<p>Carbon is like lego-technic. It connects water into organic compounds which have every chemical and physical property from volatile solvent to syrup to oil to tar and rock. </p>

<p>Silicon is like lego, try building a car out of it, it's difficult to find a good solvent for it. it can't even bond H. Silane which is the equivalent of methane doesn't occur naturally on our planet, it's too unstable. since silicon doesn't form OH compounds, it doesn't have the malleability of water, it's always rocky and unstable except at -200 degrees where it could be more versatile than carbon. silicon compounds under -180 degrees C are interesting and worth studying, they would need a kryogenic space suit to keep them cool, and if you shoot them they would spray liquid oxygen everywhere. </p>

<p>Sulphur is even less versatile than silicon. it only forms linear chains and is hyper reactive with hydrogen. </p>

<p>You'd need a solvent other than water which is used for carbon based. Ammonia, formaldehyde, sulphuric acid, liquid nitrogen, hydrogen fluoride, methane, hydrocarbons, supercritical fluids.</p>

<p>Perhaps complex robotic life forms are some of the easiest to make up. Life forms that can be designed by artificial intelligence, to use most commonly available resources at ambient temperatures are more likely than complex life forms using other than carbon. That's because a large computer can design transistors/mechanics that work with any metals and conductors in very cold temperatures (aluminium can structurally handle -130 kelvin easily), and robotic life forms would prefer to be in a stable environment without biology, clouds, rain, water, in cold environments, it would be easier for them. </p>

CC BY-SA 3.0
2017-04-29T23:17:14.867Z
Your -200 degree silicon life forms is an interesting idea.  It's usually associated with hotter in bad science fiction, but silicon life on very cold planets, lets just say I'm a tiny bit curious about the notion.
userLTK
CC BY-SA 3.0
2017-04-29T19:31:19.887Z
2017-04-29T23:17:44.503Z
2017-04-29T23:17:44.503Z
bandybabboon
<p>Carbon can hold one of the most stable 4 bond connections out of the elements. We see how life is shaped on earth and based on the properties of life here, we make educated guesses on what life must be based around in other parts of the universe, given that there is other life.</p>

<p>To answer the second part of the second, the reason we breathe in oxygen is to allow certain cellular functions to take place, the most important reason is respiration. C6H12O6 + O2 yields CO2 + H2O + 36-38 ATP. ATP is the molecule the body uses as energy. Oxygen is essential for us because it's used in the chemical reaction that gives our body energy. If an alien were to be breathing chlorine, it would need to have a chemical process that uses the chlorine in their body. 
As for there being sulphur-based life, I don't know whether sulfur is stable enough to hold the same bonds as carbon. From what scientists know, carbon is the best universal organic base, because of its stable 4 bonds. </p>

CC BY-SA 3.0
2017-05-12T17:47:56.800Z
There may be a case where the initial ingredients of a solar system are lacking in carbon for whatever reason and silicon may be chosen just because of abundance. Or maybe the system could be blessed with an unusually high boron abundance for some weird reason and life could be boron based.
Jack R. Woods
CC BY-SA 3.0
2017-05-13T01:45:22.523Z
Jack R. Woods - Boron is a lot less likely to be a base for life. Silicon and carbon are the only elements that can hold 4 stable bonds. Boron can only hold 3. If a solar system were to lack both carbon and silicon, odds are it won't produce life at all, even if there's a planet in the right spot. Again, that's why it's commonly believed that any other life would be made from carbon or silicon, never anything else.
bkrumins
CC BY-SA 3.0
2017-04-27T14:38:46.030Z
2017-05-03T11:56:28.587Z
2017-05-03T11:56:28.587Z
bkrumins
<p>An oxygen-based life form may also be possible, given that oxygen readily bonds with other molecules. In addition, how do scientists actually know that the laws of physics, chemistry, and life apply everywhere? Why not just Earth or our solar  system? Why not just the Milky Way? There could be different laws and possibilities in different galaxies, which humans will most likely never discover. Who are we to decide what id true and what is not when it will never actually be known? For all I care, life on Earth could be the most simple and unimpressive in the entire universe. We could have the intelligence and lifespan of a maggot in comparison to some form of existence. Life is, after all, just a machine. What should we care anyway? We have to; that's the only known reason as to why life exists. To fill the void. To be there. What's beyond existence? Maybe that's where a black hole leads. In all honesty, this world is far beyond any conceivable understanding, far beyond the reach of any of us. At the end of the day, we are all bodies in the inconceivably large world of being. A fleck of dust in two-dimensional space. In reality, we are only worth something to ourselves.</p>

