Stars and Planets » Planets

During star formation a disc of gas and dust form around the new born star. It is from this chaos that planets form. The disc is called a protoplanetary disc – in the case of the Solar System it is referred to as the protosolar disc. By a series of millions of collisions small rocks form which by further colllisions go on to form small asteroids called planetisimals. Eventually a few large bodies are formed and these are the planets. In the case of the Solar System we know that even after the initial formation, the planets suffered a number of catastrophic events – The Earth was hit by a small planet and the Moon was created, Mercury was stripped of some of the outer layers of its outer crust, Venus was caused to spin backwards, Uranus was knocked on to its side and Neptune aquired a moon, Triton. There is now evidence that Jupiter may have suffered giant impacts (New Scientist 14 August 2010). It took millions of years for the Solar System to settle down and even now catastrophic collisions can occasionally occur. Planets can be divided into a number of types.

  1. SMALLROCKY PLANETS such as the asteroid Ceres. The mass of such bodies is too small so form an atmosphere.
  2. SMALL PLANETS made of rock and a high proportion of ices (water,ammonia, methane carbon dioxide). These usually make up the outer regions of the Solar System. Examples are Pluto and Sedna. These are too small to hold an atmosphere.
  3. SMALL TERRESTRIAL PLANETS such as the Moon and Mercury. Although more massive than Ceres and Pluto they are still not massive enough to hold an atmosphere.
  4. TERRESTIALPLANETS such as Earth and Venus and Mars (just about) which are able to hold an atmosphere of all gases except hydrogen and helium.
  5. TERRESTIAL PLANETS considerably more massive than the Earth but still not able to hold hydrogen and helium. These are called Super-Earths. A newly discovered planet light-years away is just four times the mass of our home planet making it the second smallest extrasolar planet to be found to date. Astronomers using the Keck Telescope in Hawaii discovered the alien world, called HD156668b. Its discovery was announced on 7 January 2010 The planet sits in a star system about 80 light-years from Earth in the constellation Hercules and orbits its parent star once every four days or so.
  6. GAS GIANTS At a certain point when a super-earth crosses the threhold when it is able to hold on to hydrogen and helium it may then attract these gases in the interplanetary environment and grow into a Gas Giant. All gas giants have a super-earth core at their heart. Saturn and Jupiter are thought to have begun as rocky worlds with the mass of at least a few Earths. Their gravity then pulled in gas from their birth nebula giving them dense atmospheres. Neptune and Uranus are sometimes referred to as Ice Giants because of the large quanrities of water ammonia and methane in their atmospheres.

It should be clearly understood that MOONS ARE REALLY PLANETS but just happen to be orbiting other primary planets. This is important since a large enough moon such as Pandora may support life as in the science-fiction film Avatar! (A shame about the name chosen since there is a tiny satellite of Saturn a few kilometres in diameter called Pandora.)

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.