When astronomers look for planets that could harbor liquid water on their surface, they start by examining a star’s habitable zone. Water is a key element for life, and on a planet too close to its star, the water on its surface can boil; too far, you could freeze. This area delimits the intermediate region.
But being in this optimal zone does not automatically mean that a planet is habitable. Other factors, such as geological activity or the existence of processes that regulate gases in its atmosphere, also influence.
The habitable zone provides a useful guide for searching for signs of life on exoplanets: planets outside our solar system that orbit other stars. But the composition of these planets’ atmospheres offers the next clue about whether liquid water—and possibly life—exists beyond Earth.
On Earth, the greenhouse effect, caused by gases such as carbon dioxide and water vapor, keeps the planet warm enough for liquid water and life as we know it to exist. Without an atmosphere, the Earth’s surface temperature would be around zero degrees Fahrenheit (minus 18 degrees Celsius), well below the freezing point of water.
The limits of the habitable zone are defined by the magnitude of the greenhouse effect necessary to maintain surface temperatures that allow the persistence of liquid water. It is a balance between solar radiation and atmospheric warming.
Many planetary scientists, including myself, seek to understand whether the processes responsible for regulating Earth’s climate operate on other worlds in the habitable zone. We use our knowledge of the Earth’s geology and climate to predict how these processes might manifest elsewhere, and this is where my background in geosciences comes into play.
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Why the habitable zone?
The living zone is a simple and powerful concept, and with good reason. It provides a starting point, directing astronomers to where they might find planets with liquid water, without needing to know every detail about the planet’s atmosphere or history.
Its definition is partially based on the knowledge that scientists have about the Earth’s neighboring rocky bodies. Mars, located just outside the outer limit of the habitable zone, shows clear evidence of ancient rivers and lakes where liquid water once flowed.
Similarly, Venus is currently too close to the Sun to be within the habitable zone. However, some geochemical evidence and modeling studies suggest that Venus may have had water in the past, although the amount and duration remain uncertain.
These examples demonstrate that while the habitable zone does not perfectly predict habitability, it provides a useful starting point.
Planetary processes can provide information on habitability
What the habitable zone does not determine is whether a planet can maintain habitable conditions for long periods of time. On Earth, a stable climate allowed life to arise and persist. Liquid water was able to remain on the surface, giving slow chemical reactions enough time to form the molecules of life and allowing early ecosystems to develop resilience to change, reinforcing habitability.
Life emerged on Earth, but continued to transform the environments in which it evolved, making them more conducive to life.
This stability likely developed over hundreds of millions of years, as the planet’s surface, oceans, and atmosphere interacted as part of a slow but powerful system to regulate Earth’s temperature.
A fundamental part of this system is how the Earth recycles inorganic carbon between the atmosphere, surface and oceans over millions of years. Inorganic carbon refers to carbon present in atmospheric gases, dissolved in seawater, or trapped in minerals, rather than in biological matter. This part of the carbon cycle acts as a natural thermostat.
When volcanoes release carbon dioxide into the atmosphere, the carbon dioxide molecules trap heat and warm the planet. As temperatures rise, rain and erosion pull carbon out of the air and store it in rocks and oceans.
If the planet cools, this process slows down, allowing carbon dioxide, a greenhouse gas that contributes to global warming, to build up again in the atmosphere. This part of the carbon cycle helped the Earth recover from past ice ages and avoid runaway warming.
Even as the Sun has gradually increased in brightness, this cycle helped keep temperatures on Earth within a range where liquid water and life can persist for long periods of time.
Now, scientists are wondering whether similar geological processes could operate on other planets and, if so, how they could detect them. For example, if researchers could observe enough rocky planets in the habitable zones of their stars, they could look for a pattern relating the amount of sunlight a planet receives to the amount of carbon dioxide in its atmosphere. Finding such a pattern could indicate that the same type of carbon cycling process could be operating elsewhere.
The composition of gases in a planet’s atmosphere is determined by what happens on or beneath its surface. A study shows that measuring atmospheric carbon dioxide on several rocky planets could reveal whether their surfaces are fragmented into multiple moving tectonic plates, like Earth’s, or if their crusts are more rigid. On Earth, the movement of these plates drives volcanism and rock erosion, key processes for the carbon cycle.
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Monitoring distant atmospheres
The next step will be to obtain a population-level perspective of the planets in the habitable zones of their stars. By analyzing atmospheric data from numerous rocky planets, researchers can look for trends that reveal the influence of underlying planetary processes, such as the carbon cycle.
Scientists could then compare these patterns to a planet’s position in the habitable zone. This would allow them to check whether the zone accurately predicts where habitable conditions are possible, or whether some planets maintain conditions suitable for the existence of liquid water beyond the limits of the zone.
This approach is especially important given the diversity of exoplanets. Many exoplanets belong to categories that do not exist in our solar system, such as super-Earths and mini-Neptunes. Others orbit stars smaller and cooler than the Sun.
The data sets needed to explore and understand this diversity are just around the corner. NASA’s upcoming Habitable Worlds Observatory will be the first space telescope designed specifically to search for signs of habitability and life on planets orbiting other stars. The observatory will directly image Earth-sized planets orbiting Sun-like stars to study their atmospheres in detail.
The observatory’s instruments will analyze starlight passing through these atmospheres to detect gases such as carbon dioxide, methane, water vapor and oxygen. As starlight filters through a planet’s atmosphere, different molecules absorb specific wavelengths of light, leaving a chemical fingerprint that reveals which gases are present. These compounds offer insights into the processes that shape these worlds.
The Habitable Worlds Observatory is in the scientific and engineering development phase, with a possible launch planned for the 2040s. Combined with today’s telescopes, increasingly capable of observing the atmospheres of Earth-sized worlds, scientists will soon be able to determine whether the same planetary processes that regulate Earth’s climate are common throughout the galaxy or unique to our own.
*Morgan Underwood She holds a PhD in Earth, Environmental and Planetary Sciences from Rice University.
This article was originally published in The Conversation
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