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Asked by jess4coop to Adam, Catherine, Karen, Leila, Nazim on 16 Mar 2012.
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Adam Stevens answered on 16 Mar 2012:
That’s how they formed. They would have started off a lot smaller and slowly collected other bits of planet, gradually growing. They don’t really any more though, unless you count the odd meteorite.
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Karen Masters answered on 17 Mar 2012:
Yes. They grow by asteroids and comets and things falling onto them.
Right now Earth grows only very slowly as there are not many asteroids and comets hitting it. But in the past there were periods when this happened much faster. And in the first place Earth formed entirely by collecting up bits of rock and stuff from the debris disc around the baby Sun.
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Leila Battison answered on 19 Mar 2012:
Planets can grow if they have other matter added to them from incoming objects, like meteors or asteroids or comets. In the early stages of a solar system, this will happen a lot because there are lots of smaller rocks floating around in space in the way of an orbiting planet.
Once all the nearby rocks have been swept up and absorbed into a planet, it can only get more if a rogue comet or rock comes flying in from outer space. Without them, the planet can’t change its volume or mass, even though the amounts of the different kinds of rocks might change because of plate tectonics or volcanoes.
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Nazim Bharmal answered on 20 Mar 2012:
Once planets have formed, they don’t grow or shrink much because there isn’t much dust, ice, or gas left over in the solar system (which is what they were mainly made out of). In theory two planets can collide when they are still forming and you can get one new bigger (or smaller) one: we think that is how the moon was made when a (bigger) planet hit the Earth and what was left over became the moon, the rest went into growing the Earth.
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