Deciding what is and isn’t a planet is a problem on which the International Astronomical Union has generated a large amount of hot air. The challenge is to find a way of defining a planet that does not depend on arbitrary rules. For example, saying that bodies bigger than a certain arbitrary size are planets but smaller ones are not will not do. The problem is that non-arbitrary rules are hard to come by.

In 2006, the IAU famously modified its definition of a planet in a way that demoted Pluto to a second class member of the Solar System. Pluto is no longer a full blown planet but a dwarf planet along with a handful of other objects orbiting the Sun.

The IAU’s new definition of a planet isan object that satisfies the following three criteria. It must be in orbit around the Sun, have sufficient mass to have formed into a nearly round shape and it must have cleared its orbit of other objects.

Pluto satisfies the first two criteria but fails on the third because it crosses Neptune’s orbit(although, strangely, Neptune passes).

Such objects are officially called dwarf planets and their definition is decidedly arbitrary. In its infinite wisdom, the IAU states that dwarf planets are any transNeptunian objects with an absolute magnitude less than +1 (ie a radius of at least 420 km).

Today, Charles Lineweaver and Marc Norman at the Australian National University in Canberra focus on a new way of defining dwarf planets which is set to dramatically change the way we think about these obects.

The problem boils down to separating the potato-shaped objects in the Solar System from the spherical ones. What Lineweaver and Norman have done is show from first principles how this dividing line falls naturally between objects that are larger and smaller than 200 kilometers in radius.

Their approach is simply to look for a potato-sphere threshold in images of bodies in the Solar System. The empirical evidence suggests that the threshold lies at about 200 km.

Lineweaver and Norman then work out the material strength of these bodies in their early years when their shape was being determined. They calculate the other forces at work on these bodies, such as the gravitational forces and the forces associated with rapidly spinning bodies.

It turns out that when viewed from this point of view, the 200 km threshold fits pretty well. Anything smaller than this would almost certainly not have been squeezed by forces large enough to mould it into a sphere. Anything larger, on the other hand, is sufficiently squeezed t form a sphere.

Lineweaver and Norman’s conclusion is that dwarf planets are essentally anything larger than 200km in radius that have not cleared their own orbit of other bodies.

Such a definition fits most objects in the Solar System but there are one or two oddities that don’t fit the bill. the asteroid Vesta, for example, is both potato-shaped and larger than 200km across. Lineweaver and Norman explain this away by suggesting that it may have been deformed by a collision relatively late on in life.

The 200km threshold looks to be a sensible criteria that the astronomical community can rally around. The trouble is that it dramatically increases the number of objects that count as dwarf planets and that may not please everyone, particularly those who hanker for special treatment for Pluto.

On the other hand, it makes Pluto the main representative of the dwarf planets, an important but poorly studied subgroup of bodies in the Solar System. That can only increase interest in this icy object.

As astronomers are only too keenly aware, interest is more or less synonymous with funding. And, of course, that is the unspoken issue at the heart of the debate over what is and isn’t a planet.

Ref: arxiv.org/abs/1004.1091: The Potato Radius: a Lower Minimum Size for Dwarf Plan