CC BY-SA 4.0
2019-09-30T23:45:34.257Z
Doubting everything could be the answer to any and all science-based question, and it would be equally uninteresting. You could improve this answer by expanding on your very short introduction about something oxygen based, and perhaps spend a bit less space on relativism.
SE - stop firing the good guys
CC BY-SA 4.0
2019-10-01T11:42:01.240Z
There are plenty of reasons to expect the laws of physics to be the same, or nearly the same, throughout the Universe. Observations are plentiful. See [this](https://physics.stackexchange.com/questions/413886/why-do-scientists-think-that-all-the-laws-of-physics-that-apply-in-our-galaxy-ap) physics.SE thread for a discussion..
pela
CC BY-SA 4.0
2019-09-30T23:22:46.230Z
2019-09-30T23:22:46.230Z
user30540
<p><sub>I want to start with a disclaimer that I'm not a chemist and hopefully I don't screw any of the chemistry up. If I do, please let me know.</sub></p>

<p>I think <a href="https://astronomy.stackexchange.com/questions/20858/why-do-aliens-have-to-be-carbon-based-lifeforms#comment35226_20858">Bill Oertell</a> hit the nail on the head. Just because you can imagine a species which can drink lead doesn't mean it's actually physically possible. Only specific atoms can form the basis of the (extremely) complex organic molecules needed to form life and Carbon happens to be the atom which can do that. The primary reason for this is that Carbon has four valence electrons and its outer shell can hold eight, allowing it to form strong and varied bonds that most other atoms just can't form. As such, Carbon is the most logical element to form the backbone of organic molecules.</p>

<h3>Other ___-based Lifeforms</h3>

<p>Moving down the periodic table, you can see that Silicon has the same properties (4 valence elections, 8 possible) and you might be inclined to think that you could form some sort of silicon based life. This topic has been <a href="https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Silicon_biochemistry" rel="noreferrer">widely considered</a>, but you run into a number of problems that make Silicon-based life unfeasible. The primary reason is that Silicon is just much bigger and more massive than Carbon (having a nucleus with 28 nucleons compared to Carbon's 12). This makes it harder for Silicon to form the necessary bonds that Carbon can readily form and you're out of luck right form the get-go. Similar issues occur with other "<a href="https://en.wikipedia.org/wiki/Carbon_group" rel="noreferrer">Carbon group</a>" elements, except they're even further exacerbated by even higher masses.</p>

<p>There are many other "organic" configurations chemists have considered such as boron-based life or sulfur-based life but they all have some sort of problem that makes it hard for it to exist. Some are more possible than others, but Carbon-based life, as we know it, just seems to be the best solution. If you gave someone all the elements in the periodic table and said, make organic life, they'd probably come back with Carbon-based life because its the easiest. You don't have to overcome as many obstacles.</p>

<h3>Observations</h3>

<p>Aside from the impracticality of non-Carbon based life, you have to consider that everything we've seen in the universe, as far as organic chemistry goes, points to the fact that the Universe is filled with Carbon-based chemistry. Astronomers have been able to look out at comets, interstellar dust clouds, and all sorts of places and time after time, they see complex, Carbon-based, organic molecules. For example:</p>

