Rocky planets: A new target in the search for extraterrestrial life

Since its launch in 2021, NASA’s James Webb Space Telescope (JWST) has opened new possibilities for detecting signs of life on exoplanets, planets beyond our solar system. 

A prime focus in this search is rocky planets orbiting low-mass stars called M-dwarfs, the most common type of stars in the universe.

Among the nearby candidates is TRAPPIST-1, an M-dwarf located about 40 light years away, hosting a system of planets that has become a focal point in the search for extraterrestrial life.

Challenges to planet habitability

Past research raised concerns about the habitability of planets around TRAPPIST-1, suggesting that intense ultraviolet (UV) radiation could strip these planets of surface water. 

This desiccation would leave dry surfaces, and if the hydrogen from water vapor escaped into space while oxygen remained, it could result in an atmosphere rich in reactive oxygen, potentially hindering the chemistry needed for life to develop.

Rocky planets with stable atmospheres 

However, a new study published in Nature Communications suggests that certain rocky planets around M-dwarfs might have atmospheres that remain stable over time.

Study lead author Joshua Krissansen-Totton is an assistant professor of Earth and space sciences at the University of Washington (UW).

“One of the most intriguing questions right now in exoplanet astronomy is: Can rocky planets orbiting M-dwarf stars maintain atmospheres that could support life?”

“Our findings give reason to expect that some of these planets do have atmospheres, which significantly enhances the chances that these common planetary systems could support life,” said Krissansen-Totton.

Rocky planets in the habitable zone 

So far, the JWST has observed some of the hotter rocky planets close to TRAPPIST-1 and found that these do not have significant atmospheres. 

However, the telescope has not yet been able to fully study planets in the “Goldilocks zone” – regions where conditions might allow for liquid water and thus increase the potential for life.

In this zone, temperatures are just right, not too hot or too cold, making it a prime area for further exploration.

Modeling the formation of rocky planets

The new study modeled the evolution of a rocky planet from its molten beginnings to its cooling over hundreds of millions of years. 

The results indicated that while light gasses like hydrogen initially escaped into space, those further from their star retained some hydrogen through reactions with oxygen and iron in their interiors. 

This process led to the formation of water and heavier gasses, creating a stable atmosphere over time.

Searching for temperate exoplanets

The researchers also found that on planets in the Goldilocks zone, water tends to condense and rain out of the atmosphere, making it less likely to escape. This increases the possibility of maintaining a stable climate and liquid water on the surface. 

“It’s easier for the JWST to observe hotter planets closest to the star because they emit more thermal radiation, which isn’t as affected by the interference from the star. For those planets we have a fairly unambiguous answer: They don’t have a thick atmosphere,” said Krissansen-Totton.

“For me, this result is interesting because it suggests that the more temperate planets may have atmospheres and ought to be carefully scrutinized with telescopes, especially given their habitability potential.”

Future prospects in the search for life

While the JWST has not yet confirmed the presence of atmospheres on planets slightly farther from TRAPPIST-1, such a discovery would imply the potential for surface liquid water and a climate favorable for life. 

“With the telescopes that we have now, the James Webb and the extremely large ground-based telescopes coming soon, we’re really only going to be able to look at a very small number of habitable zone rocky planets’ atmospheres – it’s the TRAPPIST-1 planets and a couple of others,” Krissansen-Totton said.

He emphasizes that the study supports the value of using current technology to continue exploring the habitability of these planets rather than waiting for future, more advanced telescopes. 

“Given the huge interest in the search for life elsewhere, our result suggests that it’s worthwhile investing telescope time to continue studying the habitability of these systems with the technology we have now.”

The findings provide a more optimistic outlook for the possibility of life on exoplanets orbiting M-dwarfs, suggesting that some planets in these systems could harbor conditions suitable for life, making them compelling targets for ongoing and future astronomical research.

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