Ta planeta jest duża.

Because planeta is a feminine noun in Polish. The demonstrative this must agree with the noun’s gender, so:

• ten = masculine
• ta = feminine
• to = neuter

Since planeta is feminine, you say ta planeta = this planet.

A very common clue is the ending -a. Many Polish nouns ending in -a are feminine, and planeta is one of them.

Because it is feminine, other words describing it also need feminine forms, such as:

• ta planeta
• duża planeta

This is called agreement.

Because adjectives in Polish must agree with the noun in gender, number, and case.

Since planeta is feminine singular, the adjective also has to be feminine singular:

• masculine: duży
• feminine: duża
• neuter: duże

So:

• duży dom = a big house
• duża planeta = a big planet
• duże okno = a big window

Jest means is. It is the 3rd person singular form of the verb być = to be.

So in Ta planeta jest duża:

• ta planeta = this planet
• jest = is
• duża = big / large

Unlike Russian, Polish normally keeps the verb to be in present-tense sentences like this.

In a normal standard sentence like this, no. You usually need jest.

So:

• Ta planeta jest duża = correct
• Ta planeta duża = not standard Polish

Polish often drops personal pronouns like ja or ty, but not the verb to be in this kind of sentence.

Usually big is the most natural basic translation, but large can also work depending on context.

So duża planeta can mean:

• a big planet
• a large planet

In everyday learning, big is the simplest choice.

Yes. Wielka also means something like great / huge / enormous, depending on context. It often sounds stronger than duża.

So:

• Ta planeta jest duża = This planet is big.
• Ta planeta jest wielka = This planet is huge / enormous / great.

For a neutral basic description, duża is the safer choice.

The neutral word order is:

Ta planeta jest duża.

That is the most natural way to say This planet is big.

Polish word order is more flexible than English, so other orders are possible for emphasis, for example:

• Duża jest ta planeta.
• Ta duża planeta... (if you continue with more information)

But for a beginner, the standard pattern subject + jest + adjective is best.

A simple English-friendly approximation is:

tah planeh-tah yest DOO-zhah

A few notes:

• j in Polish sounds like English y in yes
• ż sounds like the s in measure or the g in beige
• stress in Polish is usually on the second-to-last syllable

So the main stresses are roughly:

• plaNEta
• DUża

Polish has no articles. There is no direct equivalent of English a/an/the used in ordinary grammar.

That means planeta can mean:

• a planet
• the planet

depending on context.

In this sentence, ta already makes it specific: this planet.

No, that is not a natural standard sentence.

The normal form is:

Ta planeta jest duża.

Why?

• ta should directly go with the noun: ta planeta
• jest comes between the subject and the description
• duża describes planeta

So the sentence structure is straightforward: this planet is big.

Yes. In Polish, words like ta and duża change form depending on case, and also on gender and number.

For example, in the nominative singular you have:

• ta duża planeta

But in other cases, the forms change. For example, after widzę (I see), you would say:

• Widzę tę dużą planetę.

So the forms in your sentence are specifically the nominative singular feminine forms.

Yes. Ta planeta is the subject of the sentence, so it is in the nominative case.

That is why you get:

• ta planeta
• duża

If the noun had a different role in the sentence, its form could change. For example:

• Ta planeta jest duża. = nominative
• Widzę tę planetę. = accusative

This is one reason Polish endings are so important.