May 4, 2010
Is Pluto is set to become an also-ran in the astronomical world?
Already demoted from the exalted planet club, Pluto could be joined by up to 50 other objects in the ever-expanding "dwarf planet" club if the new definitions of dwarf planet, recently proposed by Australian scientists Charles Lineweaver and Marc Norman, are accepted by the International Astronomical Union.
On the other hand, perhaps you would rather regard Pluto as the leading player in the astronomical second division. Rather than being the smallest of the planets, Pluto is set to become the charismatic king of the dwarfs.
The research, entitled The Potato Radius: a Lower Minimum Size for Dwarf Planets, suggests that the number of objects in the solar system classed as dwarf planets could grow by more than a factor of ten. One of the definitions of dwarf planet relates to its ability to exist in hydrostatic equilibrium - that is, the body must have sufficient mass for its self-gravity to pull it into a round shape. This is generally accepted to happen at a radius of around 400 km. Lineweaver and Norman found that the point at which a body loses its rugged and unshapely 'potato' appearance and becomes round depends on what the material that the body is made from. Icy objects form spheres at roughly 200 km radius, whilst rocky spheres form at 300 km. As these radii are considerably less than the generally accepted minimum radius for a dwarf planet - 400 km - the researchers believes that a whole new crop of trans-Neptunian objects should be classified as "dwarf planets."
I had a fascinating chat with Dr Lineweaver about how they went about deriving these numbers, and also about their work defining the shape and mass of other astronomical bodies. Lineweaver derived these radii from first-principles physics, and compared the results to astronomical observation. Bodies are held together by gravitational and electronic forces, and the research showed a strong correlation between the mass and shape of astronomical objects. For example, at a radius of ~ 200 km - 300 km, moons and rocky asteroids transition from a rounded potato shape to a sphere.
The work also looked at other shapes in the Universe, and Lineweaver divided these into five basic shapes - dust, potatoes, spheres, disks and halos. Each of these shapes correlates with the size and mass of the object. Most of the new dwarf planets will be out past Neptune – these are called trans-Neptunian objects. Pluto has a radius of 1,150 km, but it is not the largest known trans-Neptunian object - Eris has a radius of ~1,250 km.
The International Astronomical Union General Assembly, which makes the decisions re astronomical nomenclature, next meets in Beijing in August 2012. No matter what they decide, some people will always call Pluto a planet.