Could alien life be completely different from DNA?

Yes. Alien life could be completely different from DNA as we know it.

On Earth, all known life uses DNA and RNA for genetic information, but that does not prove those are the only possible information-carrying molecules. In the lab, scientists have made synthetic genetic polymers called XNAs with alternative backbones, and some can store information and undergo evolution-like selection. Researchers have also built Hachimoji DNA, an expanded 8-letter genetic system.

It’s also reasonable to say that if life existed in a very different environment, it might use a very different chemistry. For example, on a cold world with liquid methane or ethane rather than liquid water, Earth-style DNA would not be expected to behave the same way it does in water, so any such life would likely need a different molecular system for heredity.

That said, we should keep a few things tentative:

• We do not know whether life can actually arise in methane or ethane as a solvent.
• Silicon-based life is still speculative, not a proven alternative, and any such life would likely differ greatly from Earth life in chemistry and metabolism.

So the short answer is: yes, alien life could be radically different from DNA-based life, but that remains hypothetical.