The International Astronomical Union

It has been rather quiet on the "Pluto/planet" debate and the role and significance of the The International Astronomical Union [IAU]. As readers may know that there is one Ms. Laurel Kornfeld who is an ardent supporter of Pluto's status as a planet and has offered detailed reasoning for such support here and in our email correspondence. [Use the blog's search engine for many items on Pluto.] Many times the IAU has been the focal point of discussion. Neither Laurel nor I can agree on the merits of decisions rendered by the IAU members that realized a final decision regarding Pluto's status.

It is my opinion that the IAU is making the correct decision for the time being and that such a decision would be subject to modification. The IAU has been around for a long time making hundreds of decisions without vehement contest and as far as I am concerned the IAU is a well-established body of astronomy experts making decisions for astronomers using sound scientific criteria. But the line was crossed when Pluto was demoted based on new criteria...a true class between scientists and the populace. If the IAU is not to be trusted in their decisions, then what body of experts would?

The International Astronomical Union — the first 90 years

"Pluto and the Developing Landscape of Our Solar System"

The discovery of Pluto

Nearly eighty years ago an astronomer working at the Lowell Observatory in the United States made a discovery that would ultimately initiate a dramatic change in the way we look at our Solar System. The young astronomer was Clyde Tombaugh, an observing assistant working at the observatory made famous by the great astronomer Percival Lowell. Tombaugh was continuing the search for an elusive planet – planet X – that Lowell had believed (incorrectly) to be responsible for perturbing the orbits of Uranus and Neptune.

Within a year, after spending numerous nights at the telescope exposing photographic plates and months tediously scanning them for signs of a planet, Tombaugh saw what he was looking for. At around 4pm on the afternoon of 18 February 1930 Tombaugh began comparing two plates taken in January that year showing a region in the constellation of Gemini. As he flicked from one plate to the other, trying to see if something moved slightly between the two (the tell-tale sign of the planet he was hunting), he spotted something. In one part of the frame a small object flitted a few millimetres as he switched between the two plates. Tombaugh had found his new planet! (Stern & Mitton, 2005)

The changing landscape of the Solar System

The object Tombaugh had discovered was named Pluto, a name officially adopted by the American Astronomical Society, the Royal Astronomical Society in the UK and the IAU. It is a frigid world, billions of kilometres from Earth, and 30 times less massive than the then-smallest known planet, Mercury. But Pluto was not alone. It was found to have three satellites. The largest, Charon, was discovered in 1978. The smaller two were discovered using the Hubble Space Telescope in 2005 and officially named Nix and Hydra by the IAU in early 2006 (read more) .

The view of our solar system's landscape began to change on August 30, 1992 with the discovery by David Jewitt and Jane Luu from the University of Hawaii of the first of more than 1000 now known objects orbiting beyond Neptune in what is often referred to as the transneptunian region. More generally these bodies are often simply labelled as Trans-Neptunian Objects (TNOs).

With so many Trans-Neptunian Objects being found, it seemed inevitable that one or more might be found to rival Pluto in size. On the night of the 21 October 2003, Mike Brown from Caltech, Chad Trujillo from the Gemini Observatory and David Rabinowitz from Yale University were using a telescope and camera at the Palomar Observatory in the US to search the edge of the Solar System. That night they imaged a region of sky showing an object moving relative to the background stars. Later analysis showed that they had discovered another cold world, around 2500 km across, orbiting the Sun. Subsequent observations showed that the new object, initially named 2003 UB313 according to the International Astronomical Union's protocol on the initial designation of such objects, was more massive than Pluto and that it too had a satellite (read more) . With an object larger and more massive than Pluto now beyond Neptune and ever more of these Trans-Neptunian Objects being discovered, astronomers were beginning to ask: "Just what constitutes a planet?"

A new class of objects and how to define a planet

The IAU has been responsible for the naming and nomenclature of planetary bodies and their satellites since the early 1900s. As Professor Ron Ekers, past president of the IAU, explains:

Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned. (read the full article) .

The IAU decided to create a committee to gather opinions from a broad range of scientific interests, with input from professional astronomers, planetary scientists, historians, science publishers, writers and educators. Thus the Planet Definition Committee of the IAU Executive Committee was formed and quickly went about preparing a draft resolution to put to the members of the IAU. After the final meeting in Paris the draft resolution was completed. One crucial aspect of the resolution is described by Professor Owen Gingerich, Chair of the IAU Planet Definition Committee: "On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects". (read more) .

The final resolution

The first draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly in Prague and a new version slowly took shape. This new version was more acceptable to the majority and was put to the members of the IAU for a vote at the Closing Ceremony on the 24 August 2006. By the end of the Prague General Assembly, its members voted that the resolution B5 on the definition of a planet in the Solar System would be as follows:

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.

(read more)

Dwarf Planets, plutoids and the Solar System today

The IAU Resolution means that the Solar System officially 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 on. 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 Eris, formerly known as 2003 UB313. Eris was named after the IAU General Assembly in 2006 (read more) . Eris is the Greek god of discord and strife, a name which the discoverer Mike Brown found fitting in the light of the academic commotion that followed its discovery.

The dwarf planet Pluto is recognised as an important prototype of a new class of Trans-Neptunian Objects. The IAU has put given a new denomination for these objects: plutoids.

Today the resolution remains in place and is a testament to the fluid nature of science and how our view of the Universe continues to evolve with changes made by observations, measurements and theory.

References:

Stern, A., & Mitton, J., 2005, Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, Wiley-VCH 1997

History of the IAU. Birth and First Half-Century of the International Astronomical Union

by

Adriaan Blaauw

ISBN 0-7923-2980-5

This book describes the birth and early development of the International Astronomical Union, the world-wide union of professional astronomers that over the past 75 years played an important role in the development of astronomy. However, much attention is also paid to the Union's successful bridging political controversies and maintaining the ties between astronomers in times of war or heavy political tension. The book therefore is of interest not only to astronomers - professional as well as amateur - but also to students of the history of modern science in general. The author has witnessed the development of the Union over the past 55 years, the years 1976 - 1979 as its President.

International Astronomical Union

International Astronomical Union [Wikipedia]