Astronomical Concepts – Week 3

The main theme of this week’s session was the outer part of our solar system. This includes:

  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto
  • Asteroids
  • Comets
  • Oort Cloud
  • Kuiper Belt

Oort Cloud

This has never been directly observed but it is believed to exist and it is an area of space on the edge of our solar system between 5,000 to 100,000 AU in distance, so over a vast area. The Oort Cloud consists of millions, perhaps billions of small icy bodies. Every now and then something might disturb one of these bodies and it will become a comet falling towards the Sun. It is named after Dutch astronomer Jan Oort who predicted its existence in 1950.


Kuiper Belt

The Kuiper Belt is another far away region of space that consists of rocky and icy bodies. It extends far beyond Neptune about 30 to 55 AU. It is also predicted to contain over a trillion comets. It takes comets about 200 years to orbit the sun and they travel in a similar plane to the planets. One of the largest and well known objects of the Kuiper Belt is the dwarf planet Pluto. In 2015 the New Horizon’s spacecraft flew past Pluto making it the first mission to a KBO. Another dwarf planet, named Eric, was found in 2005. It is slightly bigger than Pluto and has its own moon. At the time astronomers were considering making Elis the tenth planet, however, in 2006 the International Astronomical Union created a new class of planet called dwarf planets and Pluto and Elis were classified in this new category. The Belt was named after Gerard Kuiper in 1951.

Orbital resonance

A concept described by Paul was orbital resonance. An example to help describe this concept is playing on a push swing. A child can swing by itself at a natural frequency, but the frequency can change by use of an external force, someone else pushing the child on the swing. If the pushes are timed correctly the pushes will build up and the swing gets amplified. With planets and moons when two bodies orbit they exert a regular gravitational influence on each other. This is due to their orbital periods being related by a ratio of two small integers. An example is the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io.


These moons are all in resonance with Jupiter. Io completes exactly 4 orbits and Europa 2 in the same time it takes Ganymede to complete one orbit around Jupiter. During their orbits they sometimes lineup exactly and a gravitational tug is exerted with stretches their orbits into ellipses.

Another example of resonance is the 2:3 resonance of Pluto with Neptune. Pluto completes 2 orbits for every 3 orbits of Neptune around the sun.


Next week … light!


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