Why do aliens have to be carbon based lifeforms?
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.
It's more a case that Earth-based life is the only example we have; we can make more educated guesses on what criteria a planet ought to have. Until we encounter sulphur-based life, we won't better know what other giveaways their planet would have.
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.
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.
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.
Because carbon and oxygen (and nitrogen) are the most common 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.
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 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, 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" 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.
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,
- Astronomers have also found things like i-propyl cyanide in meteorites and in interstellar dust $-$ an organic molecule which is a necessary ingredient in amino acids,
- Organic molecules have been found (and believed to have formed) on Ceres.
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.
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).
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.
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.