Pluto No Longer A Planet, Demoted To ‘Dwarf’

 Pluto, the ninth and smallest planet of our solar system, has just been demoted.

Astronomers gathered in Prague decided that it didn’t fit historic new guidelines on what constitutes a planet. Pluto had held that distinction since its discovery in 1930.

It has now been reclassified as a “dwarf planet” in a new category for minor planets.

“It could be argued that we are creating an umbrella called planet under which the dwarf planets exist,” said neutron star specialist Jocelyn Bell Burnell, who was overseeing the proceedings.

The International Astronomical Union made the controversial decision after a week of bickering over that issue and others relating to the cosmos.

The group came up with a set of basic rules a celestial object has to meet before it can be classified as a planet.

The new planet definition reads as follows: “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighbourhood around its orbit.”

Pluto is smaller than Earth’s moon and has what experts consider to be an eccentric orbit – overlapping with Neptune’s. That’s part of the reason for re-classifying it.

Not since the time of Copernicus have scientists had a strict definition on what makes a planet.

The change means the classical solar system now has eight planets – Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus and Neptune.

Two other objects that were flirting with ‘planethood’ have also now been classified as dwarfs – the asteroid Ceres and 2003 UB313, an icy orb larger than Pluto. Charon, the largest of Pluto’s three moons is no longer being considered for special designation.

A third class of sun-orbiting objects was also determined – “small solar system bodies” will apply to asteroids, comets and other natural satellites.

The final decision was met with applause by most of the 2,500 astronomers from 75 countries attending the conference.

Text of the resolutions approved Thursday by the International Astronomical Union which defines planets and downgrades Pluto:

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets.” The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

The IAU therefore resolves that “planets” and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A “planet” is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A “dwarf planet” is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as “Small Solar-System Bodies.”

The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The IAU further resolves:

Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.