El almuerzo que acabó en una pregunta

Earlier past action

habían hablado

had discussed

Habían hablado is the pluperfect. It places their discussion before another past event, sitting down.

Useful for ordering two events in the past.

Conditional perfect

deberían haber dejado indicios

should have left signs

Deberían haber dejado expresses an expected past result that is not observed.

Useful for reasoning from a condition to an absent result.

Hypothetical si clause

si esas civilizaciones fueran frecuentes

if those civilizations were common

Si plus the imperfect subjunctive fueran presents an unreal or uncertain hypothesis; the main clause uses the conditional.

Useful for building careful conditional arguments.

Concession with aunque

Aunque el número de planetas sea enorme

although the number of planets may be enormous

Aunque plus the subjunctive sea concedes a possibility without treating it as enough for the conclusion.

Useful for limiting a claim while acknowledging a point.

Nominal relative clause

Lo que está confirmado

what is confirmed

Lo que turns the following clause into a noun phrase meaning what or that which.

Useful for foregrounding the part of a claim that has evidence.