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 it is compatible with water, the solvent that enables many biochemical reactions.
However, carbon is not the only element that could potentially 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 alternative 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 and 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 bonding properties and chemical behaviors than carbon, and they could create novel types of organic compounds.
For example, phosphorus-based life could use phosphates instead of sugars as its main energy source, sulfur-based life could use sulfides instead of water as its solvent, nitrogen-based life could use azides instead of amino acids as its building blocks, and boron-based life could use boranes instead of hydrocarbons as its structural units.
Besides 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 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 molecule and cosmically abundant, could act as a solvent for life in colder regions of the universe, such as on icy moons or Kuiper belt objects.
Ammonia has a lower freezing point and a higher boiling point than water, which means it can remain liquid at lower temperatures and pressures.
Other potential solvents include non-polar hydrocarbons such as methane and ethane, which are known to exist in liquid form on the surface of Titan , Saturn’s largest moon.
Hydrocarbon solvents could support life that uses lipids instead of proteins as its 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.
What non-carbon life forms in the universe might look like
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 may look like crystals or rocks, with sharp edges and facets.
They may live in volcanic or geothermal regions, where they can access high temperatures and acidic solvents. Boron-based life forms may look like bubbles or spheres, with smooth surfaces and flexible membranes.
They may live in icy or gaseous regions, where they can access low temperatures and alkaline solvents. Nitrogen-based life forms may look like sparks or flashes, with transient and explosive behaviors.
They may live in electric or magnetic fields, where they can access high energies and reactive solvents. Phosphorus-based life forms may look like strands or networks, with complex and dynamic patterns. They may live in organic or mineral regions, where they can access moderate temperatures and neutral solvents.
Of course, these are only some 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 that 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.
Alternative life forms may be very different from us in appearance and behavior, but they may also share some common features and principles with us, such as self-organization, adaptation, reproduction, and communication.
By exploring the possibility of alternative life forms, we may also learn more about ourselves and our place in the cosmos.