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Asked by anon-186958 to Stewart, Miriam, Marton, Laura, Kathryn, David on 6 Nov 2018.
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David Ho answered on 6 Nov 2018: last edited 6 Nov 2018 7:23 am
This might be a little bit different to the answer you were looking for, but your question gives a good opportunity to talk about a really important idea in science — what does it mean to be big? To an ant, humans are enormous. But ants still find atoms far too small to see, just like we do. The earth is giant compared to us, but compared to a star like the sun it also seems miniscule. Compared to a galaxy like the milky way it’s barely a blip. It’s not even that big compared to other planets: this link shows a nice picture of all of the planets to scale: https://www.universetoday.com/36649/planets-in-order-of-size/
You might ask the question “why is the earth so big?”, but someone who’s been looking at a lot of galaxies could just as well ask “why is the earth so small?”
Scientists normally use words like “big” and “small” to compare things — the technical term for this is “relative size”. Most of the time relative size is a lot more useful than just looking at size on its own.
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Miriam Hogg answered on 6 Nov 2018:
David has given a really great answer here, however I’d like to add some information for you.
The Earth is a great size for our life. If we were smaller we might not have enough gravity to hold on to our atmosphere. Also if we were smaller the Earth would have cooled more and we might not have enough liquid inside to create a magnetic field (which protects us from the suns more excited particles which would damage us). If we were a lot larger we would be like the Gas giants! we would have a much larger atmosphere which would increase the pressure we feel on land and would make it harder for us grow to a big size.
We evolved to survive on Earth perfectly. If you changed the size of our planet we probably would find it hard to live. Although if we had started with a different sized planet maybe we would have evolved to survive on those, which means we would probably have looked a lot different! -
Stewart Martin-Haugh answered on 9 Nov 2018:
If the world was smaller, it wouldn’t have enough gravity to hold us in place!
Gravity is much weaker than all the other forces, so you need something as big as the Earth to keep us in place. You can tell that electromagnetism is stronger than gravity by lifting up a paperclip with a magnet: the magnet is very small compared to the Earth but it’s winning!
The weakness of gravity is a mystery: some people think it might be leaking away into another dimension. If that’s true, it might be possible to see evidence of the leak in a particle collider like the Large Hadron Collider. So far, no evidence has been found.
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