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Life as we know it is carbon-based,  but does it have to be that way?

There is another element on the  periodic table that shares some   of the key properties of carbon but  is far more abundant on most planets.

I’m talking about silicon.

So could  there silicon-based life out there?

All of the chemical machinery of life -  all biochemistry - is based around the   carboan atom.

At least as far as we know.

If  we want to understand the possible diversity   and abundance of life in the universe, we  need to ask whether this is the only option Perhaps the most popular alternative - at least in  science fiction - is silicon-based life.

We have   the xenomorphs of the Alien series to the giant  space worm in Star Wars and many, many others.

So why are scifi writers so into silicon-based  life?

Because it really is the most promising   non-carbon prospect due to its chemical  similarity.

It’s also more abundant on Earth   than carbon.

So, can life be based on silicon?

And if so, why aren’t we silicon-based?

When we say life is carbon-based, we don’t  just mean that life uses carbon a lot - we   mean that literally every aspect of the  molecular machinery that makes up life is   based around the chemistry of carbon.

Carbon  chains and rings form the scaffolding for   an enormous variety of molecules which  enable the raw machinery of life - from   DNA & RNA to amino acids & proteins  to all the structures built by these.

Biochemistry is by definition  the chemistry of Carbon.

In fact,   biochemists don't even write Carbon in their  formulas, they just draw bent lines and they   know that's supposed to be a Carbon atom.

So why  is carbon so favored by life?

Is it just that the   first life form happened to be carbon based,  so everything else got stuck with that atom?

Or is carbon-based chemistry really the  only way life can happen anywhere, ever?

Ultimately, carbon’s incredible utility comes  down to the way it bonds with other elements,   which enable just the right  balance of reactivity and   stability.

To understand this,  we need to think about energy.

Nature will always try to get to the lowest energy  state that it possibly can.

A stick of dynamite   for example, is in a high energy state due to  all the energy stored in the chemical bonds of   its nitroglycerine.

If the atoms find a way  to a lower energy state they’ll go for it,   explosively releasing that energy in the  process of converting the nitro to smaller   molecules.

Nitroglycerine is unstable  because it only takes a little energy   to break its tenuous bonds, which in turn  releases a lot more energy than went in,   initiating a chain reaction.

That reaction  only goes in one direction, because it’s   not energetically favourable to reassemble  all the smaller molecules back into nitro.

Life is basically an ongoing chemical  reaction.

You don’t want the chemistry   of life to be unidirectional.

You don’t  want all the raw ingredients to end up in   the same overly-stable molecules, with no  way back.

That means the reaction is over,   which for life means …well, death.

At the  same time, you want the molecules doing   the work of life to be stable enough - they  need to last long enough to do their jobs.

In other words, you want a balance  of reactivity and stability.

Let’s figure out what elements might  be good for reaching this balance.

And to start we’re going to open up to the entire  periodic table and see what we can rule out.

The key is in what types of chemical bonds a given  element can make.

There are three main bond types:   ionic, metallic, and covalent.

Ionic bonds are way too unstable,   while metallic bonds result in repeating  structures - crystals - which don’t have   the chemical variety needed for life.

Covalent bonds are really the only option.

In these, atoms share electrons with  one or more neighbors in order to fill   up their outer or valence shells.

For example,  carbon has 4 valence electrons.

It wants 8 for   maximum stability.

It achieves that stability  by connecting to 4 hydrogen atoms and sharing   their single electron.

This also gives each  of those hydrogen atoms one more electron   to fill up its own valence shells.

The  result of this arrangement is methane.

The cool thing about covalent bonds is that there  are countless ways to do this electron sharing,   enabling enormous diversity  and modularity.

For example,   if we remove a hydrogen from two methane  molecules and stick them together you have   ethane.

Now replace one of the hydrogens  with an oxygen and we have ethanol.

Covalent bonds are the key to the complex and  resilient molecular structures needed for life.

With this in mind we can look at the Periodic  Table and rule out all the elements that form   mostly ionic or metallic bonds, which is actually  most elements.

We’re keeping things that are good   at forming covalent bonds, even if they  also sometimes bond in those other ways.

We can also rule out Noble Gasses since they are  barely reactive at all, and the Halogens, because   they’re just too much trouble.

It's hard to get  them to form covalent bonds with more than one   element at the same time, and when they react with  something it's usually in an explosive manner.

