Could alien life be based on silicon?

Yes — in theory, alien life could be based on silicon. Silicon can form four bonds, so it has the basic chemistry needed to build complex structures.

But carbon is still far more likely. Silicon chemistry has big problems:

• Oxidation: it tends to form solid silicon dioxide, basically rock, not a gas it can easily get rid of.
• Weaker bonding: silicon-silicon chains are less stable than carbon chains.
• Water issue: water breaks down many silicon-based compounds.

If silicon life exists, it would probably need a very different world: low oxygen, little or no water, and possibly very cold conditions with another solvent such as methane or ethane.

So the short answer is: possible in principle, but much less likely than carbon-based life.