Learn Russian: Видимая часть планеты очень красивая. - The visible part of the planet is very beautiful.

Видимая is a present passive participle. It comes from the verb видеть (to see). Because видеть is an II conjugation verb (an exception, despite ending in -еть), we use the suffix -им- to form the base видимый (being seen). Here, it takes the feminine ending -ая to agree with the noun it describes.

Does видимая act more like a verb or an adjective in this sentence?

While it is technically a participle meaning 'which is being seen', in this context видимая acts exactly like a regular adjective meaning 'visible'. It declines (changes its endings) just like a standard adjective to match its noun.

Why do we use the feminine endings -ая in видимая and красивая?

They both describe the noun часть (part). Nouns ending in a soft sign (-ь) can be either masculine or feminine. Часть is a feminine noun, so any describing words attached to it must also be in their feminine forms.

Why is the word планеты in the genitive case here?

The genitive case is used here to show possession or relationship, translating to 'of the planet'. When you have a 'noun of noun' structure like 'part of the planet' (часть планеты), the second noun takes the genitive case.