Exoplanet science is shifting from finding any detectable exoplanets we can to searching for those in their stars' habitable zones. NASA's proposed Habitable World Observatory and other similar efforts are focused on these worlds. The problem is, habitable zones aren't static.
Habitable zones (HZ) aren't static because stars aren't static. As they age and evolve, their brightness and temperature change. Habitable planets on the edge of an HZ can find themselves uninhabitable. Conversely, uninhabitable planets could find themselves inside the HZ.
New research points out that to use our observation resources wisely so they have the most impact, we should search for planets that are in their stars' Continuous Habitable Zone (CHZ). The CHZ reflects what we know about life on Earth. Photosynthetic life took about two billion years to oxygenate Earth's atmosphere, so the researchers say we should focus on exoplanets in the 2 Gyr continuous habitable zone (CHZ2). The atmospheres of those planets are more likely to show signs of life.
The research is "Continuous Habitable Zone Metric for Prioritizing Habitable Worlds Observatory Targets," and it's been accepted into The Astrophysical Journal. The authors are Austin Ware and Patrick Young from the School of Earth and Space Exploration at Arizona State University. The research article is currently available at arxiv.org.
This work is all about prioritizing observation targets, and that starts with understanding how to identify CHZs. "One method of prioritization is to estimate the likelihood that an exoplanet has remained continuously within the HZ long enough for life to emerge and make a detectable impact on the atmosphere," the authors write.
This graphic shows how Earth had to remain in the habitable zone long enough for life to alter its atmosphere. If habitable exoplanets follow a similar pattern, we should prioritize our search toward terrestrial exoplanets that remain in a stable habitable zone for at least two billion years. Image Credit: By Sciencia58 - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=83905029
Our Sun is a good example of how a star's habitable zone can shift as a star evolves. When it was young, it was only about 70% as bright as it is now. The "faint young Sun paradox" says that despite its weakness, the Sun still had enough energy to maintain liquid water on Earth's surface. Earth's atmosphere was likely richer in greenhouse gases, which allowed our planet to trap more heat.
Back then, the HZ was closer to the Sun, and Venus may have been a much different world, with reasonable temperatures and liquid water.
The Sun has evolved since then, and now it's much brighter. Astrophysicists think the Sun gets about 1% brighter every 100 million years or so. So the HZ has gradually shifted outward. Now Venus suffers from the runaway greenhouse effect, and its surface is totally uninhabitable.
Over time, the Sun will continue to brighten, pushing the HZ outward. One day, Mars may be warm again.
In four billion years or so, the Sun will change dramatically. It will enter its red giant phase, expand, and become dramatically brighter. The inner planets will be enveloped, and the HZ will shift outward dramatically. Jupiter, Saturn, and their ocean moons may be in the HZ.
Our Sun isn't exceptional. It's a standard main-sequence star. Its life stages illustrate clearly how the HZ isn't static. According to the authors, it's a moving target. Not just because the star changes, but terrestrial exoplanets change over time, too, and our search for habitable planets should consider that.
"We utilize a Bayesian method to calculate the likelihood that a given orbital radius around a star is currently in the 2 Gyr continuous habitable zone (CHZ), the approximate time it took life on Earth to significantly oxygenate the atmosphere," the authors write.
This conceptual illustration shows a possible design for the Habitable Worlds Observatory. If it gets built, this research shows how it can be used most effectively to characterize complex exoplanets. Image Credit: NASA.
The Habitable Worlds Observatory is only a concept at this point, but scientists have already created a target list for the mission. It's called the Exoplanet Exploration Program Mission Star List (EMSL), and it has 164 stars on it. They are Sun-like stars with HZs that are accessible to a direct-imaging mission like the HWO. The team applied their Bayesian method to all these stars and developed a metric to see which ones have a CHZ2.
As expected, the metrics follow the trend related to stellar age and spectral type. The CHZ2 of older stars has lower metrics. In the later main sequence, their CHZ2 contracts because their luminosity changes more rapidly. "This biases the CHZ2 metric toward lower values at older ages," the researchers explain.
The researchers found that "... stars slightly younger and more massive than the Sun may be the most likely to have the widest zones of continuous habitability."
Early F-dwarfs are at the opposite end of this scale. Late F and G dwarfs are in the sweet spot and have long main-sequence lifetimes and large HZs. They dominate the highest CHZ2 metric values.
This figure shows the distribution of CHZ2 metric values separated by spectral type for the 164 EMSL stars. F and G-type stars dominate the higher metric value along the bottom of the graph. Image Credit: Ware and Young 2025. ApJ
However, it's not just about stars. Habitable zones are partnerships between stars and planets. Factors like geological activity and CO2 outgassing on terrestrial planets also change over time. "This results in planets becoming geologically dead within ~ 5 − 7 Gyr for those between 0.5 − 1 Earth masses, after which the planets rely solely on their host star's luminosity to maintain habitable surface temperatures," the authors explain.
The researchers think the average age for habitable planets is less than Earth's. Only super-Earths can maintain habitability for longer.
In their conclusion, the researchers remind us of what most of us already know. Finding terrestrial exoplanets orbiting stars like our Sun and then probing their atmospheres for biosignatures using direct imaging is an extremely difficult task. Despite our growing technological prowess, Nature doesn't give up its secrets easily. However, it's still our best bet for understanding how prevalent life might be in the Milky Way.
If we inform our efforts based on what we know about life on Earth and how long it took life to change the atmosphere, we might make this difficult task a little less challenging.
"Considering the potential for life to have made a detectable impact on the atmosphere presents a means to prioritize targets in the lead-up to future missions," the researchers write.