Beyond The Habitable Zone Exoplanet Atmospheres Are The Next Clue To Finding Life On Planets Orbiting Distant Stars

This article was originally published atThe Conversation.The publication contributed the article to Space.com’sExpert Voices: Op-Ed & Insights.

When astronomers search for planets that could host liquid water on their surface, they start by looking at a star’shabitable zone. Water is akey ingredient for lifeand on a planet too close to its star, water on its surface may “boil”; too far, and it could freeze. This zone marks the region in between.

But being in thissweet spotdoesn’t automatically mean a planet is hospitable to life. Other factors, like whether a planet is geologically active or has processes that regulate gases in its atmosphere, play a role.

The habitable zone provides a useful guide to search for signs of life onexoplanets– planets outside our solar system orbiting other stars. But what’s in these planets’ atmospheres holds the next clue about whether liquid water — and possibly life — exists beyond Earth.

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On Earth, thegreenhouse effectcaused by gases like carbon dioxide and water vapor, keeps the planet warm enough for liquid water and life as we know it. Without an atmosphere, Earth’s surface temperature wouldaverage around zero degrees Fahrenheit(minus 18 degrees Celsius), far below the freezing point of water.

The boundaries of the habitable zone are defined by how much of a “greenhouse effect” is necessary to maintain the surface temperatures that allow for liquid water to persist. It’s a balance between sunlight and atmospheric warming.

Many planetary scientists,including meare seeking to understand if the processes responsible for regulating Earth’s climate are operating on other habitable zone worlds. We use what we know about Earth’s geology and climate to predict how these processes might appear elsewhere, which is where my geoscience expertise comes in.

Why the habitable zone?

The habitable zone is a simple and powerful idea, and for good reason. It provides a starting point, directing astronomers to where they might expect to find planets with liquid water, without needing to know every detail about the planet’s atmosphere or history.

Its definition is partially informed by what scientists know about Earth’s rocky neighbors.Marswhich lies just outside the outer edge of the habitable zone, showsclear evidence of ancient rivers and lakeswhere liquid water once flowed.

Similarly,Venusis currently too close to the sun to be within the habitable zone. Yet, somegeochemical evidenceandmodeling studiessuggest Venus may have had water in its past, though how much and for how long remains uncertain.

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These examples show that while the habitable zone is not a perfect predictor of habitability, it provides a useful starting point.

Planetary processes can inform habitability

What the habitable zone doesn’t do is determine whether a planet can sustain habitable conditions over long periods of time. On Earth, astable climate allowed life to emerge and persist. Liquid water could remain on the surface,giving slow chemical reactions enough timeto build the molecules of life andlet early ecosystems develop resilienceto change, which reinforced habitability.

Life emerged on Earth, butcontinued to reshape the environments it evolved inmaking them conducive to life.

This stability likely unfolded over hundreds of millions of years, as the planets surface, oceans and atmosphere worked together as part ofa slow but powerful systemto regulate Earth’s temperature.

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A key part of this system is howEarth recycles inorganic carbonbetween the atmosphere, surface and oceans over the course of millions of years.Inorganic carbonrefers to carbon bound in atmospheric gases, dissolved in seawater or locked in minerals, rather than biological material. This part of the carbon cycleacts like 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 weathering draw carbon out of the air and store it in rocks and oceans.

If the planet cools, this process slows down, allowing carbon dioxide, a warminggreenhouse gasto build up in the atmosphere again. This part of the carbon cycle has helped Earth recover from past ice ages and avoid runaway warming.

Even as the sun has gradually brightened, this cycle has contributed to keeping temperatures on Earth within a range where liquid water and life can persist for long spans of time.

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Now, scientists are asking whether similar geological processes might operate on other planets, and if so, how they might detect them. For example, if researchers could observe enough rocky planets in their stars’ habitable zones, they couldlook for a patternconnecting the amount of sunlight a planet receives and how much carbon dioxide is in its atmosphere. Finding such a pattern may hint that the same kind of carbon-cycling process could be operating elsewhere.

The mix of gases in a planet’s atmosphere is shaped by whats happening on or below its surface.One studyshows that measuring atmospheric carbon dioxide in a number of rocky planets could reveal whether their surfaces are broken into a number of moving plates, like Earth’s, or if their crusts are rigid. On Earth, theseshifting platesdrive volcanism and rock weathering, which are key to carbon cycling.

Keeping an eye on distant atmospheres

The next step will betoward gaining a population-level perspectiveof planets in their stars’ habitable zones. By analyzing atmospheric data from many rocky planets, researchers can look for trends that reveal the influence of underlying planetary processes, such as the carbon cycle.

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Scientists could then compare these patterns with a planet’s position in the habitable zone. Doing so would allow them to test whether the zone accurately predicts where habitable conditions are possible, or whether some planets maintain conditions suitable for liquid water beyond the zone’s edges.

This kind of approach is especially important giventhe diversity of exoplanets. Many exoplanets fall intocategories that don’t exist in our solar system— such assuper Earthsandmini Neptunes. Othersorbit stars smaller and cooler than the sun.

The datasets needed to explore and understand this diversity are just on the horizon. NASA’s upcomingHabitable Worlds Observatorywill be the first space telescope designed specifically to search 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.

Instruments on the observatory will analyze starlight passing throughthese atmospheresto detect gases like carbon dioxide, methane, water vapor and oxygen. As starlight filters through a planet’s atmosphere, different molecules absorb specific wavelengths of light,leaving behind a chemical fingerprintthat reveals which gases are present. These compounds offer insight into the processes shaping these worlds.

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The Habitable Worlds Observatory is under active scientific and engineering development, with a potentiallaunch targeted for the 2040s. Combined with today’s telescopes, which are increasingly capable of observing 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 uniquely our own.