Nog wrote:
If comets are made from mostly water-ice, some dirt and organic matter they
had to come from a former water planet that blew up somewhere in the
universe.

Er, no. Stars are made of hydrogen, so when they go super nova, they
scatter a lot of hydrogen. Further, larger stars makes a lot of oxygen
(see C-N-O cycle fusion) so when bigger stars go super nova, they
scatter a lot of water. That's where the water in comets (and on Earth)
comes from: stellar nucleosynthesis. The same also applies to carbon
and nitrogen - they're generated in stars.

Comets - with water, organics, and rock dust - needn't come from
planets at all.

Mike Miller