Neighborhoods for starting a life in the galaxy
quantamagazine.org
quantamagazine.org
[1]: https://en.wikipedia.org/wiki/Galactic_habitable_zone#Histor...
It is likely that there may be other forms of life that use quite different organic molecules than the amino-acids and nucleic acids used on Earth, but it is very unlikely that they use substances made from other chemical elements than those used on Earth.
Not only the chemical elements used on Earth are precisely those which are the most abundant in the Universe, but they also have properties for which there are no alternative chemical elements. So it is likely that the bulk of any form of life is made of hydrogen, carbon, nitrogen, oxygen and sulfur, the 5 most abundant electronegative elements (which are also the constituents of the proteins; the least abundant of these 5, sulfur, is more abundant in the Universe than all the elements heavier than iron summed together). Electronegativity is necessary for the covalent bonds that bind atoms into molecules, while the substances based on ionic or metallic bonds cannot have alone a complex enough structure for evolving into a life form (because they form large homogeneous crystals instead of separate molecules with arbitrary structure).
A few days ago there was a posting on HN about a research paper that compared the alternatives to water as a biological solvent.
While no alternative to water is so ubiquitous in the Universe, the conclusion was that there might be some less usual conditions on some planets without water where an alternative to water could be used, the most likely alternatives being sulfuric acid or carbon dioxide at high pressures.
About elements heavier than calcium, a few of those are necessary for any kind of life to provide catalysts for the chemical reactions that can build complex molecules from the simple substances available in the environment.
For the appearance of life on Earth, at least iron, cobalt and nickel have been necessary from the beginning. These 3 metals could be sufficient in an environment where elemental hydrogen, carbon monoxide and ammonia would exist.
In most environments there is much less carbon monoxide than carbon dioxide and much less ammonia than elemental nitrogen. To enable the use of carbon dioxide and of elemental nitrogen, on Earth either molybdenum or tungsten has been necessary in the beginning, though perhaps in other conditions life could have evolved based on some catalyst using lighter elements, e.g. vanadium or even iron, which could have allowed the appearance of life on a planet with much less matter provided by supernova explosions (which generate most of the elements heavier than zinc) than in the Solar System.
Either claim (and many other impossibility claims like them, e.g. that an apple cannot fall upwards) could be invalidated by a single counter-example, but in none of these cases we know of any circumstance that would make the counter-example possible.
In science everything can be changed by a new discovery, but any such new discovery cannot contradict the known facts, whatever was known to be true within certain limits must remain true within those limits and can become false only outside them.
If any other chemical elements could be used to make structures complex enough to grow and multiply, that would have to happen in some conditions so different of everything that we have seen in the Solar System that we cannot imagine them.
For features of life on Earth like the use of nucleic acids it is valid to say that this is something specific to the only instance of life that we know an other forms of life could store information is some others from the millions of possible organic macro-molecules.
On the other hand, other features of life, like the use of the HCNOS elements, cannot be said to be an accidental feature of the only instance of life that we know, because their usage appears to be an unavoidable consequence of the composition of the Universe and of the chemical properties of the elements.
Some ideas that existed in various early SciFi works, like silicon-based life or other such variants are in complete contradiction with the chemistry known for now.
One could suppose that somewhere, perhaps in the depth of a giant planet, there may be ambient conditions, like very low temperatures, very high pressures and a chemical composition that could make some molecules to be stable even if they cannot be stable in the experimental conditions that can be reproduced on Earth and where some chemical reactions that cannot happen on Earth could happen there. This is not completely impossible, because for now the simulations of chemical reactions are not precise enough to predict what happens in conditions very different from those that can be verified through experiments. Nevertheless, for now there are not much more grounds to believe in things outside the known chemistry than to believe in teleportation or warp drive.
Supposing that there would be somewhere such living beings based on an unknown chemistry, it is certain that they could not be extracted from their environment and brought to the surface of a planet like those currently known without being destroyed in the process.
All life on Earth needs Phosphorus to form DNA.