What is life?

This sounds like a stupid question, but it's a necessary one if we humans are ever going to contact aliens. First we have to define what life is in order to understand what we might see on extraterrestrial planets. This is important because we may be able to locate life on other planets by obtaining spectral scans of planetary atmospheres. This will soon be happening.

The technical definition is that life is something that moves and can reproduce. Another definition is that it processes and converts energy using a biological process called metabolism. The latter definition is defined by another term that needs to be defined. A biological entity is cell or creature that contains DNA. However, that's only valid on Earth, not out in the universe.

Other definitions are that living things grow, they respond to stimuli, they can adapt, they use metabolism, they're organized into cells, they're able to regulate temperature, and they're able to reproduce.

Could we find life on other worlds armed with these definitions, or would we have to adapt these definitions? That's a good question because most of the ideas for searching for alien life involve things that we are familiar with like bacteria. The reason why this is the case is because of the theory that life as we know it should be ubiquitous in the universe. In other words, if it happened here it probably happened everywhere. Most of the missions to find life are based on Earth-like biology.
But, what if alien life is based on a different chemistry or even so alien we can't recognize it? That's where we get into the fascinating idea of hypothetical biochemistry.

Let's say for the sake of argument that amino acids could be different elsewhere in the universe. Our amino acids are L-type. This stands for the way the amino acid molecule is oriented in 3 dimensions. L stands for left form, meaning that the molecule is oriented so that another molecule with the same composition would not be superimposable. It's as if you were looking at it in a mirror.

There is a D-form of amino acids. This has the opposite chirality (orientation) from the L-form.

This idea also applies to sugars. Our biological sugars are D-form. However, L-form sugars could exist. Thus, alien life could have this different stereochemistry, but it's doubtful if that would make a big difference. Life based on these opposite configurations would still function much like ours does.

The other major possibility for alien life is biochemistry based on other elements than carbon, which is what ours is. The two possibilities are silicon and germanium. I believe that germanium is too unreactive to be useful for life.

Silicon, on the other hand, is the best choice, but it has some drawbacks. Silicon, like carbon, can form large molecular structures but it's not as good at forming bonds with a diverse group of atoms. In other words silicon's chemistry is limited. The other problem is that not much silicon-based compounds are found out in space. Another problem is that silanes, which are the equivalent of hydrocarbons, are very reactive with water. Silicon atoms are much larger than carbon atoms and have difficulty forming double bonds, which is a type of boding in the base units of DNA. The only good thing about silicon is that it's much more abundant on Earth. Perhaps silicon would be more effective on planets that are unlike Earth. It's possible that silicon based life would flourish on very cold planets because polysilanols (like sugars) are soluble in liquid nitrogen. Also, some diatoms here on Earth have silicon-based skeletons. Silicon based life is possible, but no one has discovered how this would actually work.

Everyone knows that life as we know it requires liquid water. There could be life formed in liquid ammonia, which dissolves organic molecules and even metals. However, liquid ammonia is a sissy compared to water. Its heat of vaporization is only half of water's. A planet with life that exists in liquid ammonia would be very strange. Ammonia boils at -33 C and freezes at -78 C. Chemical reactions would be slow at these temperatures and so would any life forms that develop in it.
Another possibility of a solvent for life is liquid methane, which exists on Saturn's moon Titan. Some biochemists have suggested that life could exist in liquid methane, but it would also be quite alien. They have proposed that methaneogenic life would have phospholipid cell membranes. That would be wild if it's true.

Other possible solvents for life are Hydrogen fluoride and hydrogen sulfide, both of which exist or could exist on extraterrestrial planets. The only problem with these is that they are rare out in space.

One area where alien life would differ is in plants. Some physicists suggest that there could be non-green photosynthesis. They suggest that red or yellow plants could thrive on alien planets.

The bottom line is that we may discover life that is quite different than what we have on Earth. There are many possibilities that we should be aware of. It only proves that we are novices at this new science of astrobiology.

Thanks for reading.