The same general rule that applies to ordinary language applies to scientific or technical writing: Capitalize proper nouns or adjectives but not common terms.
Spiderweb Galaxy
Saturn
Beta Centauri
Mesozoic Era
Crohn’s disease
Here are examples of common terms that are lower case:
small-world geometry
network optimization theory
second law of thermodynamics
chronic fatigue syndrome
peripheral arteriosclerosis
Scientists capitalize (and italicize) the Latin names of plants and animals, generally the first word (genus) of the two-part term: Canis lupus (gray wolf), Canis latrans (coyote). When the genus name is used alone, it is capitalized (Canis, Pinus).
Exercises:
Capitalization 1
Capitalization 2
Related topic:
Abbreviations for scientific terms