As I understood it, it would be perfectly possible to replicate most of the functionality of life (amino acids) by moving down one period in the periodic table.

As I understood it, it would be perfectly possible to replicate most of the functionality of life (amino acids) by moving down one period in the periodic table. So that would be C->Si, N->P, P->As, etc (with or without O->S substitution [0]). You can't do the same by moving left or right in the periodic table because elements are grouped vertically by their electrochemical properties (electrovalence), so moving left or right gives you a material with wildly different properties. The problem is that all of the molecules in period 3 are much more massive than their carbon-family counterparts, which means that the energy required to construct said building blocks must be equally higher. If we assume a high-entropy environment exists for these molecules to form, the first obvious question is then: how come carbon-based life didn't appear in that environment as well and outcompeted Si-based life? Would it have to be high-pressure environments like inside gas giant planets, or high-temperature environments on planets much closer to the star? And if the environment is inhospitable to carbon-based life, is there sufficient stability in that environment to produce more than just the functional building blocks of life? Next, the question of development speed is an interesting one. Of course, evolution isn't linear so everything here is conjecture, but just as a thought experiment: How long would it take for such an environment (inhospitable to carbon-based life, but capable of spawning silicon-based life) to produce single-celled organisms, and then for those organisms to meaningfully alter their biosphere (like Earth's Great Oxidation Event) so that we can detect those changes through our telescopes? Earth is 4.5 billion years old; oxygen started appearing in the atmosphere 2 billion years ago, but didn't appear in meaningful quantities until less than 1 billion years ago. That 3.5 billion years from planet formation to detectable biosphere signature for carbon-based life (n=1, of course). If we take a wild guess that silicon-based life is slower to develop by a factor 2, we'd have to find planets that are at least 7 billion years old (and have been stable for that time). Given that the lower bound for the age of the entire universe is 8 billion years, we're getting close to planets that are as old as the universe itself. Add to that that our telescopes are also looking into the past, the further out we look -- but 100 million years (the Virgo supercluster) seems negligible compared to the supposed 7 billion years projected incubation time: It seems rather optimistic to expect silicon-based life signatures to readily appear on our telescopes. [0] and then there's also https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... -- where they suggest NH3 and its ionized counterparts could serve as a replacement for H2O, OH- and H3O+