Australasian Science: Australia's authority on science since 1938

Galactic Democracy

By Stephen Luntz

Public outrage over Pluto’s demotion as a planet has inspired a unique attempt to engage the public in astronomical decision-making.

Prof Duncan Forbes of Swinburne University has teamed with Prof Pavel Kroupa of Universitaet Bonn to offer members of the public a chance to vote on the definition of a galaxy.

On its discovery, Pluto was thought to be larger than Mercury. With no clear definition of the boundary between a planet and smaller objects, its planetary status survived even as its size was revised downwards. Eventually, the discovery of increasing numbers of objects of similar size forced the astronomical community to come up with a consistent definition.

Something similar is happening with galaxies. Increasing numbers of objects have been found that fill the gap in size between dwarf galaxies and globular clusters. Meanwhile, astronomers are starting to wonder how many globulars are actually the cores of former dwarves. Forbes came up with an estimate of 25% (AS, May 2010, p.5), but other astronomers think it is possible all of them are (AS, Jan/Feb 2011, p.12).

“I think it is important to have a clear definition and well-defined process for determining what is a galaxy. Such a process may provide new insight into how galaxies form and evolve with time,” says Forbes.

However, Forbes is keen to avoid the situation where the ruling is hijacked by astronomers attending an International Astronomical Union conference, as happened in Pluto’s case. The angst created by the 2006 IAU ruling was so great that several US states have passed motions through their Congresses challenging it.

Anyone with an interest can go to, where Forbes says that “people can choose from several suggested definitions for a galaxy, based on factors such as size, motions of stars and the presence of cold dark matter and satellites. They can also suggest their own definition.”

However, Forbes and Kroupa do not want to turn the debate into the equivalent of a newspaper’s online poll, ruled by gut opinion divorced from knowledge. Participants are required to indicate whether they have read a paper the pair published in the Publications of the Astronomical Society of Australia discussing the merits of the five definitions on offer.

The vote lacks official status, but Forbes hopes that professional astronomers will take the results into account, as well as making amateurs with an interest in astronomy feel more engaged. “Many people are interested in astronomy, and opportunities like these allow them to make a real contribution,” says Forbes.