Question

The Search for Life. Based on what you have learned about life on Earth, what are we searching for when we search for life elsewhere? For example, are we searching only for worlds with surface oceans and oxygen-rich atmospheres like Earth, or for something else? Write one to three paragraphs describing the types of worlds that we can consider as potential homes for life.

Short answer

We search for Earth-like and non-Earth-like conditions including subsurface oceans and alternative biochemistries.

Step-by-step solution

Identifying Earth-like Criteria

When searching for life, scientists first look for Earth-like conditions, such as planets with liquid surface water, suitable temperatures, an atmosphere with oxygen and nitrogen, and rocky surfaces. These factors are crucial because life as we know it depends heavily on these elements.

Expanding the Search Criteria

While Earth-like conditions are a starting point, scientists also consider a broader range of environments where life might exist. These include subsurface oceans, such as those thought to exist on moons like Europa or Enceladus, which may harbor life below their icy surfaces.

Considering Alternative Biochemistries

The search for life also includes considering environments that don't exactly match Earth's conditions. Astrobiologists explore possibilities like life based on silicon instead of carbon, thriving in methane or ammonia atmospheres, or surviving in extreme conditions such as high radiation or extreme pressures.

Key concepts

Earth-like conditions

When scientists embark on the quest to find life elsewhere in the universe, they often start by searching for environments similar to Earth. This approach is based on the assumption that life, as we know it, thrives under certain conditions. These include planets with liquid surface water, which is crucial because water is a vital solvent that facilitates biochemical reactions in living organisms. Additionally, suitable temperatures, often defined by the habitable zone, allow for stable liquid water and life-friendly conditions. An atmosphere rich in oxygen and nitrogen is also seen as a promising sign since it is essential for complex life forms that rely on aerobic respiration. Furthermore, a planet with a rocky surface could support the kinds of geological processes that contribute to a stable and nurturing environment for life.
The focus on these Earth-like conditions stems from our current understanding of how life developed on our planet, and how it sustains itself through an intricate balance of climate, geology, and available resources.

alternative biochemistries

Beyond the Earth-centered view of life, astrobiologists are openly considering the potential for life forms based on biochemistries unlike our own. One concept is the exploration of life based on silicon rather than carbon. Silicon shares some chemical properties with carbon, allowing for the possibility of forming complex molecules. Even though silicon-based life is theoretical, it broadens the scope of life's potential diversity. Additionally, some scientists ponder the idea of life thriving in methane or ammonia-based atmospheres instead of oxygen-rich environments. This is inspired by the known adaptability of extremophiles on Earth.
These alternative chemistry-based conceptions allow us to imagine organisms capable of surviving in extreme environments, such as high radiation zones, super-hot temperatures, or exceptionally high pressures, which stray far from the conditions traditionally thought necessary for life.

subsurface oceans

One thrilling aspect of the search for extraterrestrial life lies beneath the icy crusts of celestial bodies, prompting scientists to look at subsurface oceans as potential cradles of life. Moons such as Europa (around Jupiter) and Enceladus (around Saturn) have shown evidence of vast oceans under their icy shells. These zones are intriguing because, despite their cold and dark nature, they might offer geologically driven heat and nutrients from the core, akin to hydrothermal vents on Earth's ocean floor, where thriving ecosystems can be found despite the lack of sunlight.
Subsurface oceans represent a different kind of environment where life could potentially arise. They challenge our conventional ideas of habitability by offering conditions that are isolated from the atmosphere above yet provide chemical interactions necessary for life underwater. This notion stretches the imagination of scientists to consider life forms starkly different from terrestrial beings.