Astronomy Without A Telescope – One Potato, Two Potato

Sometimes it’s good to take a break from mind-stretching cosmology models, quantum entanglements or events at 10-23 seconds after the big bang and get back to some astronomy basics. For example, the vexing issue of the potato radius. 

At the recent 2010 Australian Space Science Conference, it was proposed by Lineweaver and Norman that all naturally occurring objects in the universe adopt one of five basic shapes depending on their size, mass and dynamics. Small and low mass objects can be considered Dust – being irregular shapes governed primarily by electromagnetic forces. 

Next up are Potatoes, being objects where accretion by gravity begins to have some effect, though not as much as in the more massive Spheres – which, to quote the International Astronomical Union’s second law of planets, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

Objects of the scale of molecular dust clouds will collapse down into Disks where the sheer volume of accreting material means that much of it can only rotate in a holding pattern around and towards the centre of mass. Such objects may evolve into a star with orbiting planets (or not), but the initial disk structure seems to be a mandatory step in the formation of objects at this scale. 

At the galactic scale you may still have disk structures, such as a spiral galaxy, but usually such large scale structures are too diffuse to form accretion disks and instead cluster in Halos – of which the central bulge of a spiral galaxy is one example. Other examples are globular clusters, elliptical galaxies and even galactic clusters. 

The proposed five major forms that accumulated matter adopts in our universe. Credit: Lineweaver and Norman.

The authors then investigated the potato radius, or Rpot, to identify the transition point from Potato to Sphere, which would also represent the transition point from small celestial object to dwarf planet. Two key issues emerged in their analysis. 

Firstly, it is not necessary to assume a surface gravity of a magnitude necessary to generate hydrostatic equilibrium. For example, on Earth such rock crushing forces only act at 10 kilometres or more below the surface – or to look at it another way you can have a mountain on Earth the size of Everest (9 kilometres), but anything higher will begin to collapse back towards the planet’s roughly spheroid shape. So, there is an acceptable margin where a sphere can still be considered a sphere even if it does not demonstrate complete hydrostatic equilibrium across its entire structure. 

Secondly, the differential strength of molecular bonds affects the yield strength of a particular material (i.e. its resistance to gravitational collapse). 

On this basis, the authors conclude that Rpot for rocky objects is 300 kilometres. However, Rpot for icy objects is only 200 kilometres, due to their weaker yield strength, meaning they more easily conform to a spheroidal shape with less self-gravity. 

Since Ceres is the only asteroid with a radius that is greater than Rpot for rocky objects we should not expect any more dwarf planets to be identified in the asteroid belt. But applying the 200 kilometre Rpot for icy bodies, means there may be a whole bunch of trans-Neptunian objects out there that are ready to take on the title.

Minor Planets

Main Belt Asteroids

Minor planet is a term used to refer to a celestial object – that is not a planet or comet – which orbits the Sun. Found in 1801, Ceres, also known as a dwarf planet, was the first minor planet discovered. The term minor planet has been in use since the 1800’s. Planetoids, asteroids, and minor planets have all been used interchangeably, but the situation became even more confusing when the International Astronomical Union (IAU) committee reclassified minor planets and comets into the new categories of dwarf planets and small solar system bodies. At the same time, the IAU created a new definition of what a planet is, and Pluto was reclassified as a dwarf planet. Hydrostatic equilibrium – the ability to maintain a roughly spherical shape – is what separates dwarf planets from the more irregularly shaped small solar system bodies. The names become even more confusing because the IAU still recognizes the use of the term minor planets.

Minor planets are extremely common with over 400,000 registered and thousands more found each month. Approximately 15,000 minor planets have been given official names while the rest are numbered. When asteroids were first discovered, they were named after characters from Greek and Roman mythology like Ceres was. At first, astronomers thought that the asteroids, especially Ceres and Pallas were actually planets. Astronomers also created symbols for the first asteroids found. There were symbols created for 14 asteroids and some of them were very complex like Victoria’s symbol, which looks like a plant with three leaves growing out of an off center starburst. Soon, astronomers ran out of mythological names and started christening asteroids after television characters, famous people, and relatives of discoverers. Most names were feminine, attesting to an unnamed  tradition. As the numbers ran into the thousands, scientists started using their pets as inspiration. After an asteroid was named 2309 Mr. Spock, pet’s names were banned. That did not stop the oddness though because names such as 9007 James Bond and 6402 Chesirecat have been suggested and actually accepted.

There are a number of different categories that minor planets fall into including asteroids, Trans-Neptunian objects, and centaurs. There  are various types of asteroids, although most of them can be found in the asteroid belt, which is the region of space between Mars and Jupiter. Trans-Neptunian objects are celestial bodies found orbiting beyond Neptune, and centaurs are celestial bodies with unstable orbits located between Jupiter and Neptune. The categories also overlap, making classifying things a nightmare. For example, Ceres is a dwarf planet and minor planet, additionally it can also be classified as an asteroid.

Universe Today has a number of articles including astronomers find new minor planet and why Pluto is no lone a planet.

You can also check out these articles on asteroids and the solar system.

Astronomy Cast has an episode on the asteroid belt you will want to listen to.