The International Astronomical Union’s General Assembly have been voting, in fact they still are [
for now], on the proposals for the definition of a planet listed here. And, as the BBC breaking news report put it “Pluto loses status as a planet” [breaking news? – Ed] and here. I’m trying to confirm the voting, but it seems proposal 5A was passed, and 5B was voted down.. which means, as suggested yesterday, we have lost Pluto.. Updated below Updated againThe definition of a planet
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
They’ve just voted to accept 6A.. 6B
to follow has been voted down.
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.
The following sentence is added to Resolution 6A:
This category is to be called “plutonian objects.”
Update 6B was voted down which means there is no name at present for the new category of trans-Neptunian objects.
Jocelyn Bell Burnell, a specialist in neutron stars from Northern Ireland who oversaw the proceedings, urged those who might be “quite disappointed” about Pluto’s diminished status to look on the bright side.
“It could be argued that we are creating an umbrella called ‘planet’ under which the dwarf planets exist,” she said, drawing laughter by waving a stuffed Pluto of Walt Disney fame beneath a real umbrella.
As renowned astronomer Jocelyn Bell Burnell, formerly of this parish (Northern Ireland) and currently the house patron of Burnell House at Cambridge House Grammar School in Ballymena, comforts Pluto *ahem* the BBC notes that there could be trouble ahead…
Is this the end of the matter?
Unlikely. The Principal Investigator on Nasa’s $700m New Horizons mission to Pluto has lambasted the decision to demote Pluto. Alan Stern told BBC News the decision was “embarrassing” and the criteria amounted to “sloppy science”. He said like-minded astronomers would try to get Pluto reinstated.