Learn Spanish (Latin America): ¿Cuántos planetas habremos colonizado para el próximo siglo? - How many planets will we have colonized by next century?

Because we are talking about an action that will already be completed before a certain point in the future. This is called the future perfect tense. We form it using the future tense of the helping verb haber (here, habremos meaning 'we will have') followed by the past participle colonizado ('colonized').

Why is it Cuántos instead of Cuántas if planetas ends in an 'a'?

The word planeta is an exception to the usual gender rules. It is a masculine noun despite ending in '-a'. Because of this, the question word for 'how many' must be the masculine plural Cuántos to match it.

Why does the sentence use para instead of por to translate the word 'by'?

In Spanish, para is used to indicate deadlines or a specific target time in the future. Since the speaker is setting el próximo siglo ('the next century') as a deadline for when this colonization will be finished, para is the required preposition.

Does the ending of colonizado need to change to match the plural noun planetas?