written by: jessica wang and sorina andrei
graphics by: yunyi cui
In our search for life-supporting exoplanets, the Earth is the best (and only) model available that helps us understand the development of life. Researchers tend to look for Earth-like biosignatures, such as oxygen, methane, and carbon dioxide, when observing potentially habitable planets (1). Liquid water in particular has been the focus for many researchers as it is essential for sustaining life on our home planet. Life on Earth, afterall, is hypothesized to have originated from the ocean (2). Water has special properties, resulting from its molecular structure, that granted it its nickname, the “universal solvent” (3).
A water molecule is composed of one negatively charged oxygen atom bonded with two positively charged hydrogen atoms (3). The asymmetry of this structure produces an imbalance in charge (giving water both a positive side and a negative side), making water polar. The polarity of water allows it to form strong bonds with other polar molecules as the positive side of each molecule will be attracted to the negative side of the other. This is also the case for molecules composed of ions, which are charged particles. Because most biological molecules are either polar or ionic, water is able to interact with many substances. Water can seep into the spaces between molecules, surrounding them, thus dissolving whatever larger substance it came into contact with. As the universal solvent, water transports a variety of substances (oxygen and nutrients, for example) within individual cells and through the body to the necessary locations (3).
Water molecules are also attracted to other water molecules, resulting in a property known as cohesion (3). By sticking together tightly, water is able to travel up plant roots (a necessary action for plants to survive). Water’s high boiling point can be attributed to cohesion, allowing water to remain in liquid form from 0°C to 100°C. Among other things, this high boiling point allows animals to regulate body temperature (3).
On a cellular level, water maintains the shape of cells by generating pressure, contributes to the creation of membranes, and influences the folding of proteins and DNA (3). All three are important building blocks for life.
With all this in mind, researchers compiled a list of 60 potential habitable exoplanets based on whether or not the planets are within the habitable zone relative to their stars (4). What makes researchers so optimistic about these planets is that they are (theoretically) able to preserve water in its liquid state. Unfortunately, we have yet to see if these planets actually contain liquid water.
Planet K2-18b is among one of the most promising exoplanets in this list as water vapour has been detected in its atmosphere (5,6). The data collected on the planet’s atmospheric pressure and temperature profile suggest that clouds are able to form and even produce liquid rain (6). Whether life can be supported on K2-18b is still unknown, however, researchers are eager to use this planet to obtain more information on the composition and climate of potentially habitable exoplanets (5).
Some of the most groundbreaking discoveries were found within our own solar system. In 2004, the Mars Exploration Rover, Opportunity, discovered hematite and jarosite on mars (7). These minerals are found in acidic water bodies on Earth, such as hot springs, which indicate that similar bodies of water exist (or used to exist) on Mars. What’s exciting about this finding is that the microorganisms that thrive in these environments on Earth might be found on Mars (7). While Mars is very cold, it’s still possible for life to exist on the planet. Extremophiles, as the name suggests, are organisms that are able to survive in extreme conditions, including low temperatures (8). Mars may contain groundwater beneath its surface, potentially housing extremophiles like the ones we have on Earth (9). This suggests that, while alien life may not be visible on the surface, they could be found in the ground.
Water is the first factor to consider in our search for life, and for good reason. Through these water-focused discoveries, researchers can gain a better understanding of what facilitates the development of life and what limits it. As we learn more about extraterrestrial ecosystems, the boundaries of life become more apparent, influencing the direction of future research and bringing us one step closer to finding a habitable exoplanet (8).
Glossary
Biosignature: a sign that provides evidence of past or present life.
Cohesion: the property that makes water molecules attracted to each other.
Exoplanet: a planet orbiting a star outside the solar system.
Extremophile: organisms that can survive in extreme conditions.
Habitable zone: the distance from a star at which liquid water could exist on a planet.
Polarity: a separation of electric charge of a molecule in which one portion of the molecule is positively charged and the other portion is negatively charged.
Comments