La atmosfero estas ĉirkaŭ la planedo.

• La = the
• atmosfero = atmosphere
• estas = is / are / am
• ĉirkaŭ = around
• la = the
• planedo = planet

So the sentence is built very straightforwardly: The atmosphere is around the planet.

In Esperanto, la is the definite article, meaning the.

It appears before both nouns because both are definite:

• la atmosfero = the atmosphere
• la planedo = the planet

Unlike English, Esperanto has only one article form: la. It does not change for gender, number, or case.

So:

• la knabo = the boy
• la knabino = the girl
• la planedoj = the planets

Because -o is the normal noun ending in Esperanto.

This is one of the most important regular features of the language:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

So:

• atmosfero = atmosphere
• planedo = planet

If you changed the ending, the word’s grammatical role would change:

• planeda = planetary
• plane­de would be an adverbial form, though that specific form is not especially common

This regular system makes Esperanto much more predictable than English.

Estas is the present-tense form of esti, meaning to be.

A very important point for English speakers: Esperanto verbs do not change according to the subject.

So:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ŝi estas = she is
• ĝi estas = it is
• ili estas = they are

It is always estas in the present tense.

That makes Esperanto much simpler than English in this area.

Here, ĉirkaŭ is a preposition, meaning around.

It connects the idea of location to la planedo:

• ĉirkaŭ la planedo = around the planet

So the sentence says where the atmosphere is located.

Esperanto prepositions work a lot like English ones:

• sur la tablo = on the table
• sub la seĝo = under the chair
• apud la domo = next to the house
• ĉirkaŭ la planedo = around the planet

Because after a preposition like ĉirkaŭ, you normally do not add -n.

So:

• ĉirkaŭ la planedo = around the planet

The -n ending is mainly used for the direct object, and sometimes for direction or motion in certain expressions. But here, la planedo is simply the object of the preposition ĉirkaŭ, not the direct object of the verb.

Also, this sentence describes location, not movement.

Compare:

• La atmosfero estas ĉirkaŭ la planedo. = The atmosphere is around the planet.
  
  • static location, so no -n

But in some motion contexts Esperanto may use -n to show direction:

• Li kuras en la ĝardenon. = He runs into the garden.

That is a different grammar point. In your sentence, la planedo is correct.

Yes, but only in a different kind of meaning.

With some prepositions, Esperanto may use -n to show direction toward something rather than simple location. With ĉirkaŭ, you may also see -n in expressions of movement around something.

For example:

• Li promenis ĉirkaŭ la domo. = He walked around the house.
  This can simply describe the route.
• Li kuris ĉirkaŭ la domon. = He ran around the house.
  This may emphasize movement around it as a path or extent.

However, in your sentence:

• La atmosfero estas ĉirkaŭ la planedo.

the idea is purely positional, so la planedo without -n is the normal choice.

Esperanto word order is fairly flexible, but the standard, neutral order here is:

La atmosfero estas ĉirkaŭ la planedo.

This is:

• subject: La atmosfero
• verb: estas
• prepositional phrase: ĉirkaŭ la planedo

You could move parts for emphasis, for example:

• Ĉirkaŭ la planedo estas la atmosfero.

That is grammatically possible, but it sounds more marked or literary, as if you are emphasizing around the planet.

For learners, the original order is the best and most natural one to use.

A helpful rough pronunciation is:

• ĉ sounds like ch in church
• i sounds like ee in see
• r is usually rolled or tapped
• k sounds like k
• aŭ sounds roughly like ow in cow

So ĉirkaŭ is approximately: CHEER-kow

More carefully, it is something like: CHEER-kah-oo, blended smoothly at the end.

The special letter ĉ is one of Esperanto’s accented letters and always has the same sound.

A rough English-friendly pronunciation is:

lah aht-moh-SFEH-ro EH-stahs CHEER-kow lah plah-NEH-doh

A few pronunciation notes:

• a is like a in father
• e is like e in bet, but often a bit clearer
• o is like o in for, but without strong English diphthonging
• stress in Esperanto usually falls on the second-to-last syllable

So:

• atmosfero → at-mo-SFE-ro
• planedo → pla-NE-do
• ĉirkaŭ → ĈIR-kaŭ

Yes, la atmosfero refers to the atmosphere, meaning a specific, identifiable one.

In this sentence, the listener naturally understands that it is the atmosphere belonging to or surrounding the planet being discussed.

If you said just atmosfero without la, that would sound more like an atmosphere or atmosphere in a more general sense, depending on context.

So:

• La atmosfero estas ĉirkaŭ la planedo. = The atmosphere is around the planet.
• Atmosfero estas grava por vivo. = Atmosphere is important for life.
  Here the meaning is more general.

Yes. It is very typical and very regular.

It shows several core Esperanto patterns:

1. la for the
2. noun ending -o
3. unchanged present-tense verb estas
4. a preposition followed by its noun phrase
5. simple, clear word order

That is one reason Esperanto is often easier for beginners: sentences like this are built from very regular parts, with few exceptions.