Planetary Systems

At Armagh we are investigating the following questions:

How did our solar system form?

How many planets can support life?

Are planetary systems stable?

For a list of published papers on these subjects, go to John Chambers homepage.

To find out about other planetary systems, here is a site on extra-solar planets.

How did our solar system form?

The solar system probably formed about 4.5 billion years ago from a giant disk of gas, dust and ice in space. The central region collapsed due to gravity to become the Sun, while the rest of the disk went on to form the planets, comets and asteroids.

Planet formation began with the grains of dust and ice colliding and sticking together to form larger objects. We are using computer simulations to study how these larger objects accreted one another to form planets and asteroids.

Here are some examples of planetary systems produced by the computer simulations. For comparison, the middle picture shows the inner planets in the solar system (Mercury, Venus, Earth and Mars).

How many planets can support life?

All forms of life on Earth require liquid water to survive, so the presence of liquid water is a good criterion for deciding whether other planets in the universe could be habitable.

Earth is the only habitable planet in the solar system. Venus, closer to the Sun, is too hot to have liquid water. Planets and moons in the outer solar system are too cold for liquid water. In between is a Habitable Zone, where any large, rocky planet should be habitable. Mars lies inside the habitable zone, but it is too small to have a thick atmosphere or liquid water.

Here are 7 planetary systems generated using the computer simulations described above. The red planets are uninhabitable, the yellow ones probably habitable, and the green ones definitely habitable (provided they are large enough):

Are planetary systems stable?

The planets in the solar system have remained on stable orbits for almost 4.5 billion years. However, we don't know whether this will always be true. It is possible that two of the planets could collide at some point in the future.

A system of 2 planets is always stable unless the planets have orbits very close together. For 3 or more planets the situation is more complicated, and has to be investigated using computer simulations.

We find that systems with 3 or more planets are generally unstable if you wait long enough. There is a relationship between the spacing of planetary orbits and the length of time it takes for the orbits to become unstable:

Last Revised: 5th February 1999
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