I just read an interesting article from the Astronomical Society of the Pacific (an international astronomy education organization), it’s a take on the whole Pluto debate – from the point of view of someone defending Ceres, rather than someone defending Pluto. My opinion remains strong: I think that the IAU reclassification of Pluto is great, engenders wonderful classroom debates, and doesn’t demean Pluto in any way. It’s just an opinion though.

Mark Sykes, the author of the article, raises one very good point though. He claims that most planetary scientists do not belong to the IAU, and they’re the ones who should be classifying objects as planets, plutoids, and whatever. He’s right – there’s no reason for planetologists to join the IAU. And, they are the ones studying planets.

What is a Planet?

IAU Definition – August 24, 2006

The International Astronomical Union (IAU) voted on the definition of planet on August 24, 2006.

A Planet is:

1. Orbiting a star.

2. Big enough to be spherical – an IAU process will decide if any new bodies discovered are planets or not.

3. Massive enough to clear its orbit of similar bodies.

A Dwarf Planet is:

4. Orbiting a star.

5. Big enough to be spherical.

6. Not massive enough to clear its orbit of similar bodies.

7. Not a moon.

In addition, other categories can be used to be more specific:

A Plutonian (a new official category, but not an official name):

8. A dwarf planet similar to Pluto in size and orbit.

A Small Solar System Body is:

9. Anything that’s not a planet.

What does this mean?

Planets (as currently recognized by the IAU):

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Plutonians:

Pluto, Eris

Dwarf Planets:

Ceres, Pluto, Eris

Planets that could be added under this definition:

The bodies discovered over the last few years: “Sedna,” “Quaoar,” and many other recently discovered Trans-Neptunian Objects.

The current asteroids Vesta, Pallas, Hygeia