Nasa planetary scientists want to promote Pluto to the status of a planet again

If the proposed definition becomes the accepted scientific convention, then that could mean that there are over 110 planets in the solar system.

Planetary Scientists, including those working on the New Horizons mission by Nasa to explore Pluto want to redefine the definition of a planet. The current definition by the International Astronomical Union (IAU) has stopped considering Pluto as a planet, and recognises that the solar system has eight planets. Pluto, and other objects like it in the outer solar system, have been designated a class of their own, known as dwarf-planets. The IAU definition is not enforceable by international law, and as such is a scientific convention.

There are a number of shortcomings with the definition, as pointed out by researchers who specialise in studying planets, as against astronomers who study planets as well as other celestial objects. A planet can only be in orbit around the Sun, and so extra solar planets in orbit around other stars are not technically qualified to be called planets. The planets are expected to clear all the other objects in their orbit, which no planet in the solar system does, as new objects constantly cross the orbits of planets. The farther away a planet is from the sun, the bigger it has to be to clear the orbit, which means that an Earth sized object in the asteroid belt would not be considered as a planet under the current definition.

 Nasa planetary scientists want to promote Pluto to the status of a planet again

The New Horizons mission by Nasa is exploring Pluto and other Kuiper belt objects.

Additionally, the definition is inconsistent as giant planets are considered as planets, but dwarf planets are not considered as planets. The scientists have proposed to redefine what is a planet according to the inherent characteristics of the object, instead of external factors. The suggested definition is "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

The proposed redefinition excludes stellar objects such as stars, white dwarfs, black holes and neutron stars. Brown dwarfs, which are on the larger end of the planetary scale and on the smaller end of the stellar scale is excluded from the definition, and left for the future when the contentious issue of the planetary status of brown dwarfs is resolved. The suggestion given for a simple definition suitable for use in school textbooks is "round objects in space that are smaller than stars".

Alan Stern, the lead scientist on the New Horizon mission by Nasa has told Tech Insider that the definition of a planet currently used by the IAU  is "Bull****". Stern points out the complex geography on Pluto, including an atmosphere thicker than mercury, water ice mountains that are bigger than the Rocky mountains on Earth, and ridges on frozen plains that are signs of geological activity. "You really should listen to planetary scientists that know something about this subject. When we look at an object like Pluto, we don’t know what else to call it."


The 110 spherical objects in the solar system that would be considered planets according to the proposed definition. Image:

If the proposed definition becomes the accepted scientific convention, then that could mean that there are over 110 planets in the solar system, with new ones being constantly discovered. The many moons in the solar system, including that of the Earth, will all be considered as planets according to the new definition. In scientific literature, there are instances in scientific literature where large spherical asteroids are described as "planets of the Kuiper belt" and the phrase "planet-wide" is used to report conditions across the entire surface of a moon.

For schoolchildren, this does not mean that they will have to learn the names of all the 110 planets in the solar system. It is sufficient to learn the names of only the 12-25 most important objects, similar to how students typically focus on only a few elements of the periodic table, and a handful of the 88 official constellations.

A better approach would be to classify the solar system into zones, and describe the objects typical in those zones. The inner solar system has rocky planets (Mercury, Venus, Earth), the middle zone has gaseous (Saturn, Jupiter), rocky (Mars) and Icy (Europa) planets, whereas the outer solar system has icy (Pluto) planets.

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