The phrase "Life as we know it" is often spoken with a knowing wink, and implies the speaker is in on the big secret. Life on Earth is carbon based and every form of life we know of is on Earth. Which leads to Life as we know it, being carbon based.

Science Fiction occasionally offers Silicon based life as an option.

  • Has there been an reliable studies, or factual based speculation on what a non-carbon based (or possibly radically different carbon approach) life might be?
  • Would non-carbon based life leave different markers to search for?
  • Are there any programs searching for these markers?
  • $\begingroup$ Your previous title sounded excessively broad, but your question is actually a good one. I edited your title to make it sound a little more clear. $\endgroup$
    – called2voyage
    Commented Feb 28, 2014 at 17:19
  • $\begingroup$ Since hydrogen, carbon and oxygen are three of the four most common elements in the universe, in Earth, in life and in human beings, it seems to me that even if something like silicon based life could exist, it would be much more rare than carbon based life. Life as we know it exists of the very most common elements around everywhere. Under what conditions would silicon be more available than carbon? I'd bet that carbon life simply ate and exterminated all silicon life if it ever existed on earth. $\endgroup$
    – LocalFluff
    Commented Feb 28, 2014 at 17:56
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    $\begingroup$ I believe that we could. It depends on how you define life, but I'd say it's reasonable to assume it'll absorb and store some of the energy provided by its environment, and later use these energy reserves when it's sparse, producing metabolic byproducts that might be detectable. If such life can move about, it would also have migratory patterns following their source of nutrients. This doesn't really require of us to have deeper understanding of such lifeforms inner workings. It would just require dismissing all other possibilities first. $\endgroup$
    – TildalWave
    Commented Mar 1, 2014 at 13:12
  • $\begingroup$ Also relevant regarding search for intelligent life: Technosignature. And we're also listening, so if we can confirm another WOW! signal, we wouldn't really know what lifeform sent the signal, unless they chose to disclose that somehow. If it had undisputable intelligent origins, we wouldn't even know if it was sent by something alive, or simply AI of some civilisation reaching technological singularity. So yes, we are definitely searching for life "as we might not know it". $\endgroup$
    – TildalWave
    Commented Mar 1, 2014 at 13:35

4 Answers 4


Some rarely discussed option also include naturally occuring self-organising and self-replicating helical plasma structures, at least in theory: DOI:10.1088/1367-2630/9/8/263

This article studies the theoretical possibility that some plasma structures with properties usually used to describe living organism. Given the right environment, these structure could duplicate themselves (reproduction), preserve information for a given time (memory), and most importantly, they exibit the same thermodinamic properties as life: they are able to reduce their local enthropy (by incrasing their environment's one).

An interresting feature of these plasma structures is that they are shaped as helical strings, much like the DNA in our own cells.

This would difinately not be classified as "life as we know it"!

EDIT: Here is a more direct answer to the original question, based on the info above and my opinion:

The sentence "life as we know it" is said in a cautionary way, meaning "everything we know and is defined as life". It implies that there may be weird lifeforms we don't know about which we are not searching for.

As for these plasma structures, there is no space exploration program dedicated to searching for them, but results from dust experiments in microgravity (onboard the ISS) could be used to test the plasma structure hypothesis.

  • $\begingroup$ Would these leave signitures you could detect, without being in contact with them? If so are we doing anything to search for the markers? $\endgroup$ Commented Feb 28, 2014 at 18:47
  • $\begingroup$ From what I understand, in order to exist, these structure need a huge plasma environment to exist. The conditions would be similar to the universe right before the recombinaison era (en.wikipedia.org/wiki/Recombination_(cosmology)). It would be hard to pierce such a plasma with today's remote sensing techniques... But some clever physicists could find some way, we never know! $\endgroup$
    – PhilMacKay
    Commented Feb 28, 2014 at 19:33

Wikipeida has a decent article about hypothetical forms of life. Silicon shares many chemical properties with Carbon, making it a good candidate. However, carbon is much more versatile than other similar elements in terms of what they can bond to. Silicon has limitations and interacts with much fewer elements than Carbon.

One characteristic of "life as we know it" is that it requires some sort of solvent for biochemical reactions take place. All life we have studied is based on water, but there are many liquids that have the potential to provide life with a solvent. Currently when scientists study the makeup of different planets and moons, some markers of interest are non-water solvents, and this includes ammonia, hydrocarbons (methane), and many others. The presence of these liquids can be detected by using remote sensing and light analysis.

As an aside, "life as we know it" took on a very different meaning once scientists began discovering and classifying Extremophiles. The "range" of known life increased dramatically. We now know that life is extremely resilient and robust, and these findings are very exciting. However, this doesn't tell us much about non-carbon based life, it really only describes the variability in environments that can support life.

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    $\begingroup$ No, they didn't. $\endgroup$
    – Mark Adler
    Commented Feb 28, 2014 at 18:04
  • $\begingroup$ Woah, what?? I feel stupid, having told people that story before. Thanks, question updated. $\endgroup$
    – Stu
    Commented Feb 28, 2014 at 18:11
  • 2
    $\begingroup$ Don't feel stupid. The "amazing discovery" story always gets much, much bigger headlines than the "well, maybe not" story. $\endgroup$
    – Mark Adler
    Commented Feb 28, 2014 at 21:51

Are we searching for life “as we don't know it”?

Of course not. How could we?

We are (barely) searching for life by looking for electromagnetic signals from some distant civilization, but since we use electromagnetic signals for communication, that is life "as we know it". That parenthetical "barely": The organization that is doing this is SETI. NASA along with other space agencies have pretty much divorced themselves from the goings on of SETI.

We are (barely) searching for life by looking for signatures of biological life as we know it on Mars, and there are plans to look for signatures of biological life as we know it on the slew of recently discovered exoplanets. That parenthetical "barely": NASA and other space agencies until recently have paid rather short shrift to looking for signs of life on Mars, past or present. With regard to exoplanets, right now we're still in the process of finding planets that might support life as we know it. Looking in detail at whether some of those planets do support life as we know it: that is future work. Hopefully, it's near future work rather than the work of some future generation.

With regard to life as "we don't know it" -- how do we look for something with regard to which we have no clue what it looks like?

  • $\begingroup$ Strictly technically, we're so far only searching for conditions for life as we know it on Mars. Granted, tongue-in-cheek, if something jumped on MSL and waved into one of its onboard cameras, that'd clearly qualify, but otherwise, real in-situ search for life will only start with NASA's Mars 2020 and ESA's ExoMars missions. $\endgroup$
    – TildalWave
    Commented Mar 1, 2014 at 13:25

To conduct a successful search for extraterrestrial life, one would have to keep an open mind to the existence of life based on alternative chemistries, but focus a search effort on that which is most likely to occur, and therefore be easiest/most likely to find. The point of this Wikipedia article is that alternative chemistries are theoretically possible, but each of the theoretical alternatives is ultimately less viable or probable than the one we know. As stated in the article, it comes down to abundancies of the elements involved, the properties of liquid water as a solvent, carbon's rich chemical potential, and the other elements being most compatible with a carbon and water based chemistry.

It isn't that an alternative couldn't exist, just that there is one and only one chemistry which seems far more likely to occur than any other.


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