How likely is it that this sphere of water actually collided with our Earth billions of years ago? What is the possibility of even more such "water meteoroids" flying around nearby galaxies?
Water does not stay liquid in a vacuum. It is either solid or gaseous, depending on the ambient conditions. It is not well established where the primordial Earth gained its water. The matter in the protoplanetary disc from which the Earth coalesced probably didn't contain much water or other volatiles because of the proximity to the Sun. Instead, it is thought that water arrived afterwards, via numerous collisions - then commonplace - with icy comets that originated beyond the frost line.
Yes, sort of. I believe the current theory is that much of Earth's water came from thousands or millions of meteorites that carried ice and/or hydrogen and oxygen compounds that later stabilized to water. This would have happened during the solar system's infancy, when there were far more asteroids flying around our solar system. In fact, it is thought that the planets started this way. Lots of free flying rock colliding together, gaining mass, gravity, etc. over billions of years.
http://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29 http://en.wikipedia.org/wiki/Origin_of_water_on_Earth