne-Gaposchkin |
| Related | Pluto; Eris; Kuiper belt; Solar System |
IAU 2006 definition of planet The International Astronomical Union (IAU) 2006 definition of planet is a formal resolution adopted at the IAU General Assembly in Prague that established quantitative and categorical criteria for classifying bodies within the Solar System. The resolution arose amid debate following discoveries in the Kuiper belt and beyond, notably the identification of Eris and refinement of observations from projects like the Hubble Space Telescope and surveys led by astronomers such as Mike Brown. The decision reclassified several bodies and created new terminology affecting historical and contemporary practice across institutions including the Jet Propulsion Laboratory and university departments.
In the early 21st century, observational programs using instruments like the Subaru Telescope and the Palomar Observatory uncovered trans-Neptunian objects including Eris, Haumea, and Makemake that rivaled Pluto in size. Discoveries by teams led by Mike Brown and analyses supported by facilities such as the Keck Observatory and the Sloan Digital Sky Survey intensified debate at conferences and within bodies like the International Astronomical Union about whether the historical category of Planet remained adequate. Influential voices included astronomers from institutions such as the Smithsonian Astrophysical Observatory, European Southern Observatory, and university researchers influenced by precedents from the Copernican Revolution and discussions reminiscent of classifications used by Johannes Kepler and Isaac Newton. Policy and educational implications prompted national academies and agencies including the National Aeronautics and Space Administration and the Royal Astronomical Society to seek clarity.
At the XXVI General Assembly of the International Astronomical Union in Prague, delegates debated Resolution B5, which was proposed following motions by commissions and working groups that included representatives from the International Planetary Data Alliance and committees chaired by prominent astronomers. The assembly adopted a three-part definition distinguishing bodies that orbit the Sun and either clear their orbital neighborhood or do not, thereby formally introducing the term dwarf planet. The vote and subsequent statements involved delegations from national bodies including the Royal Astronomical Society and the American Astronomical Society, and proceedings were covered in reports circulated among observatories like Mauna Kea Observatories and research institutes such as the Max Planck Institute for Astronomy.
The resolution set out three criteria: (1) the body must orbit the Sun; (2) it must be in hydrostatic equilibrium (nearly round) as understood from studies influenced by work at the California Institute of Technology and the University of Cambridge; and (3) it must have cleared its neighboring region of other objects, a dynamical concept studied with methods developed at the Jet Propulsion Laboratory and informed by simulations from groups at the Massachusetts Institute of Technology and the Harvard–Smithsonian Center for Astrophysics. Bodies meeting (1) and (2) but not (3) were designated dwarf planet, and other small bodies were classified as Small Solar System body. The nomenclature intersected with historical terminology tied to figures like William Herschel and classification practices used in planetary science by teams at institutions such as the European Space Agency.
The IAU decision directly led to the reclassification of Pluto from full planet to dwarf planet, affecting educational materials, museum exhibits at institutions like the Smithsonian Institution, and curricula in departments at universities including Princeton University and the University of Oxford. Catalogs maintained by the Minor Planet Center and mission planning at agencies such as NASA were updated, influencing missions like New Horizons and studies at facilities including the Applied Physics Laboratory. The scheme provided a working taxonomy used by pedagogical bodies such as the International School for Advanced Studies and regulatory frameworks within professional societies like the International Astronomical Union's commissions.
The resolution provoked criticism from astronomers including Alan Stern and commentators in outlets tied to institutions such as the European Southern Observatory, who argued that the Sun-centric wording excluded exoplanetary contexts studied at programs like the Kepler Space Telescope mission and by teams at Caltech. Critics questioned the operational meaning of "clearing the neighborhood" and cited dynamical studies from groups at the Max Planck Institute for Solar System Research and the University of Arizona indicating gradations rather than binary thresholds. Others pointed to procedural concerns about the voting process at the Prague General Assembly and debated historical continuity comparing the decision to nomenclatural reforms discussed at past congresses such as sessions of the Royal Society.
Following 2006, multiple proposals surfaced from researchers at institutions including Cornell University, Southwest Research Institute, and the University of Colorado Boulder to revise or replace the definition with mass-based, formation-based, or dynamical criteria. Workshops convened at bodies like the International Astronomical Union and meetings of the American Geophysical Union continued to address exoplanet classification issues raised by missions such as Spitzer Space Telescope and teams involved in the Transiting Exoplanet Survey Satellite. Scholarly articles from researchers affiliated with the Max Planck Institute for Astronomy and proposals by consortia including the International Planetary Data Alliance keep the debate active, while observational programs continue to discover new trans-Neptunian objects that test classification thresholds.