When we eventually find life on other planets, we won't find plants and animals. We might find stationary, multicellular, vascular organisms that use chlorophyll for photosynthesis, but they won't be plants. We might find complex, heterotrophic organisms with bones, nerves, and sense organs, but they won't be animals.
A plant is the descendent of a species that lived hundreds of millions of years ago. As such, it shares certain traits with other descendents of the same species. An animal is the descendent of another species that also lived hundreds of millions of years ago. Creatures on other planets aren't descendents of these species, so they're not plants and not animals.
Earlier in the development of modern thought, "plant" and "animal" were names of categories. Creatures with certain characteristics were placed in one category or the other. According to this way of understanding kingdoms of organisms, a creature on another planet that had the right array of characteristics would be a "plant" or an "animal."
But that's not how we think any more. A species' taxonomic identity is a matter of lineage, not characteristics.
Creationism would imply that taxonomy is subjective. Since taxonomy, to a creationist, doesn't represent a history of descent, the categories are arbitrary. For a scientist, whether a whale is a fish or a mammal is a real question. For a creationist, whether a whale is a fish or a mammal is simply a matter of definition, since whales are not related by descent to other animals. Taxonomy is one case in which it's the fundamentalists, not the natural scientists, who are relativists. Here's a page on baraminology, creationism's relativistic taxonomy.
"we": While Homo sapiens might not find life on other planets, our biological and digital descendents almost certainly will.
Twilight of the Reptiles: taxonomy, subjectivity
GenCon SoCal 2004