Learn Hardcore Portuguese (Portugal): O planeta descrito no painel é muito distante. - The planet described on the panel is very distant.

O planeta descrito no painel = The planet described on the panel (a specific one, already identified by the description in the panel).
Um planeta descrito no painel would sound like a planet described on the panel, suggesting it’s one among several and not a specific, uniquely identified planet.

Because the phrase descrito no painel identifies a specific planet, the definite article o is the natural choice.

What exactly is descrito here? Is it a verb or an adjective, and why does it end in -o?

Descrito is the past participle of the verb descrever (to describe), but here it’s being used as an adjective.

Literally: O planeta descrito no painel = The planet (that is) described on the panel.
Portuguese often uses a past participle directly before a noun as a kind of “reduced” relative clause.

Agreement:

planeta is masculine and singular.
So the participle-adjective must match: descrito (masc. sg.).
Feminine: a estrela descrita
Plural: os planetas descritos, as estrelas descritas

Could I say O planeta que é descrito no painel instead of O planeta descrito no painel? Is there a difference?

Yes, you can say:

O planeta que é descrito no painel é muito distante.

Meaning-wise, it’s almost the same. Differences:

O planeta descrito no painel is more compact and sounds more natural in written Portuguese and in slightly formal speech.
O planeta que é descrito no painel is a full relative clause (with que é spelled out). It’s correct but a bit heavier and more “spelled out”, and in many contexts people would prefer the shorter version.

So, descrito before the noun is like a reduced form of que é descrito.

What is no in no painel? Why not just em o painel?

No is a contraction:

em (in/on) + o (the, masculine singular) → no

So no painel literally is em o painel, but in normal Portuguese you must contract them:

em + o → no (in/on the, masc. sg.)
em + a → na (in/on the, fem. sg.)
em + os → nos
em + as → nas

Saying em o painel sounds wrong; the contraction no painel is obligatory in standard usage.

What exactly does painel mean here, and how is it different from words like quadro or ecrã?

Painel in Portuguese (PT) usually means:

a panel (information board, wall display, control panel, or a large board with text/images).

Possible contrasts:

painel – a flat surface used for displaying information, images, controls, or diagrams.
quadro – can mean a picture frame, a painting, or a blackboard/whiteboard (in schools).
ecrã – the screen of a computer, TV, or phone (European Portuguese; in Brazil they say tela).

In your sentence, no painel suggests something like an information panel, display board, or similar surface where the planet is described.

Why is it é muito distante and not está muito distante?

Ser (é) and estar (está) are both “to be”, but:

ser is used for more permanent, inherent characteristics.
estar is used for temporary states or locations.

Distance, as a property of where a planet is in space, is treated as a stable characteristic. So:

O planeta é muito distante. = The planet is very distant (as a general fact).

Saying O planeta está muito distante could be understood, but it sounds odd for an astronomical statement, because it suggests a temporary or changing state, which doesn’t fit well with this context.

What’s the difference between distante and longe? Could I say O planeta é muito longe?

You cannot say O planeta é muito longe; that’s incorrect.

distante is an adjective: distant.
- O planeta é distante. (The planet is distant.)
longe is normally an adverb: far.
- O planeta está longe. (The planet is far away.)

So:

Correct with adjective + ser:
O planeta é muito distante.
Correct with adverb + estar:
O planeta está muito longe.

Mixing é with longe is what makes é muito longe wrong in standard Portuguese.

Why is the word order planeta descrito and not descrito planeta like in English with “described planet”?

In Portuguese, adjectives (and participles used as adjectives) usually come after the noun:

um planeta distante = a distant planet
um planeta descrito no painel = a planet described on the panel

Putting descrito before planeta (descrito planeta) is not natural in Portuguese and sounds ungrammatical in this context.

There are cases where adjectives can go before the noun, but that usually adds a stylistic or meaning nuance (often poetic, emotional, or idiomatic). Descrito planeta is not one of those accepted patterns.

Why is distante not changed to distanta to agree with planeta?

Adjectives ending in -e in Portuguese typically have:

one form for both masculine and feminine,
but they change in the plural.

So:

Masculine singular: distante – o planeta distante
Feminine singular: distante – a estrela distante
Masculine plural: distantes – os planetas distantes
Feminine plural: distantes – as estrelas distantes

So we never say distanto/distanta; distante is the correct singular form for both genders.

How should I pronounce planeta, descrito, painel, and distante in European Portuguese?

Approximate European Portuguese pronunciation (not IPA, just guidance):

planeta → plah-NEH-tuh
- Stress on NE: pla-NE-ta.
- Final -a is a reduced, almost “uh”-like sound.
descrito → dish-KREE-too
- Initial de- sounds more like “dsh” or “dish” in rapid European Portuguese (due to the ‘e’ reduction).
- Stress on CRI: des-CRI-to.
painel → pah-EE-NEHL (or blended pa-NEHL)
- ai like English “eye”.
- Final -el like “ehl”. Stress on NEHL.
distante → dish-TÃN-t(uh)
- di- often sounds like “dsh”.
- The an in -tante is nasal (like French “an”).
- Final -e reduced, almost “uh”. Stress on TAN (nasal): dis-TAN-te.

Spoken quickly in European Portuguese, many unstressed vowels become quite reduced and softer than in English.