I still don't get this at all. 'dessen' is a relative pronoun, like 'that' or 'which' or 'who' in English. In this case it's in the genitive. So I would expect is to be used in something like 'the oceans of earth, which is the third planet from the sun, cover most of the planet', with 'which' being a genitive relative pronoun - it would become 'dessen'.
I don't see how this sentence has anything at all to do with relative pronouns. What am I missing? Someone parse it for me?
The word 'which' in your phrase is not a genitive relative pronoun, but a nominative relative pronoun. The case is indicated by the function of 'which' in the secondary phrase. Thus, "which is the third planet" means "the earth is the third planet", so 'which' is a nominative relative pronoun (it plays the role of the subject). In German it would be: "die Ozeane der Erde, die der dritte Planet von der Sonne ist, bedecken etc etc".
To construct a phrase with a genitive relative pronoun, you need the pronoun to fulfil a possessive function in the secondary phrase, such as: "the oceans of earth, whose temperature ensures the existence of liquid water, cover 71% of its surface". In German: "die Ozeane der Erde, deren Temperatur die Existenz von flüssigem Wasser sicherstellt, bedecken etc etc".
(my comment offers no clarification of the problem posed by the Duolingo sentence, which is, why should it use 'desen' and not 'sein')