*The article was written by Morgan Underwood, PhD candidate in Earth, Environmental and Planetary Sciences, Rice University, and published on The Conversation Brasil.
When astronomers look for planets that might have liquid water on their surfaces, they start by looking at the star’s habitable zone. Water is an essential element for life, and if a planet is too close to its star, the water on its surface can “boil”; Or if it’s too far away, it might freeze. This habitable zone represents the central region.
But being in that sweet spot doesn’t automatically mean the planet is suitable for life. Other factors also play an important role, such as whether the planet is geologically active or has processes that regulate gases in its atmosphere.
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The habitable zone provides a useful guide to searching for signs of life on exoplanets, which are planets outside our solar system orbiting other stars. But what’s in the atmospheres of these planets holds the next clue about whether liquid water — and perhaps life — exists beyond Earth.
On Earth, 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. Without an atmosphere, Earth’s average surface temperature would be about -18°C, well below the freezing point of water.
The boundaries of the habitable zone are determined by the amount of “greenhouse effect” needed to maintain surface temperatures that allow liquid water to persist. It is a balance between sunlight and warming the atmosphere.
Many planetary scientists, myself included, seek to understand whether the processes responsible for regulating Earth’s climate operate on other worlds located in the habitable zone. We use what we know about the Earth’s geology and climate to predict how these processes might play out elsewhere, and that’s where my background in geosciences comes in.
Image of the habitable zone of a similar star system, with planets similar to Venus and Mars outside the “right temperature” zone.
Why the habitable zone?
The idea of a habitable zone is simple and powerful, and for good reason. It provides a starting point, directing astronomers to where they can expect to find planets with liquid water, without needing to know every detail about a planet’s atmosphere or history.
Its definition depends in part on what scientists know about Earth’s rocky neighbors. Mars, which is located outside the outer limit of the habitable zone, shows clear evidence of the existence of ancient rivers and lakes in which liquid water once flowed.
Likewise, Venus is currently too close to the Sun to fall within the habitable zone. However, some geochemical evidence and modeling studies suggest that Venus may have had water in the past, although how much and for how long is not known with certainty.
These examples show that although the habitable area is not a perfect indicator of habitability, it does provide a useful starting point.
Operations may indicate habitability
What the habitable zone does not do is determine whether a planet can maintain habitable conditions for long periods of time. On Earth, a stable climate allowed life to emerge and persist. Liquid water was able to remain on the surface, giving slow chemical reactions enough time to build the molecules of life and allowing the first ecosystems to develop the ability to adapt to change, enhancing habitability.
Life emerged on Earth, but it continued to reshape the environments in which it evolved, making them more suitable for life.
This stability likely developed over hundreds of millions of years, as the planet’s surface, oceans and atmosphere worked together as part of a slow but powerful system to regulate Earth’s temperature.
A key part of this system is the way Earth recycles inorganic carbon between the atmosphere, surface and oceans over millions of years. Inorganic carbon refers to carbon associated with atmospheric gases, dissolved in seawater or trapped in minerals, not biological materials.
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 weather pull carbon from the air and store it in rocks and oceans.
If the planet cools, this process slows, allowing carbon dioxide, a greenhouse gas, to build up back into the atmosphere. This part of the carbon cycle helped Earth recover from past ice ages and avoid runaway warming.
Even as the Sun gradually increased in brightness, this cycle helped keep temperatures on Earth within a range where liquid water and life could be sustained for long periods.
Now scientists are wondering whether similar geological processes might be occurring on other planets, and if so, how they might detect them.
For example, if researchers could observe enough rocky planets in the habitable zones of their stars, they could look for a pattern linking the amount of starlight a planet receives with the amount of carbon dioxide in its atmosphere. Finding this pattern may indicate that the same type of carbon cycling process could be occurring elsewhere.
The mixture of gases in a planet’s atmosphere is shaped by what happens on or beneath its surface. A study has shown that measuring atmospheric carbon dioxide on several rocky planets can reveal whether their surfaces are divided into several moving plates, like Earth’s, or whether their crusts are more solid. On Earth, these moving plates stimulate volcanic activity and rock weathering, which are essential to the carbon cycle.
A simulation of what space telescopes like the Habitable Worlds Observatory would capture when observing distant star systems
Remote skies monitoring
The next step will be to get a population-level perspective of planets in the habitable zones of their stars. By analyzing atmospheric data from many rocky planets, researchers can look for trends that reveal the influence of fundamental planetary processes such as the carbon cycle.
Scientists can then compare these patterns to the planet’s position in the habitable zone. This would allow them to test whether the region accurately predicts places where habitable conditions are possible or whether some planets maintain conditions suitable for liquid water outside the region’s boundaries.
This type of approach is especially important given the diversity of exoplanets. Many exoplanets fall into categories not found in our solar system, such as super-Earths and minor Neptunes. Others orbit stars that are smaller and cooler than the Sun.
The datasets needed to explore and understand this diversity are about to emerge. NASA’s Habitable Worlds Observatory will be the first space telescope designed specifically to look for signs of habitability and life on planets orbiting other stars. It will directly image Earth-sized planets around 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. When starlight is filtered through a planet’s atmosphere, different molecules absorb specific wavelengths of light, leaving behind a chemical fingerprint that reveals the gases present. These compounds provide insight into the processes that shape these worlds.
The Habitable Worlds Observatory is undergoing active scientific and engineering development, and is scheduled to launch in the 2040s. Combined with today’s telescopes, which are increasingly capable of observing the atmospheres of Earth-sized worlds, scientists may soon be able to determine whether the same planetary processes that regulate Earth’s climate are common throughout the galaxy or unique to our planet.
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