The March issue of National Geographic magazine offers an article called “Life probably exists beyond Earth. So how do we find it?” It’s an excellent overview of current research into the subject, especially the technology scientists are using to hunt for extraterrestrial life. As the article points out, it’s estimated that there are at least a billion stars in our galaxy and possibly trillions of galaxies in the universe. Thanks to the Kepler space telescope, we’ve learned that most stars have multiple planets (Kepler’s findings have helped confirm the existence of roughly four thousand planets beyond our solar system, and it was only able to survey a small slice of the sky). Mainly from information gathered as planets pass in front of their sun—speed of the transit, how much the star’s light dims and, in some cases, spectroscopic analysis of the light—researchers can make good guesses about how close the planet orbits its star, how large the planet is, and whether it’s a rocky world like Earth or a gas giant like Jupiter. A planet is considered a good candidate to host life if it’s a rocky world of a certain size (similar to Earth) and orbits in the so-called “habitable zone”—the right distance from its sun to allow liquid water. About a quarter of Kepler’s exoplanets meet these qualifications, which could mean there are twenty-five billion habitable planets in our Milky Way galaxy alone.
Yet those criteria are rather “Earth chauvinist” as Carl Sagan might have said. They’re the requirements for life forms that we would recognize: life based on the element carbon, using water as a solvent. There are other possibilities.
Carbon is an excellent basis for life-serving (we call them organic) compounds because it bonds to other carbon atoms and many other elements well to produce complex and versatile molecules, and is very welcoming to oxygen which Earth life uses in producing energy. The result of their pairing in combustion is CO2, carbon dioxide gas, which is easily disposed of, for instance in our exhaled breath. Carbon is also very common in the universe. But scientists have speculated for a long time that similar elements like silicon could also form the basis for life (although this Scientific American article makes a good case against silicon). Another possibility is metals like iron, magnesium, or aluminum, which are more common than carbon even on Earth, though not as adaptable.
Although we call water (H2O) the “universal solvent”, it’s not the only solvent that could be used by a life form. Scientists have proposed methane and similar hydrocarbons as a possibility, especially since lakes of methane were discovered on Saturn’s moon Titan. Ammonia is another suggestion, as are other hydrogen compounds like hydrogen sulphide, hydrogen chloride, and hydrogen fluoride.
There are arguments against all of these when compared to the biological and biochemical processes we’re familiar with, but who knows what Nature might have dreamed up? What makes these speculations even more interesting is that these different elements would provide an environment friendly to life at very different temperature ranges than are suitable for carbon-based life. So if we accept that there could be creatures made of silicon, or boron, or iron compounds, using methane or ammonia in whatever passes for blood, then the potential “habitable zone” of stars increases a lot. Let’s also not forget that gas giant planets might have habitable moons. So that means many more than 25 billion places in our galaxy that could support some form of life. That’s exciting!
I haven’t even touched on the fact that there are other chemicals capable of facilitating photosynthesis, rather than just the chlorophyll that Earth plants use. And although almost all Earth life uses DNA for the “master blueprint” that determines structure and function, and DNA uses only four chemicals (guanine, cytosine, adenine and thymine) as the “letters” that encode genetic information, scientists have now created viable synthetic DNA that uses four different “letters”. This opens the door for even more possible forms of life.
For science fiction writers, all of this is both a blessing and a curse. It’s fun to imagine the different forms that aliens from other planets could take. It’s mostly pleasurable to work out the implications of these imagined features (would a four-legged being drive something like a car? What would a methane-based alien drink to have a good time at a party?) But it’s a major headache to get the chemistry and physiognomy right. Most of us aren’t Biochemistry PhDs or xeno-anatomy experts.
What all of this says to me is that, when we finally do get “out there” discovering new forms of life in all their variety, the universe will be an even stranger place than we can imagine.