<ul>
<li><a href="http://www.astrochem.org/docs/IAU251_Salama.pdf" rel="noreferrer">We've found Polycyclic Aromatic Hydrocarbons (PAHs) all over the solar system and galaxy</a>,</li>
<li><a href="http://www.bbc.com/news/science-environment-29368984" rel="noreferrer">Astronomers have also found things like i-propyl cyanide</a> in meteorites and in interstellar dust $-$ an organic molecule which is a <a href="https://en.wikipedia.org/wiki/Isopropyl_cyanide#Importance_in_life.27s_origin" rel="noreferrer">necessary ingredient in amino acids</a>,</li>
<li><a href="http://www.popularmechanics.com/space/solar-system/news/a25235/ceres-organics/" rel="noreferrer">Organic molecules have been found (and believed to have formed) on Ceres.</a></li>
</ul>

<p>The point I'm driving at here is that the universe seems ready-made to start forming Carbon-based life everywhere. Everywhere we look, we see organic molecules which have Carbon as the backbone of their structure. Nowhere do we see complex silicon based molecules (or something even crazier). This is highly suggestive that Carbon is the way to go if you want to quickly and easily form life.</p>

<p>There are probably a 100 other reasons to favor Carbon over something else, but that delves into more complicated chemistry than I've touched on here and I'm not as knowledgeable about that. I would think this question would get a more in-depth chemistry answer on the <a href="https://chemistry.stackexchange.com/">chemistry stack exchange</a>.</p>

CC BY-SA 3.0
2017-04-28T13:33:02.683Z
Another point would be abundancy. Carbon is much more abundant in the universe than any other element except Hydrogen, Helium and Oxygen due to stellar fusion processes. So even if bio-chemistry would be possible with other elements, they are statistically less likely to happen. This would in turn favour development of carbon based life throughout the universe over silicon based (or any other element capable of forming complex molecules).
Adwaenyth
CC BY-SA 3.0
2017-04-29T23:28:35.750Z
indeed, CO2 has two double bonds only possible with C and dissolves in water and is very reactive, SiO2 is one the the most stable and unreactive minerals on the planet, quartz. it's an example of the general solvent abilities of carbon, it can form millions of types of solvents, whereas Si almost only forms minerals in space, in comets and on planets.
bandybabboon
CC BY-SA 3.0
2017-05-03T23:06:35.433Z
Another point is that Silicon - Silicon bonds are very weak (See: https://physics.stackexchange.com/questions/127890/why-are-silicon-silicon-bonds-weak and https://chemistry.stackexchange.com/questions/28702/how-come-there-arent-any-silicon-analogs-of-alkenes-or-alkynes) and for Silicon based life, Silicon - Oxygen bonds should be better than Silicon -Silicon.
Juan T
CC BY-SA 3.0
2017-04-26T21:08:49.180Z
2017-04-26T21:08:49.180Z
zephyr
<p>So correct me if I'm wrong, but when searching for extra terrestrial life, scientists usually search for signs they would expect for life on earth to thrive in. We look for conditions on planets that meet certain criteria e.g close enough to a sun for water, environments for carbon based life forms to exist. I was wondering; why cant an alien be a sulphur based life form that for the sake of argument drinks lead and breathes in chlorine like we do oxygen.</p>

