Question

"Complete a Punnett square for the following genetics problem and provide the genotypic and phenotypic ratios of the predicted offspring.

A heterozygous, three-horned alien from Mercury has met and fallen in love with a four-horned alien from Jupiter. What would happen if there was an intergalactic mating between the pair?"
Ok, the three horned alien is heterozygous I get that. Wouldn't I need to know what the second aliens traits are to even try to solve and for what is this punnet square trying to figure out? Can some one explain this?