Finally, we’re going to scrap elements in rows 4  and below.

Although these may form covalent bonds,   these atoms are so large that the  nucleus doesn't hold the electrons   with enough strength, leading  to dangerously weak molecules By the way, this is the reason Arsenic is toxic.

Cells can accidentally build molecules using   Arsenic in place of Phosphorus due to them having  the same number of valence electrons.

When that   happens the resulting molecules will break,  destroying important structures in the cell,   or interrupting important processes, and  if this happens too much the cell will die.

In the end we are left with only seven elements:  Hydrogen, Carbon, Nitrogen, Oxygen, Silicon,   Phosphorus, and Sulfur.

Of these, all but silicon  are considered fundamental building blocks of life   on Earth.

Other elements are used by life for  very specific functions, but hydrogen, carbon,   nitrogen, oxygen, phosphorus, and sulfur are  the most important.

And of these, carbon is   by far the more critical because it provides the  scaffolding of all biochemistry.

The reason it’s   good for this is that it forms strong bonds with  itself, and does so in a variety of ways.

In fact,   of the 6 building blocks of life, only carbon  can form both 1-D and 2-D structures with itself.

In our process of identifying elements that  may be useful for life we got to the 6 main   building blocks plus silicon.

But why  did silicon get dropped by evolution?

This seems weird because silicon  is chemically similar to carbon,   also having 4 valence electrons.

Silicon  can imitate Carbon and form long chains,   and rings, and the resulting molecules often have  similar properties to molecules made with Carbon.

To give you just one example we can make  Silicon oils, and they feel and look just   like other kinds of oil.

It’s the only  other element capable of forming complex   covalent scaffolds and supporting a large  diversity of molecules.

Despite all of this,   Earth life makes less use of silicon than it  does of many other trace elements like iron.

This seems extra strange when you remember  that silicon accounts for more than 28% of   the mass of Earth’s crust, in all them silicate  rocks.

On the other hand, Carbon makes up only   around 0.02% of the crust.

This tells us that  carbon must have substantial advantages over   silicon.

So let’s talk about some of the  issues with silicon as a basis for life.

One huge one is that silicon-based molecules  tend to be far more reactive to water than   carbon-based.

In fact, although  silicon chemistry is extremely   diverse, most complex silicon  molecules are unstable in water.

You can see how this would be a problem for  silicon-based life on our very watery Earth.

In general, life needs a solvent - a  liquid medium that allows molecular   machinery to move around and interact.

Water  is incredible for this.

Its thermal stability,   power as a solvent, and abundance  through the universe make it the best,   and perhaps even the only decent choice of the  base solvent for life.

The instability of most   silicon molecules in water is actually a huge  blow to the viability of silicon-based life.

However water is not the only option.

A much  greater variety of silicon-based molecules   are stable in liquid hydrocarbons - which  are present on the moons of the gas giants,   and even in sulphuric acid, which  is abundant in Venus’s atmosphere.

Both cases have their issues - liquid hydrocarbons  are cold, which among other things means they have   trouble holding in solution the large, complex  molecules that known life relies on.

Sulphuric   acid is chemically extremely aggressive,  perhaps too much so for life to get started.

On the other hand, xenomorphs are silicon-based  and they do have acidic blood - makes sense that   they’re silicon -in-sulphuric-acid-based.

If that’s what Venusian life is like,   let’s cancel the cloud city construction  program before the space horror begins.

There is one way to potentially get silicon-based  life in water - and that’s by coupling silicon   with either carbon or oxygen for our scaffolding  molecule.

Chains of these pairs of atoms can   produce some chemical diversity.

However neither  case seems to provide significant advantage over   just using carbon chains.

Another reason that  silicon seems implausible as a scaffolding   element for life is that the stuff really,  really loves forming bonds with oxygen.

Silicon forms stronger bonds with oxygen than  with any of the other “building blocks of life”,   and certainly more so than  with other silicon atoms.

Silicon-based molecules are  easily broken by oxygen,   and silicon-oxygen bonds are hard to  break in turn.

That means reactions   tend to go in one direction when there’s  oxygen available.

And the strength of the   silicon-oxygen bond is perhaps the main reason  that silicon is rarely used by life on Earth.

Although there’s an awful lot of  it, Earth’s silicon is locked up in   rocks and is inaccessible to life.

