I linked it to the existing Russian sentence, though it means literally "Zemlja to je planeta.", or something like that. Maybe, it would be more exact to unlink and link it to "Земля - планета".? (We don't use the verb "to be" in the Present tense in Russian")

How would you write in this context: There are many planets in our galaxy. For example: Earth is a planet. ?

"В нашей галактике много планет. Например, Земля".

I think if the sentence "Zemlja to (ovo) je planeta." is really exist in Serbian, it's the better translation for the Russian sentence. If no, the existing translation matches enough.

The only thing I can think of is something when we negate something else.
Somthing like this:

Zemlja, ona je planeta, a ne planetoid.
Earth, it is a planet, and not a planetoid.
Jorden, den er en planet, og ikke en småplanet.
Земля - это планета, а не планетка.

Should we add those, if they are good example for your language, as well?

Do you mean these?

Zemlja, ona je planeta, a ne planetoid.
Earth, it is a planet, and not a planetoid.
Jorden, den er en planet, og ikke en småplanet.
Земля - это планета, а не планетка.

Yes, we can, but in Russian it should be "астероид" (a small planet, one of those between Mars and Jupiter).

Ok, but you add Russian variant, but please look at
http://en.wikipedia.org/wiki/Planetoid
I couldn't find Russian translation for the page, so I used some other language.

Related:

[#3368102]

I really can't understand modern classifications of planets. "Minor planet" (redirected from "planetoid" should be "Малая планета" in Russian, but Wiki says "Малая планета" is the same as "астероид". But asteroids is only between Mars and Jupiter, and minor planets (planetoids) also included Pluto and some other objects.
So, I would like translate as "планетоид", though Wiki says it's not the strict scientific term.

Yes, and since we are writing common language, not scientific papers, we may use such phrases/words (just it may prove useful to check their usage over google).
I usually type something like this in google:
"planetoid" site:uk

Quotes are necessary only for phrases. And just look few results, are they OK, and when comparing various usages, look for number of returned results.