The field of exoplanet exploration is undergoing a transformation, shifting from a pursuit of discovering extraordinary new worlds to a quest for comprehending and characterizing these planets. Of particular interest to astrobiologists are planets that have the potential to sustain life.

A crucial factor in determining a planet’s habitability is the presence of liquid water, a fundamental requirement for life as we know it on Earth. Consequently, astrobiologists have directed their efforts towards locating Earth-like planets within the habitable zone, the region surrounding stars where liquid water could potentially exist. This focused search has yielded numerous potential candidates, with the promise of many more discoveries as our observatories continue to advance in capability.

Nevertheless, the vast number of identified exoplanets poses a challenge for astronomers, as the allocation of observation time is a valuable and limited resource. This raises the question of how to effectively identify the most promising exoplanets worthy of further in-depth investigation.

Oxygen signature

Cassandra Hall and her colleagues from the University of Georgia in Atlanta emphasize the importance of energy in sustaining life. They argue that oxygenic photosynthesis serves as the primary energy source for the majority of biomass on Earth. Based on this understanding, Hall and her team propose that the search for potentially habitable planets should be limited to those capable of supporting this type of photosynthesis. Through their calculations, they have determined that this new criteria significantly narrows down the potential zone around a star where such planets could exist. They have identified a few promising candidates that possess conditions suitable for Earth-like photosynthesis, suggesting that these locations should be the focus of the search for extraterrestrial life.

Photosynthesis is a process that converts starlight into chemical energy. On Earth, this reaction is powered by the sun and involves the conversion of carbon dioxide and water into glucose and oxygen. Plants utilize glucose as a source of energy for various life processes.

The ability to support photosynthesis on other planets depends not only on temperature but also on the intensity and spectra of available light. Additionally, the presence of water is crucial for photosynthesis to occur.

Hall and her team simulate conditions on other planets to determine their potential for supporting photosynthesis. They take into account factors such as the distance from the parent star, the spectra of light emitted by the star, and the presence of atmospheric gases that may obstruct the necessary light.

As a result, the photosynthetic habitable zone is much narrower compared to the conventional habitable zone. Only a limited number of planets meet the criteria for supporting photosynthesis.

This finding has significant implications for the range of planets discovered by astronomers thus far. Many exoplanets within the habitable zone are found orbiting red dwarf stars, primarily due to the ease of detection using current planet hunting techniques. These stars have lower masses, allowing planets to orbit closely with faster orbits, making them more observable. However, planets in close proximity to their parent stars are more likely to be tidally locked, always presenting the same face to their star.

The existence of liquid water on these bodies has sparked a heated discussion regarding their potential to support life. This is due to the fact that liquid water would only be present in a limited area along the terminator. However, Hall and his colleagues argue that the debate becomes irrelevant when considering the photosynthetic habitable zone, as it typically extends beyond the tidal lock zone.

Red dwarf problem

Most of the planets orbiting red dwarfs can be disregarded as potential subjects for further investigation, based on this criterion. Specifically, this eliminates the four planets located in the habitable zone around a nearby star known as Trappist 1. These planets are some of the closest to Earth and have received significant attention. However, their significance is likely to diminish due to their tidal locking.

Nonetheless, Hall and his colleagues have identified five planets that consistently fall within the photosynthetic habitable zone. These planets are Kepler-452 b, Kepler-1638 b, Kepler-1544 b, Kepler-62 e, and Kepler-62 f.

For instance, Kepler-452 b has a radius approximately 50 percent larger than Earth’s and orbits a star similar to our Sun every 385 days. Without any greenhouse effect, its surface would have an average temperature of -8 degrees Celsius. Therefore, even a slight greenhouse effect would enable the existence of liquid water.

This firmly places Kepler-452 b within the habitable zone and now, within the photosynthetic habitable zone as well.

Hall and his team believe that Kepler-452 b and its counterparts are the most promising targets for future research. They conclude: “We propose that the search for signs of life elsewhere in the Universe should earnestly begin on the candidate planets we have identified.”

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