CC BY-SA 3.0
2017-04-26T12:40:48.967Z
@Kozaky, or photino birds :-)
Carl Witthoft
CC BY-SA 3.0
2017-04-26T12:58:27.813Z
Carbon is remarkably good at what's required for life.  It makes strong bonds and forms long chain molecules that are both stable and flexible.    That's not to say it couldn't be done with other atoms, but Carbon is ideal.  I don't think any other atoms come close.
userLTK
CC BY-SA 3.0
2017-04-26T20:04:59.613Z
Chemistry is chemistry; it doesn't matter if it's here on earth or some planet on the other side of the galaxy or universe. Chemistry and physics are going to obey the same laws. We know that carbon readily forms long chain molecules, and the only other element we know of that comes even close is silicon, which doesn't get anywhere close to carbon.
BillDOe
CC BY-SA 3.0
2017-04-26T21:13:00.780Z
@kozaky No, it really isn't. It's chemistry.
ProfRob
CC BY-SA 3.0
2017-04-30T15:06:28.463Z
Sulfur plays with a lot of difference valence states: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/group6.php That means that sulfur based life would have to spend a lot of time and energy trying to regulate redox conditions.
Wayfaring Stranger
CC BY-SA 3.0
2017-05-03T18:59:56.790Z
Because carbon and oxygen (and nitrogen) are the [most common elements](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements) in the universe. Any lead based life form would have to compete against that. And at least on Earth they were all extinguished by the superior carbon/oxygen life forms.
LocalFluff
CC BY-SA 3.0
2017-05-03T20:39:20.910Z
You might be able to get away with substituting or Boron-Nitrogen combo for a Carbon-Carbon.  Maybe.
David Elm
<astrobiology>
Why do aliens have to be carbon based lifeforms?
mystery man
# Why do aliens have to be carbon based lifeforms?

(Asked by: username_0 on 2017-04-26)

So correct me if I'm wrong, but when searching for extra terrestrial life, scientists usually search for signs they would expect for life on earth to thrive in. We look for conditions on planets that meet certain criteria e.g close enough to a sun for water, environments for carbon based life forms to exist. I was wondering; why cant an alien be a sulphur based life form that for the sake of argument drinks lead and breathes in chlorine like we do oxygen.

**Tags:** astrobiology

> **Comments:**
> **username_1**: @Kozaky, or photino birds :-)  
> **username_2**: Carbon is remarkably good at what's required for life.  It makes strong bonds and forms long chain molecules that are both stable and flexible.    That's not to say it couldn't be done with other atoms, but Carbon is ideal.  I don't think any other atoms come close.  
> **username_3**: Chemistry is chemistry; it doesn't matter if it's here on earth or some planet on the other side of the galaxy or universe. Chemistry and physics are going to obey the same laws. We know that carbon readily forms long chain molecules, and the only other element we know of that comes even close is silicon, which doesn't get anywhere close to carbon.  
> **username_4**: @kozaky No, it really isn't. It's chemistry.  
> **username_5**: Sulfur plays with a lot of difference valence states: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/group6.php That means that sulfur based life would have to spend a lot of time and energy trying to regulate redox conditions.  
> **username_6**: Because carbon and oxygen (and nitrogen) are the [most common elements](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements) in the universe. Any lead based life form would have to compete against that. And at least on Earth they were all extinguished by the superior carbon/oxygen life forms.  
> **username_7**: You might be able to get away with substituting or Boron-Nitrogen combo for a Carbon-Carbon.  Maybe.  

---

## 4 answers

### Answer by username_14 on 2017-04-26. Score: 21 | Accepted answer

I want to start with a disclaimer that I'm not a chemist and hopefully I don't screw any of the chemistry up. If I do, please let me know.

I think [Bill Oertell](https://astronomy.stackexchange.com/questions/20858/why-do-aliens-have-to-be-carbon-based-lifeforms#comment35226_20858) hit the nail on the head. Just because you can imagine a species which can drink lead doesn't mean it's actually physically possible. Only specific atoms can form the basis of the (extremely) complex organic molecules needed to form life and Carbon happens to be the atom which can do that. The primary reason for this is that Carbon has four valence electrons and its outer shell can hold eight, allowing it to form strong and varied bonds that most other atoms just can't form. As such, Carbon is the most logical element to form the backbone of organic molecules.

### Other \_\_\_-based Lifeforms

Moving down the periodic table, you can see that Silicon has the same properties (4 valence elections, 8 possible) and you might be inclined to think that you could form some sort of silicon based life. This topic has been [widely considered](https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Silicon_biochemistry), but you run into a number of problems that make Silicon-based life unfeasible. The primary reason is that Silicon is just much bigger and more massive than Carbon (having a nucleus with 28 nucleons compared to Carbon's 12). This makes it harder for Silicon to form the necessary bonds that Carbon can readily form and you're out of luck right form the get-go. Similar issues occur with other "[Carbon group](https://en.wikipedia.org/wiki/Carbon_group)" elements, except they're even further exacerbated by even higher masses.

