Ĉiu planedo havas graviton, sed ne ĉiu havas atmosferon.

Because graviton is the direct object of havas (has).

In Esperanto, direct objects usually take the -n ending:

• La planedo havas atmosferon. = The planet has an atmosphere.
• Mi vidas la stelon. = I see the star.

So in:

• Ĉiu planedo havas graviton
  the thing being had is graviton, so it gets -n.

The same is true for atmosferon.

Both are possible in Esperanto, but they are used a little differently.

• ĉiu planedo = each planet / every planet
• ĉiuj planedoj = all planets

In many cases they are very close in meaning. Here, ĉiu planedo havas graviton emphasizes the idea one by one: every single planet has gravity.

So:

• Ĉiu planedo havas graviton = Every planet has gravity.
• Ĉiuj planedoj havas graviton = All planets have gravity.

The first version is often the more natural match for English every planet.

Because ne ĉiu means not every.

• ne ĉiu havas atmosferon = not every one has an atmosphere

If you said ĉiu ne havas atmosferon, that would suggest:

• everyone/every one does not have an atmosphere which sounds like none of them have an atmosphere

That is a very different meaning.

So:

• Ne ĉiu havas atmosferon = Not every planet has an atmosphere.
• Neniu havas atmosferon = No planet has an atmosphere.

Because ĉiu can stand by itself when the noun is understood from context.

In:

• Ĉiu planedo havas graviton, sed ne ĉiu havas atmosferon.

the second ĉiu means not every planet. Since planedo was already mentioned, Esperanto does not need to repeat it.

This is very similar to English:

• Every planet has gravity, but not every one has an atmosphere.

Esperanto often omits the repeated noun when it is obvious.

Ĉiu means each, every, or sometimes everyone/everything, depending on context.

Examples:

• Ĉiu planedo = every planet / each planet
• Ĉiu studento = every student
• Mi konas ĉiun. = I know everyone.
  Here ĉiun stands alone, so it means everyone.

In your sentence, ĉiu refers to individual planets:

• Ĉiu planedo = every planet
• ne ĉiu = not every one / not every planet

Because havas is the present-tense finite verb, meaning has.

Esperanto verb endings are very regular:

• -i = infinitive, like to have → havi
• -as = present tense → havas
• -is = past tense → havis
• -os = future tense → havos

So:

• havi = to have
• havas = has / have

Since the subject is ĉiu planedo, the sentence needs the present-tense verb havas.

No. Esperanto verbs do not change according to person or number.

So all of these use havas:

• Mi havas = I have
• Vi havas = you have
• Li havas = he has
• Ili havas = they have

That is why:

• Ĉiu planedo havas graviton and
• La planedoj havas graviton both use havas.

Sed means but.

It connects two contrasting ideas:

• Ĉiu planedo havas graviton = Every planet has gravity
• ne ĉiu havas atmosferon = not every one has an atmosphere

So:

• ..., sed ... = ..., but ...

Its position works much like English but:

• Mi volas iri, sed mi estas laca. = I want to go, but I am tired.
• La libro estas interesa, sed longa. = The book is interesting, but long.

Yes. Gravito means gravity.

It is a noun:

• gravito = gravity
• graviton = gravity, as a direct object

So:

• La Tero havas graviton. = The Earth has gravity.

A learner might notice that in physics English also has the word graviton, but that is a different specialized term in English. In Esperanto here, graviton is just gravito + -n, not the English particle name.

Yes.

• atmosfero = atmosphere
• atmosferon = atmosphere as a direct object

Both nouns take -n because they are the things being had:

• havas graviton
• havas atmosferon

So the pattern is exactly parallel.

Ĉiu is pronounced roughly like CHEE-oo.

A few details:

• ĉ sounds like English ch in church
• i sounds like ee
• u sounds like oo

So:

• ĉiu ≈ CHEE-oo

Also, Esperanto stress normally falls on the second-to-last vowel:

• ĈI-u

Yes, the first half could often be translated that way:

• Ĉiu planedo havas graviton = Every planet has gravity
• also possibly: All planets have gravity

But the second half is especially important:

• sed ne ĉiu havas atmosferon = but not every planet has an atmosphere

If you translated the second half as but all do not have an atmosphere, that would be wrong. It sounds like none of them have one.

So the best natural English translation keeps the contrast:

• Every planet has gravity, but not every planet has an atmosphere.

Yes. Ĉiu works for both people and things.

Examples:

• Ĉiu studento legas. = Every student is reading.
• Ĉiu domo havas pordon. = Every house has a door.
• Mi vidis ĉiun. = I saw everyone.

In your sentence it is used with a thing:

• ĉiu planedo = every planet

So the word itself is very flexible; context tells you what ĉiu refers to.