Sure, all life on Earth is carbon-based, water-using and cell-based, but we’re talking about aliens here, right? Aren’t we being a little ethnocentric?

This is a serious issue in exobiology – if we’re only out looking for carbon-based life, we might miss out on other (incredibly interesting and scientifically educational) forms of life.

Alternatives to carbon-based life, however, are improbable at best. Life must both reproduce and metabolize. Both functions require large, flexible molecules to store a host of information (reproduction) and to manipulate other molecules in their environment (metabolism). There are at most 94 naturally occurring elements in the universe, so your large molecules must be made of them. Carbon is the atom that is best for making large, flexible molecules on at least two counts: 1) chemical properties and 2) abundance.

First, the chemical properties of carbon – it is one of the few atoms with four free slots to grab another atom (technically, ‘four unbound outer shell electrons’). Put another way, it has four locations where it can simultaneously bond with other elements to make interesting chemicals. This means it can grab, for example, another carbon atom, which can itself grab another carbon atom, etc. and form a long chain. The middle atoms in the chain can still each grab two other atoms. So carbon can potentially form a long strand that can curl into many shapes, and still have fun and interesting things hanging off of it. Atoms with less than four bonding slots are much more restricted; no atoms have more than four.

Second, carbon is the most abundant element with four bonding slots – when we look into the universe, carbon is actually the fourth most abundant element overall. So there is a lot of it around – that makes it easier for carbon atoms to find each other and do their funky dance. The next most abundant element with four bonding slots is silicon (if you want to know why silicon is a bad bet for life, see Why can’t life be silicon-based?).

Having said all of this, it's possible we're missing something. Which is why missions searching for life do try to keep an eye out for anything 'out of the ordinary'. For more on this, see How would you look for non-carbon-based life?

Why couldn’t alien life be, for example, orbs of light or electrical fields?

Using the most minimal definition of life, there is a requirement that life engage in chemical processes, or metabolism. Chemistry requires atoms. It is not possible for individual light waves (photons) or electrical fields to engage in chemistry (or metabolism).