There are many other "organic" configurations chemists have considered such as boron-based life or sulfur-based life but they all have some sort of problem that makes it hard for it to exist. Some are more possible than others, but Carbon-based life, as we know it, just seems to be the best solution. If you gave someone all the elements in the periodic table and said, make organic life, they'd probably come back with Carbon-based life because its the easiest. You don't have to overcome as many obstacles.

### Observations

Aside from the impracticality of non-Carbon based life, you have to consider that everything we've seen in the universe, as far as organic chemistry goes, points to the fact that the Universe is filled with Carbon-based chemistry. Astronomers have been able to look out at comets, interstellar dust clouds, and all sorts of places and time after time, they see complex, Carbon-based, organic molecules. For example:

* [We've found Polycyclic Aromatic Hydrocarbons (PAHs) all over the solar system and galaxy](http://www.astrochem.org/docs/IAU251_Salama.pdf),
* [Astronomers have also found things like i-propyl cyanide](http://www.bbc.com/news/science-environment-29368984) in meteorites and in interstellar dust $-$ an organic molecule which is a [necessary ingredient in amino acids](https://en.wikipedia.org/wiki/Isopropyl_cyanide#Importance_in_life.27s_origin),
* [Organic molecules have been found (and believed to have formed) on Ceres.](http://www.popularmechanics.com/space/solar-system/news/a25235/ceres-organics/)

The point I'm driving at here is that the universe seems ready-made to start forming Carbon-based life everywhere. Everywhere we look, we see organic molecules which have Carbon as the backbone of their structure. Nowhere do we see complex silicon based molecules (or something even crazier). This is highly suggestive that Carbon is the way to go if you want to quickly and easily form life.

There are probably a 100 other reasons to favor Carbon over something else, but that delves into more complicated chemistry than I've touched on here and I'm not as knowledgeable about that. I would think this question would get a more in-depth chemistry answer on the [chemistry stack exchange](https://chemistry.stackexchange.com/).

> **Comments:**
> **username_15**: Another point would be abundancy. Carbon is much more abundant in the universe than any other element except Hydrogen, Helium and Oxygen due to stellar fusion processes. So even if bio-chemistry would be possible with other elements, they are statistically less likely to happen. This would in turn favour development of carbon based life throughout the universe over silicon based (or any other element capable of forming complex molecules).  
> **username_8**: indeed, CO2 has two double bonds only possible with C and dissolves in water and is very reactive, SiO2 is one the the most stable and unreactive minerals on the planet, quartz. it's an example of the general solvent abilities of carbon, it can form millions of types of solvents, whereas Si almost only forms minerals in space, in comets and on planets.  
> **username_16**: Another point is that Silicon - Silicon bonds are very weak (See: https://physics.stackexchange.com/questions/127890/why-are-silicon-silicon-bonds-weak and https://chemistry.stackexchange.com/questions/28702/how-come-there-arent-any-silicon-analogs-of-alkenes-or-alkynes) and for Silicon based life, Silicon - Oxygen bonds should be better than Silicon -Silicon.  

---

### Answer by username_8 on 2017-04-29. Score: 4

Carbon is like lego-technic. It connects water into organic compounds which have every chemical and physical property from volatile solvent to syrup to oil to tar and rock.

Silicon is like lego, try building a car out of it, it's difficult to find a good solvent for it. it can't even bond H. Silane which is the equivalent of methane doesn't occur naturally on our planet, it's too unstable. since silicon doesn't form OH compounds, it doesn't have the malleability of water, it's always rocky and unstable except at -200 degrees where it could be more versatile than carbon. silicon compounds under -180 degrees C are interesting and worth studying, they would need a kryogenic space suit to keep them cool, and if you shoot them they would spray liquid oxygen everywhere.

