Learn Hardcore Italian: Quel pianeta distante brilla poco, ma la galassia vicina è chiarissima. - That distant planet shines faintly, but the nearby galaxy is very bright.

quel before most consonants (e.g. quel pianeta)
quello before z or s+consonant (e.g. quello zaino, quello studente)
quell’ before vowels (e.g. quell’amico)
Since pianeta begins with a simple consonant P, the correct form is quel.

Why is distante placed after pianeta? Could I say quel distante pianeta?

In Italian, most descriptive adjectives normally follow the noun. So quel pianeta distante is the neutral, standard order. You can put distante before the noun (quel distante pianeta) but it sounds more literary or poetic and can add emphasis. Generally:

Noun + adjective = neutral description
Adjective + noun = stylistic/emphatic effect

What does brilla poco literally mean, and why is poco used instead of an adjective?

brilla is the present tense of brillare (“to shine”). poco here is an adverb meaning “a little” or “sparingly.” So brilla poco literally means “shines little,” i.e. “shines dimly.” If you tried to use poco as an adjective, you’d need a linking verb, for example è poco brillante (“it is not very bright”), which changes the structure and nuance.

Why is there a comma before ma, and is it always required?

The comma separates two independent clauses joined by ma (“but”), marking the contrast:
“…brilla poco, ma la galassia vicina è chiarissima.”
In Italian, a comma is standard before ma when each clause has its own subject and verb. In very short pairs you might sometimes drop it, but here the comma is correct to signal the break.

In la galassia vicina, why does vicina come after galassia? Could it go before?

Just like distante, vicina is a descriptive adjective and normally follows the noun: la galassia vicina. You could say la vicina galassia, but it gives a more poetic or emotional tone. The everyday, neutral word order is noun + adjective.

What is chiarissima, and how do you form such superlatives in Italian?

chiarissima is the absolute (suffix) superlative of chiara (“clear/bright”). You form it by adding -issimo/a/i/e to the adjective stem and agree in gender/number:

chiaro → chiar
- -issimo → chiarissimo (m. sg.)
change ending for fem. sg.: chiarissima
It expresses “very” or “extremely” (here: “extremely bright”). An alternative is molto chiara, but chiarissima is stronger.

Why are the verbs in the present tense (brilla, è) instead of past or future?

The present indicative is used for general truths or ongoing facts. The sentence describes how that distant planet and the nearby galaxy are, in general and right now:

quel pianeta distante brilla poco = that distant planet generally shines dimly
la galassia vicina è chiarissima = the nearby galaxy is extremely bright
Using past or future would change the time frame and the sense of a general property.