I dunno what the percentage is, but, yes, there is some good evidence that at least some water on the earth came from comets.

The earth's EM field doesn't repel large chunks of non-magnetic, non-metallic material. It's not like it's some sort of Star Trek force field shield system. Charged particles from the sun are spun into the field. Uncharged particles are not. Gigantic uncharged chunks of stuff are entirely unaffected.

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