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.
