Pluto’s Planetary “Demotion”

August 24, 2006 on 11:10 am | In Science News & Hot Topics |

From news @ nature.com, Planet loses planet status.

In the end it was decided that to qualify as a planet in orbit around our Sun, a chunk of rock must have been made round by its own gravity; have cleared its neighbourhood of other debris; and not be a satellite of another planetary body.

Pluto’s membership of the trans-neptunians disqualifies it from being a fully fledged planet because it has not ‘cleared its orbit’.

Instead, Pluto is one of a new category of object to be known as ‘dwarf’ planets (which, not to be confusing, don’t fall under an umbrella term of ‘planets’, and must, by definition, be written with single quote marks around ‘dwarf’). These objects satisfy the other criteria, in being round and not a satellite. Ceres, which lies in the asteroid belt between Mars and Jupiter, is also now a ‘dwarf’ planet.

‘Dwarf’ planets in Pluto’s neighbourhood, including the object nicknamed Xena (UB313), will be given a category of their own.

The author’s conference blog at the IAU General Assembly gives a far more detailed account of what transpired at the various meetings.

MORE RESOURCES AND INFORMATION:

• Press releases from the International Astronomical Union (IAU)

  This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

  Also, excerpts from a 1999 IAU press release: Recent news reports have given much attention to what was believed to be an initiative by the International Astronomical Union (IAU) to change the status of Pluto as the ninth planet in the solar system. …. No proposal to change the status of Pluto as the ninth planet in the solar system has been made by any Division, Commission or Working Group of the IAU responsible for solar system science. Accordingly, no such initiative has been considered by the Officers or Executive Committee, who set the policy of the IAU itself.

• From the IAU General Assembly’s newspaper: August 16 (pg. 4-5 on the IAU Planet Definition Committee), August 21 (pg. 3 has comments/responses on the Planet Defenition Proposal), an accompanying electronic supplement to the discussion, August 24 (pg. 8), and August 25 (pg. 1 has the final votes on the resolutions).
• Michael Brown, the Caltech astronomer whose discovery of UB313 (”Xena”) contributed to the “What’s a Planet?” debate, writes about UB313 on his website.
• News Articles: New York Times, Washington Post, Los Angeles Times
• Pluto is a planet, but one of 12?, Astronomy Magazine
• From Sky & Telescope: “The Day We Lost Pluto,” Planet Debate Heats Up, and What’s a Planet?