Life as we know it is entirely carbon-based (made of molecules mostly composed of carbon atoms). Though we cannot presently imagine life forms that are not carbon-based, when searching for alien life it is good to keep our eyes open, in case our imagination has failed us!

By definition, life must engage in some form of metabolism - that is, it must engage in chemical reactions that run counter to equilibrium, or the balance, of what would happen if the lifeform was not there. This means we can search for life by looking for chemistry that is out of equilibrium or out of balance. For example, if aliens looked at Earth, they might notice that our atmosphere contains oxygen, as well as methane. These are two gases that normally would react with each other very quickly, leaving none in our atmosphere. In fact, oxygen and methane do react in our atmosphere all the time. The only reason that there is any of these two gases left on Earth is because something is constantly adding more oxygen and more methane back into our atmosphere. That something is life (specifically plants and other photosynthesizing organisms - oxygen - and bacteria and other methanogenic organisms - methane)!

This is not to limit ourselves to just looking for oxygen and methane, either. If we found a planet with substantial amounts of fluorine, for example, in its atmosphere, we would be highly suspicious that life might be present there, since fluorine is a very reactive gas, which does not survive for long in natural equilibrium. This concept was first put forth by James Lovelock in the context of searching for life on Mars, and was the basis for his controversial 'Gaia hypothesis'.

Though all life on Earth is carbon-based, it has occasionally been suggested that silicon-based life could have arisen (or could arise in the future) elsewhere in the universe. Silicon is an interesting atom because, like carbon, it has four bonding slots, so it might be possible to form long-chain, flexible silicon-based molecules, like carbon does.

Silicon, however, has a nasty habit of forming very strong bonds with another element, oxygen. You may have seen large collections of silicon-oxygen molecules – they’re called sand! Carbon will bond with oxygen under the right conditions, but it would generally rather bond with itself. Silicon, on the other hand, prefers to bond with oxygen and not with itself, so it just ends up in a tiny, boring molecule surrounded by oxygen. This is not very conducive to carrying information or manipulating other molecules, the most basic functions of life.

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?