Este planeta é pequeno.

Este means this.

In Brazilian Portuguese, the three main demonstratives are:

• este = this
• esse = that
• aquele = that over there / that one over there

In everyday Brazilian speech, people often use esse where older grammar would prefer este, so you may hear both in similar situations. But in a sentence like Este planeta é pequeno, este is a perfectly correct and natural way to say This planet is small.

Because é is the verb ser in the he/she/it form, meaning is.

• é = is
• e = and

So the accent is very important here:

• Este planeta é pequeno. = This planet is small.
• Este planeta e pequeno. = incorrect, and it would also create confusion with and

In Portuguese, adjectives often come after the noun, much more often than in English.

So:

• planeta pequeno = small planet
• literally, planet small

In your sentence, the structure is:

• Este planeta = this planet
• é pequeno = is small

That is a very normal Portuguese word order.

Because planeta is a masculine noun in Portuguese, so the adjective must agree with it.

• planeta = masculine
• pequeno = masculine singular
• pequena = feminine singular

So:

• Este planeta é pequeno.
• Esta casa é pequena.

Even though planeta ends in -a, it is still masculine. That is something learners often notice because many -a words are feminine, but not all of them.

You usually have to learn the gender of each noun, although endings often give you a clue.

Many nouns ending in -a are feminine, but some are masculine, including:

• o planeta
• o problema
• o sistema
• o programa

A good habit is to learn nouns together with their article:

• o planeta = the planet

That helps you remember the gender more easily.

Yes, but it means something slightly different.

• Este planeta é pequeno. = This planet is small.
• O planeta é pequeno. = The planet is small.

So este points to a specific planet as this one, while o just means the.

Because demonstratives like este, esse, and aquele can directly modify the noun without an article.

So this is normal:

• este planeta
• essa casa
• aquele carro

In Brazilian Portuguese, you may sometimes hear forms like esse meu amigo, where a demonstrative appears with another determiner, but in your sentence este planeta is already complete and natural by itself.

A simple learner-friendly pronunciation is:

ES-che pla-NE-ta eh pe-KE-no

A few notes:

• este in Brazil often sounds close to ES-che in many accents
• planeta has stress on ne: pla-NE-ta
• é sounds like eh
• pequeno has stress on que: pe-KE-no

Pronunciation varies by region, but that version will help you sound natural in Brazil.

Yes, but the tone changes.

• pequeno = small
• pequenino / pequeninho = very small / tiny / little

So:

• Este planeta é pequeno. = neutral statement
• Este planeta é pequeninho. = more expressive, often affectionate or emphatic

For basic learning, pequeno is the safest and most neutral choice.

The pattern is:

demonstrative + noun + verb + adjective

So here:

• Este = demonstrative
• planeta = noun
• é = verb
• pequeno = adjective

This is a very useful pattern you can reuse:

• Este carro é rápido. = This car is fast.
• Essa cidade é grande. = That city is big.
• Aquele livro é interessante. = That book over there is interesting.

You need este planeta if you want to say this planet.

Portuguese often drops subject pronouns like eu, ele, nós, when the meaning is clear, but este planeta is not just a pronoun subject like it. It is the actual noun phrase you want to talk about.

So:

• Este planeta é pequeno. = complete and correct
• É pequeno. = It is small / [This] is small, but only if the context already makes clear what you mean

Without context, É pequeno sounds incomplete.

Yes. It is grammatically correct and natural.

In very casual speech, many Brazilians might more often say:

• Esse planeta é pequeno.

But Este planeta é pequeno is still fully correct and understandable. It may sound a bit more careful, formal, or textbook-like depending on the context, but it is absolutely good Portuguese.