Learn Finnish: Onko avaruudessa kaukaisia planeettoja? - Are there distant planets in space?

This is an existential sentence (a "there is/are" sentence). In Finnish, existential sentences always use the third-person singular verb (on), even when the subject (planeettoja) is plural. We just add the question particle -ko to make it Onko.

Why are kaukaisia planeettoja in the partitive plural instead of the genitive plural we are currently learning?

In existential sentences ("There are..."), the plural things that exist take the partitive plural case to show an unknown or indefinite amount (some distant planets). The genitive plural is used for possession (like "the planets' rings"), not for simply stating that they exist.

How do we get the form kaukaisia from the basic word for distant?

The base word is kaukainen. For adjectives ending in -nen, the stem changes to -se- before adding endings. To form the partitive plural, we add the plural marker -i- and the partitive ending -a, resulting in kaukaisia.

What happens to the word planeetta to make it planeettoja?