The International Astronomical Union (IAU) is not often in the news, but its proposed redefinition of the word “planet” hass certainly caught the public’s eye. We’ve been waiting for the final decision to be made, and we’re glad we waited, because there have been some important last minute changes.
Science textbooks will have to be ripped up - the solar system is about to get a bunch of new planets.
Astronomers want to redefine our home in the Milky Way as a place with 12 - rather than nine - planets orbiting the sun.
The proposal comes from a two-year project by the International Astronomy Union (IAU) to create the first ever scientific definition for the term planet.
It will also mean that Pluto keeps its status as a planet, despite calls from many astronomers, revealed in the Guardian on Monday, that it should be demoted because of its diminutive size.
If the ideas are approved at the general meeting of the IAU in Prague next week, schoolchildren will, in future, have to learn that the solar system has 12 planets: eight classical ones that dominate the system - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus - and four in a new category called plutons.
These are Pluto, its moon Charon, a spherical asteroid that sits between Mars and Jupiter called Ceres, and an object called 2003 UB313 but nicknamed Xena by American astronomers who found it.
(There’s one mistake there: since “plutons” were going to be defined as having orbits lasting more than 200 years, Ceres – with an orbital period of 4.6 years – is certainly not a “pluton”.)
There were at least three major issues involved:
(1) Bringing Ceres, and possibly other asteroids such as Pallas and Vesta, back into the fold as planets. (When they were first discovered around 200 years ago, they were called planets, but as more were discovered they were put into a new category as “asteroids”)
(2) Treating the Pluto-Charon system as a double planet system, and hence Charon as a planet, because they orbit around a centre of mass which is outside Pluto and between the two objects.
(3) Adding 2003 UB313 (popularly called Xena, but more likely to be called Proserpine or Persephone in the future), together with potentially many other trans-Neptunian objects (Kuiper belt objects)
In them, the proposals now define:
(1) A "planet" as having "cleared the neighbourhood around its orbit" – which excludes Ceres and Pluto, and leaves us with just 8 planets in the solar system
(2) A "dwarf planet" – which will include Ceres, Pluto, Charon, "Xena" and possibly other asteroids and KBOs.
(3) "Small Solar System Bodies" for the other asteroids and KBOs.
In addition, they are now refering to KBOs as "Plutonian objects" rather than "Plutons" (which would have conflicted with a different meaning of the same term in geology).
And breaking news in the Chicago Tribune says that the proposed resolution has just now been adopted.
We’re still looking at what this means for 523.4 Planets of solar system and for 523.48 Trans-Uranian planets in Dewey. In particular, we will need to decide exactly where Charon and other KBOs ("Plutonian objects") go in the classification, and whether “Trans-Uranian planets” is still the best caption.