Life as we know it on Earth is based on carbon, a versatile element that can form complex molecules with various other elements. Carbon is also abundant in the universe and is compatible with water, the solvent that enables many biochemical reactions.
However, carbon is not the only element that could support life. Scientists have speculated about alternative biochemistries that could exist in different environments, using different elements and solvents.
One of the most popular candidates for an alternate element is silicon, which belongs to the same group as carbon on the periodic table and has four valence electrons. Silicon can form long chains and rings like carbon, but it is less stable and more reactive.
Silicon also tends to form strong bonds with oxygen, resulting in silicates and silica, which are rigid, crystalline structures that are not conducive to life.
However, some scientists have suggested that silicon-based life could exist in environments where oxygen is scarce, such as on planets with reducing atmospheres or under the surface of gas giants.
Another possibility is that life could use elements other than carbon or silicon as the main backbone of its molecules, such as phosphorus, sulfur, nitrogen, or boron.
These elements have different binding properties and chemical behaviors than carbon, and could create new types of organic compounds.
For example, phosphorus-based life might use phosphates instead of sugars as its main source of energy, sulfur-based life might use sulfides instead of water as a solvent, nitrogen-based life might use azides instead of amino acids like its building blocks, and boron-based life could use boranes instead of hydrocarbons as its building blocks.
In addition to the choice of element, another factor that could affect the possibility of alternative life forms is the choice of solvent. Water is a polar molecule that can dissolve many substances and facilitate chemical reactions.
It also has a wide range of temperatures and pressures where it remains a liquid, which is important for maintaining a stable environment for life. However, water is not the only possible solvent for life.
Some scientists have proposed that ammonia, which is also a polar and cosmically abundant molecule, could act as a solvent for life in the coldest regions of the universe, such as icy moons or Kuiper belt objects.
Ammonia has a lower freezing point and a higher boiling point than water, meaning it can remain a liquid at lower temperatures and pressures.
Other potential solvents include nonpolar hydrocarbons such as methane and ethane, which are known to exist in liquid form on the surface of Saturn’s largest moon Titan.
Hydrocarbon solvents could support life that uses lipids instead of proteins as the main macromolecules, since lipids are insoluble in water but soluble in hydrocarbons. Hydrocarbon solvents could also allow for more complex organic chemistry than water, since water tends to hydrolyze many organic compounds.
We can’t even imagine what other life forms would be like.
What non-carbon life forms would look like in the universe
Based on these challenges and possibilities, we can imagine some of the possible shapes and appearances of non-carbon life forms in the universe. Silicon-based life forms can look like crystals or rocks, with sharp edges and facets.
They can live in volcanic or geothermal regions, where they can access high temperatures and acidic solvents. Boron-based life forms can resemble bubbles or spheres, with smooth surfaces and flexible membranes.
They can live in icy or gaseous regions, where they can access low temperatures and alkaline solvents. Nitrogen-based life forms can appear to be sparks or flashes, with transient and explosive behavior.
They can live in electric or magnetic fields, where they can access high energies and reactive solvents. Phosphorus-based life forms can resemble threads or webs, with complex and dynamic patterns. They can live in organic or mineral regions, where they can access moderate temperatures and neutral solvents.
Of course, these are just a few of the possible scenarios for non-carbon life forms in the universe. There may be other elements or combinations that can support life in ways we have not yet imagined.
The search for alternative life forms in the universe is not only a scientific curiosity, but also a philosophical challenge. It forces us to rethink our definition of life and our assumptions about its origin and evolution. It also expands our imagination and our sense of wonder about the diversity and complexity of nature.
Alternate life forms may be very different from us in appearance and behavior, but they may also share some common characteristics and principles with us, such as self-organization, adaptation, reproduction, and communication.
By exploring the possibility of alternative life forms, we can also learn more about ourselves and our place in the cosmos.