As of question writing, all known life is carbon-based, in the sense that it needs to contain carbon atoms to survive.
But life could take many forms:
- Wikipedia has a handy list of hypothetical types of biochemistry, notably silicon biochemistry.
- Life could be based on non-organic chemistry (e.g. inorganic chemistry, or nuclear chemistry in the degenerate crust on the surface of a neutron star*).
- Life could also not be chemistry based at all. It could be electrical (e.g. Ems) or mechanical (e.g. clockwork).
- Life could operate on vastly different time / space scales from us (e.g. a cloud of interstellar stuff somehow consistently implementing a sentient computation).
These examples are not necessarily mutually exclusive, and I obviously make no claim regarding their respective feasibility/likelihood. They are rather meant to suggest the vastness of design-space.
If we encounter a phenomenon that is widely considered by the scientific community to be an alien life-form, will all simple life-forms we discover be carbon-based?
- The life-form has to have originated independently from earth life. That is: earth life can be a consequence of the alien life-form, they can share a cause, but earth life cannot have caused the alien life.
- The life-forms that count for this question are ones on the complexity level of our single-celled organisms or lower (as determined by a poll of xeno-biologists if there is any ambiguity). If there are none, then the simplest life-forms we have found are taken for resolution.
- The life-form has to need less than 1% of its atoms to be carbon atoms in order to keep being alive. It can incidentally contain carbon atoms, as long as they could theoretically be absent and the life-form still be alive.
- The scientific community has to have reached a consensus as judged by Metaculus admins.
- This resolves positive if any life-form we encounter satisfies points 1. 2. and 3.
- This resolves negative if all the life-forms we encounter that satisfy points 1. and 2. do not satisfiy point 3.
- This resolves ambiguous if no life-form that satisfies point 1. is found before 2500, or if before then we have conclusive evidence that none exists in the observable universe.
- This resolves 50 years after we first discover an alien life-form that satisfies condition 1., to give time for consensus forming.
* My thanks to @(Uncle Jeff) for this example.
Note that in this sense Humans are only "based" on hydrogen (60%), oxygen (25%), carbon (10%) and nitrogen (1.5%).