Usage of 'y compris'

Question

I'm aware that the English translation of this expression is simply including. However, I came across this article in which they explained that although in English it is correct to use including to make an enumeration, in French, y compris doesn't quite work like that.

This year my students are studying several planets, including Mars, Venus and Earth.

Cette année, mes élèves étudient plusieurs planètes, *y compris Mars, Vénus et la Terre.

Nonetheless, I'm not entirely sure if I understand why? There is this example that shows the correct use of the expression:

Il a servi à boire à tout le monde, y compris à ses ennemis.

If I rephrase the original sentence about planets & make it similar to the one above so it looks like this, would it be okay?

This year my students are studying several planets, including the planets of the solar system.

Cette année, mes élèves étudient plusieurs planètes, y compris aux planetes du systeme solaire.

But, if that is correct, how come it turns out to be incorrect when I replace "planets of the solar system" for the actual planets?

Answer 1

Il a servi à boire à tout le monde, y compris à ses ennemis

In this example, “y compris” underlines the fact that you would not normally expect the agent to offer a drink to his enemies (but only to his friends). In English this use of “y compris” could be rendered by the word “even”.

By contrast, “dont” does not have this connotation, as in

Cette année, mes élèves étudient plusieurs planètes, dont Mars, Vénus et la Terre.

Mars, Venus and the Earth are not unexpected in a list of planets, there are even prototypical planets, which is why “dont” is a better fit. Using “y compris” in this case would be grammatically correct but semantically inappropriate.

That's what the article you found is getting at.

[…] contrairement à l’anglais including, la tournure française y compris « comporte généralement une petite connotation de “contrairement à ce qu’on pourrait croire” ».

Or, in English,

[…] unlike the English [word] including, the French phrase y compris “generally includes a slight hint of ‘in spite of what you might think’”

The same article offers a couple of other possible translations for “including”: “notamment” (literally especially) and “parmi lesquels/lesquelles”.

Answer 2

There are several points here.

Is your first sentence incorrect?

You said y compris cannot be used for an enumeration:

Cette année, mes élèves étudient plusieurs planètes, y compris Mars, Vénus et la Terre.

I see no grammatical error here and this seems perfectly fine to me. If you are unsure, you can use another translation of including. The first to come to my mind is dont:

Cette année, mes élèves étudient plusieurs planètes, dont Mars, Vénus et la Terre.

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Side note: about your adaptation

In your example from the Wiktionary, you have à after y compris, but it is because it is necessary after the verb the proposition is related to. This is a form of emphasis

Il a servi à boire à tout le monde, y compris à ses ennemis.
Il a servi à boire à tout le monde. Il a même servi à boire à ses ennemis.

This works for other examples on the page:

Il fume comme un pompier, y compris lorsqu’il a une bronchite.
Il fume comme un pompier. Il fume comme un pompier même lorsqu’il a une bronchite.

In your adaptation, you used à after y compris. This is not correct here. The full and repetitive sentence would be:

Cette année, mes élèves étudient plusieurs planètes. Ils étudient en particulier les planètes du système solaire.

Therefore:

Cette année, mes élèves étudient plusieurs planètes, y compris les planètes du système solaire.