On the  other hand, most of Earth’s carbon is in   CO2 in the atmosphere, which is relatively  easily accessible through photosynthesis.

There is one sense in which the strength of the  silicon-oxygen bond could be useful for life,   and that’s in energy generation.

Let’s  think about where our energy comes from.

It’s from the conversion of oxygen molecules  - which contain a ton of energy in their   bonds - into CO2 molecules - which contain very  little.

The simplest way to get that energy is   to set something on fire, which works great  for steam engines but not so great for cells.

The mitochondria in our eukariot cells  liberate that energy with a few more steps,   finally storing it in the ATP molecule.

So  could silicon-based aliens get energy by   converting O2 into silicon-dioxide?

On the surface this looks good,   because SIO2 is even more stable than  CO2, so there’s more energy to liberate.

The problem is that while CO2 is  a gas that can be expelled easily,   silicon-dioxide is silica … it’s sand.

It’s  coarse and rough and gets everywhere, I hate it.

It’s not something you want building up in your  cells or your lungs.

And this wouldn't be a small   amount of sand.

A regular person exhales about  a kilogram of Carbon Dioxide throughout the day,   meaning Silicon aliens would need to cough out  a good amount of sand.

Or they poop glass beads   or something - probably there’s a way around the  issue.

But it’s certainly not as convenient and   energy-cheap as exhaling CO2.

And with life,  any little saving in energy is a big deal.

Let’s summarize.

In certain conditions silicon  potentially has the chemical diversity for an   alien biochemistry, but those conditions are  pretty specialized, and are far less ideal   that the ones that support carbon-based life.

It  seems that if life has the option of using carbon   then it should do so.

But it’s not impossible that  in highly unusual environments silicon-based life But what would that life even look like?

For decades science fiction authors have  imagined silicon based life as variations   of living crystals or rocks, that's because that’s  the common form of silicon on Earth is just that.

But we carbon-based creatures don’t look  like walking diamonds or pencil leads,   so there’s no reason to think of silicon-based  aliens would look particularly crystally.

Well, actually.

There is a reason.

And that’s  because living silicon crystals already exist   on Earth.

Let me present you to Diatoms.

These  are single celled organism that have incorporated   Silicon into their cell walls, making them  rigid, and basically just Silica crystals   that encapsulate all the parts of the cell inside  them.

These beautiful living crystals live simple   lives.

They just float around in the ocean  in long chains photosynthesizing, producing   half of the oxygen that we breath.

Now to be  clear, Diatoms are not Silicon based life,   it's just their cell walls.

Everything inside  them is based on Carbon.

But they are at least   a proof of concept that one important component of  life - the cell wall - can be built from silicon.

OK, so what have we learned?

Well, silicon-based  aliens are probably slightly over-represented in   fiction compared to their likely frequency  out there in the universe.

In fact,   carbon-in-water is such an astoundingly good  option for biochemistry that there may be no   other game in town.

Even Carl Sagan called  himself a carbon-chauvinist for this reason.

We’ll probably be more similar than different to  the alien species of our first-contact.

But let’s   not ignore the hydrocarbon lakes of Titan or the  acid cloud-scapes of Venus, where silicon-based   critters may expand our notion of what is  possible in the variety of life across space time.

Thank you to Brilliant.org for supporting PBS.

Brilliant is an online learning platform for STEM  with hands-on, interactive lessons.

Brilliant is   for curious learners, both young and old,  professional and inexperienced.

Brilliant   courses teach you how to think via interactive  lessons and problem-solving activities/exercises.

For example, Brilliant.org offers  a course called Quantum Objects.

The principles of quantum mechanics came  from investigating microscopic phenomena,   the bizarre behavior of quantum objects like atoms  and elementary particles that often appear to   contradict classical mechanics and probability.

In this course, you'll explore experiments of   quantum objects and use them to construct new  equations of motion, new laws of physics, and   a new system of measurement based not on numbers  but on algebras.

By the end, you’ll gain a new   appreciation for how the physics of the small  enables lasers, transistors, and other modern   technologies that define our world.

Then you'll  be ready to dive into the ongoing revolution of   quantum information and computing.

To learn more  about Brilliant, go to brilliant.org/spacetime Today we’re doing comment responses for the  last two episodes - the one about the new   simulations that show how our moon formed from  a giant impact with higher resolution than ever   before.

