• Question: It is believed that our moon was born when another planet collided with the Earth , the Earth survived but the other planet did not, this sent out lots of material which came together and formed our moon. If there are many moons are they all formed from the collisions of planets?

    Asked by amyjohns to Adam, Catherine, Karen, Leila, Nazim on 12 Mar 2012.
    • Photo: Adam Stevens

      Adam Stevens answered on 12 Mar 2012:

      That is a very explanation of what scientists think happened. There’s lots of evidence for this, so it’s a pretty good theory.

      However, most of the moons we know about were probably made in a different way – they were just small planets that got ‘trapped’ by the gravity of their planet. Jupiter and Saturn has so many moons because they’re so big and their gravity is very powerful.

      The moons of Mars, Phobos and Deimos, look so much like asteroids that it’s hard to believe that they’re anything other than captured asteroids.

    • Photo: Leila Battison

      Leila Battison answered on 12 Mar 2012:

      When our moon was formed, over 40% of the outside of Earth was liquedised and mixed with the destroyed planet Thea. So the Earth didn’t quite survive! The moon is made up of the melted mixture of the outside of Earth and all of Thea.

      But the chances of two huge planets colliding like this is quite rare, because most ‘rogue’ bodies are much smaller. Lots of moons are made of ice, that are crystallised out from the gases at the edge of the solar system, or much smaller asteroids that have been trapped by a big planet’s gravity.

    • Photo: Karen Masters

      Karen Masters answered on 12 Mar 2012:

      I don’t think that’s the idea. We think the gas giant planets (Jupiter, Saturn etc) might have had disks around them when they formed and that’s how they got their big moons. Some smaller moons are probably captured asteroids.

      The Earth and the Moon are quite special as they are actually quite similar in size to each other – in some ways more like a binary planet system than a planet moon system (or so I’ve been told).

    • Photo: Catherine Rix

      Catherine Rix answered on 12 Mar 2012:

      I have heard this theory before, I don’t know very much about it – the planetary scientists are the best people for this one I think!

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