An explorer would discover new plants or animals. He wouldn't discover the new plants - that would imply the plants had already been found and he had merely come across them again. Same with "elementary particles".
"....buried in the earth (ground)"
"....transmitted from Earth (the planet)
If "earth" is used it usually means "ground" and needs "the".
If "Earth", it's usually the planet, so doesn't need "the".
We plant seeds in the earth, but we launch rockets into space from Earth.
Hope this helps,
- For Teachers