And the one on Max Tegmark’s mathematical  universe hypothesis which proposes that the most   fundamental reality is a platonic realm of  mathematics.

We’ll start with the latter.

onebronx opines that the basic math of our  Universe is likely not the SM lagrangian   and the GR equation, but the principle of  a stationary action and a specific set of   mutually-consistent symmetries.

I absolutely  agree on the core point - which is that IF   the mathematical universe hypothesis is true,  a given universe probably shouldn’t defined   by the platonic existence of a complicated  set of equations, but rather a very simple   set of rules.

In the mathematical universe  hypothesis, the physical world is emergent   from a more fundamental mathematical world.

But that doesn’t mean that complex mathematical   structures can’t also be emergent from simpler,  more fundamental mathematical structures.

S. Lee Odegard mentions that we should have  included Sir Rodger Penrose's three worlds   ontology, in which reality is described as three  realms of existence, each merging from the other.

I agree, and this was totally going to be in the  episode but we ran out of space for it.

To give an   overview - Penrose proposes this way of thinking  about the circular emergence of reality.

We have   the material world as emerging from a platonic  mathematical world - in the sense that without   the possibility of mathematical regularities,  the material world couldn’t exist; and then   the mental world emerges from the material - in  the sense that material brains generate mind;   and finally the platonic world emerges from  the mental - in the sense that minds are able   to conceive of the full language of mathematics  even beyond its manifestation in the physical.

This is pretty different from Tegmark’s idea  that the platonic math world is the ontological   precursor to everything else.

Penrose doesn’t  grant primacy to the platonic - which is why   I ultimately decided to leave that  out - perhaps for a future episode.

If anyone wants to read about this, get Penrose’s  book Road to Reality - but fair warning,   reading it is a full semester intensive physics  course.

Which is, by the way, a recommendation.

OK, and now on to the far less abstract  idea that the moon formed when a planet   that we name Theia smashed  into the newly formed Earth markcentral mentioned an idea I’d never  heard before.

That lumps of relatively   intact Theia material may still  be present in the earth’s mantle.

There are these high density blobs of material  that sit at the boundary between Earth’s mantle   and core called the Large low-shear-velocity  provinces.

Some scientists have suggested that   this is material from the planet that collided  with Earth in the Moon’s formation.

There is some   loose evidence that isotopic ratios in basalt  thought to be from lava plumes rising from the   regions above the LLSVPs have “primitive” isotopic  ratios consistent with material that wasn’t mixed   in the giant impact.

Very cool possibility,  though obviously a lot more work is needed.

Alice asks: If the moon is a very unlikely event,  and an unusually strong iron core is necessary   to protect the development of life, does this  provide a sufficiently hard filter to resolve   the Fermi paradox?

The answer is: we don’t know,  but people have speculated that this might be the   case.

My personal intuition is that the extremely  hard filter needed to explain the Fermi paradox   is actually a chain of somewhat hard filters,  and the large moon may be one of these.

Others   include having a planetary composition like ours  - large protective gas giants in the outer solar   system but only small terrestrial planets in  close, a star of a very particular quality,   and so on.

Check our video on the rare  earth hypothesis for more on this.

I’d like to address a comment about my  comment response from the supercritical   fluids episode where I joked that there are  animals with hands large enough to swim in   gas - they’re called birds.

Several people noted  that bats would be a better example here because,   as Xovvo notes, and I quote “Bat wings are formed  from membranous extensions from palm to fingertip   between digits two through five.

Bird hands  are actually very small and have completely   fused together into a structure called the  carpometaparpus, tipped by greatly reduced   digits II and III (digit I bearing the alula,  anterior to the rest of the carpometacarpus).

Birds just have very long feathers to form  their flight surface.

So … I’ve learned a   lot from my mistake - both regarding the size  of flighted animal hands, and that even casual   jokes should be subject to rigorous research and  fact-checking.

My sincere apologies to everyone Several people noticed that I flubbed when  naming Neptune’s moon Triton.

I misspoke   and said Titan, which is of course Saturns moon.

Thanks to those in the comments who wondered if  it might just be my Australian accent - like,   we don’t pronounce r’s after t’s or something.

Yeah, that’s it - just remember any time you hear   me misspeak it’s actually just how Aussies  pronounce the actual correct word.

Also,   all Australian birds have gigantic hands,  unlike everywhere else in the world.