The geophysical planet definition is an alternate definition of what is and is not a planet that was proposed in response to criticism of the definition adopted by the International Astronomical Union. The geophysical planet definition states: a planet is a sub-stellar mass object that has never undergone nuclear fusion that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid, regardless of its orbital parameters. This is equivalent to the second clause of the IAU definition. It excludes the first clause and the third clause. This definition results in dwarf planets and round moons being counted as planets. Five bodies are currently recognised as dwarf planets by the IAU: Ceres, Pluto, Haumea, Makemake, and Eris. It has been suggested that for icy bodies the limit at which objects are likely to be in hydrostatic equilibrium is around 400 km. This would mean there are many more dwarf planets in the Kuiper belt, and would make dwarf planets the most common type of planet in the Solar System under the geophysical planet definition. The proposed geophysical planet definition allows objects in the solar system to be grouped together on the basis of common physical properties regardless of location. An examination of spacecraft imagery of objects in the solar system reveals that the threshold at which an object is large enough to be round by self-gravity is also likely to be where geological activity becomes prevalent. However, some "round" satellites such as Mimas and Callisto are geologically inactive. Others, such as Io, Europa and Enceladus are geologically active due to tidal heating, rather than the gravitational energy more closely associated with "roundness". Early proponents of the geophysical planet definition include the lead organisers of a petition shortly after the IAU vote, protesting the definition adopted by the IAU. The petition attracted more than 300 signatures. Proponents of the geophysical planet definition have published papers reviewing the planetary science literature, which shows this definition has occasionally been used by some planetary scientists for decades, including after the IAU definition was established, and that asteroids have routinely been regarded as "minor" planets though the usage is somewhat inconsistent and varies considerably. Others have expressed general support for a definition based on intrinsic values, particularly one that includes Pluto as a planet, consistent with the reasoning behind the geophysical planet definition, albeit without explicitly endorsing a specific proposed definition. Since the geophysical planet definition does not specify that planets orbit the Sun, it also applies to exoplanets. It excludes objects that have ever undergone nuclear fusion so it may exclude some of the higher mass objects included in exoplanet catalogs and brown dwarfs. The Extrasolar Planets Encyclopaedia, Exoplanet Data Explorer and NASA Exoplanet Archive all include objects significantly more massive than the theoretical 13 Jupiter mass threshold at which deuterium fusion begins for reasons including: uncertainties in how this limit would apply to a body with a rocky core, uncertainties in the masses of exoplanets, and discussions about whether deuterium burning or formation mechanism is the most appropriate criterion to use to define an upper cut-off. Nonetheless, both the geophysical planet definition and the IAU definition require a planet to be "round". Determining the roundness of a body requires measurements across multiple chords but exoplanet detection techniques provide only the planet's mass, the ratio of its cross-sectional area to that of the host star, or its relative brightness. However, as of 2019, the smallest detected exoplanet has a mass 0.02 times that of the Earth, and analogy to objects within our own solar system suggests that is enough.
