“Demotion” planned for Pluto?

Kuiper Belt Objects

The International Astronomical Union (IAU), the body that names and defines celestial objects, starts a marathon 12-day meeting, the IAU XXVIth General Assembly, in Prague today.

One of the subjects to be discussed is the need for a formal IAU definition of planet. (Believe it or not, no formal IAU definition has ever existed before.) This discussion will decide the fate of Pluto’s status as a planet.

The ninth planet just isn’t like the others. When it was discovered, scientists didn’t understand how its orbital eccentricities separated it from the other planets, and thought it was several times larger than Earth (it’s smaller than the moon).

The Guardian writes about Pluto and the meeting here.


When is a planet not a planet? Judgment day looms for Pluto

Solar system Astronomers to vote on new definition
Celestial bodies may be cut to eight – or raised to 50

LiveScience’s Robert Roy Britt blogs about Pluto’s status here, mentioning a recent NPR segment where reporter David Kestenbaum interviews several “experts” who had recently gathered in Paris for an IAU-sponsored discussion of planet nomenclature. Their consensus? Pluto is a planet.


There seems to be a catch. There’s talk of still calling Pluto a planet, yes, but of “demoting” it down into a new class of smaller planets with odd orbits. Britt writes:

If all goes as it should—meaning if astronomers can put aside their quibbles and vote “yes”—in 20 years Pluto will probably still be popular with children, but rather than being known as the 9th planet, it’ll be known as the first object ever discovered in that sea of dozens or maybe even hundreds of dwarf planets that will have been found by then.

So far I’m behind this new definition, this breaking down of our local planets into categories. Despite our cultural understanding of the nine planets, Pluto’s not a planet like the others (and if it is, what is Xena?). It’s something else. Based on my admittedly nonexpert understanding of the facts, I believe that Pluto needs to lie within a different category than the other eight planets. The category where I hope that science will put Sedna, Quaoar, Xena, and the other bodies pictured above.

I imagine and even hope that our old cultural definition of Pluto as a planet like the others will fade, but that Pluto will yet be known as a planet because there will be several kinds of planets. I really like the idea below (it’s Britt paraphrasing what Kestenbaum said):

Several of the panel members favor dividing round objects up as terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto.

A third class of very small rocky worlds with eccentric orbits — the category “dwarf planets” is being bandied about. And that’s where Pluto and the others would go.

One response to ““Demotion” planned for Pluto?

  1. There is a prior definition, though in issuing it, the IAU included a cop-out, stating that it was merely a “working definition,” meant to cover only the known bodies.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s