There may already be microbes on Earth that could survive on Mars. Bacteria from the Dead Sea and the Arctic tundra have been shown to survive in a simulated Martian atmosphere.
Venus would be trickier, even in its cooler upper atmosphere, because this planet has no ice or water. Alien life might have its own completely different biochemistry, but we couldn’t genetically engineer it, because DNA molecules themselves require water.
For more complex, multicellular life, the lack of atmospheric oxygen on Mars would probably rule out this planet. Earth organisms that don’t need oxygen are almost all single-celled because anaerobic metabolisms produce much less energy.
But Jupiter’s moon Europa has a liquid water ocean underneath its icy crust, and in 2009, researchers at the University of Arizona suggested that there might be oxygen too. How survivable this ocean is for Earth life will depend on what other toxins and nutrients are dissolved in it.
Deep-sea fish and invertebrates would be good colonisation candidates, though, and genetic engineering might be useful to give them improved cold and pressure resistance.
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