A while back I wrote some very simple programmes to calculate different characteristics of planets depending on their size and average density.

For example, Mars has a surface gravity which is only 38% of Earth's, partly because it is smaller but also because it is less dense (3.9 tonnes per cubic metre cf 5.5T), due to being made more of rock and less of iron. If you wanted to design a bigger planet with the same surface gravity as Earth, then one with the same density of Mars would have a diameter of 17,900 km, twice the surface area, and an escape velocity 20% higher.

Our Moon is even less dense, at 3.3T/m3. So an Earth-gravity planet made up like the Moon (assuming that is possible) would have a diameter of 21,260 km, a surface area 2.8x larger and an escape velocity 30% higher than Earth's.

Obviously, I can do other calculations for planets of different surface gravities.

Earth is actually the most dense planet we know of, closely followed by Mercury and Venus. It will be interesting to see if the planet-hunters find rocky planets orbiting other stars which lie outside the 3.3-5.5T/m3 range.

 

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Tony Williams
Scales (2007)
The Foresight War (2004)
http://www.quarry.nildram.co.uk