De or het planetologie?

The correct Dutch article is:

de planetologie

Rule Words ending in -logie: always de. See the rule
Meaning (Dutch) de wetenschap die zich bezighoudt met de systematische studie van planeten

How to use planetologie

the planetologiede planetologie
this planetologiedeze planetologie
that planetologiedie planetologie
our planetologieonze planetologie
your planetologiejouw planetologie
every planetologieelke planetologie
which planetologie?welke planetologie?
a big planetologie / the big planetologieeen grote planetologie, de grote planetologie

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).