Euglena are a unicellular organism. They are classified into the Kingdom Protista. They are then further classified into the Phylum<span> Euglenophyta. All euglena can make their own food by photosynthesis.</span>
Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
To be honest, since we live on Earth (assuming you do as well), Earth is the easiest planet to study. We don't know much about the other planets because we've never been to any of them. I believe that the second easiest planet to study is either Uranus or Saturn.