Sulphur is even less versatile than silicon. it only forms linear chains and is hyper reactive with hydrogen.

You'd need a solvent other than water which is used for carbon based. Ammonia, formaldehyde, sulphuric acid, liquid nitrogen, hydrogen fluoride, methane, hydrocarbons, supercritical fluids.

Perhaps complex robotic life forms are some of the easiest to make up. Life forms that can be designed by artificial intelligence, to use most commonly available resources at ambient temperatures are more likely than complex life forms using other than carbon. That's because a large computer can design transistors/mechanics that work with any metals and conductors in very cold temperatures (aluminium can structurally handle -130 kelvin easily), and robotic life forms would prefer to be in a stable environment without biology, clouds, rain, water, in cold environments, it would be easier for them.

> **Comments:**
> **username_2**: Your -200 degree silicon life forms is an interesting idea.  It's usually associated with hotter in bad science fiction, but silicon life on very cold planets, lets just say I'm a tiny bit curious about the notion.  

---

### Answer by username_9 on 2017-04-27. Score: 2

Carbon can hold one of the most stable 4 bond connections out of the elements. We see how life is shaped on earth and based on the properties of life here, we make educated guesses on what life must be based around in other parts of the universe, given that there is other life.

To answer the second part of the second, the reason we breathe in oxygen is to allow certain cellular functions to take place, the most important reason is respiration. C6H12O6 + O2 yields CO2 + H2O + 36-38 ATP. ATP is the molecule the body uses as energy. Oxygen is essential for us because it's used in the chemical reaction that gives our body energy. If an alien were to be breathing chlorine, it would need to have a chemical process that uses the chlorine in their body.
As for there being sulphur-based life, I don't know whether sulfur is stable enough to hold the same bonds as carbon. From what scientists know, carbon is the best universal organic base, because of its stable 4 bonds.

> **Comments:**
> **username_10**: There may be a case where the initial ingredients of a solar system are lacking in carbon for whatever reason and silicon may be chosen just because of abundance. Or maybe the system could be blessed with an unusually high boron abundance for some weird reason and life could be boron based.  
> **username_9**: username_10 R. username_10 - Boron is a lot less likely to be a base for life. Silicon and carbon are the only elements that can hold 4 stable bonds. Boron can only hold 3. If a solar system were to lack both carbon and silicon, odds are it won't produce life at all, even if there's a planet in the right spot. Again, that's why it's commonly believed that any other life would be made from carbon or silicon, never anything else.  

---

### Answer by username_11 on 2019-09-30. Score: -5

An oxygen-based life form may also be possible, given that oxygen readily bonds with other molecules. In addition, how do scientists actually know that the laws of physics, chemistry, and life apply everywhere? Why not just Earth or our solar system? Why not just the Milky Way? There could be different laws and possibilities in different galaxies, which humans will most likely never discover. Who are we to decide what id true and what is not when it will never actually be known? For all I care, life on Earth could be the most simple and unimpressive in the entire universe. We could have the intelligence and lifespan of a maggot in comparison to some form of existence. Life is, after all, just a machine. What should we care anyway? We have to; that's the only known reason as to why life exists. To fill the void. To be there. What's beyond existence? Maybe that's where a black hole leads. In all honesty, this world is far beyond any conceivable understanding, far beyond the reach of any of us. At the end of the day, we are all bodies in the inconceivably large world of being. A fleck of dust in two-dimensional space. In reality, we are only worth something to ourselves.

> **Comments:**
> **username_12**: Doubting everything could be the answer to any and all science-based question, and it would be equally uninteresting. You could improve this answer by expanding on your very short introduction about something oxygen based, and perhaps spend a bit less space on relativism.  
> **username_13**: There are plenty of reasons to expect the laws of physics to be the same, or nearly the same, throughout the Universe. Observations are plentiful. See [this](https://physics.stackexchange.com/questions/413886/why-do-scientists-think-that-all-the-laws-of-physics-that-apply-in-our-galaxy-ap) physics.SE thread for a discussion..  

---

