How do you know if there are other planets past our galaxy ?
There is as yet no solid evidence for planets around other stars.
Other stars are so far away that we can't see their planets directly.
Astronomers hope to infer the existence of extra-solar planets by
carefully measuring the slight wobble of a star that has a large
planet orbiting it. Jupiter could be detected this way from nearby
stars by looking at our sun. This is because Jupiter does not orbit
around the exact center of the sun: both the sun and Jupiter orbit
around their "center of mass" (we can ignore the other planets
because they are so much less massive than Jupiter).