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Science has no standard definition of a galaxy, and a good one is needed because recent observations have found objects in space that “challenge traditional notions of both galaxies and star clusters.”

So say researchers Beth Willman and Jay Strader in a new draft paper, “'Galaxy', Defined” which says that the terms “galaxy” and “cluster” have both been used for a century or more and applied with an “' I know it when I see it' size-based distinction.”

The pair's proposed definition is:

“A galaxy is a gravitationally bound collection of stars whose properties cannot be explained by a combination of baryons and Newton’s laws of gravity.”

The non-Newtonian aspect of the definition is important, as it allows the inclusion of star groups that may or may not include dark matter.

The paper includes very detailed explanations of how candidate galaxies could be qualified, and in its conclusion makes the case that the proposed new definition is needed “To facilitate comparisons between dwarf galaxy predictions and the increasingly complex sets of observations of candidate dwarf galaxies, the field needs to move forward with an agreed-upon definition for galaxy.”

There's no word in the paper on whether the definition could ever be put to the vote, as happened when Pluto lost its planetary status. ®

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