Exoplanets, or planets outside our solar system, are common in our galaxy.
Some of these planets orbit in the “habitable zone” of their star, a region where the temperature might allow liquid water to exist.
NASA’s James Webb Space Telescope (Webb) is observing several of these potentially habitable planets, and astronomers are now working hard to analyze the data.
Drs. Knicole Colón and Christopher Stark, two Webb project scientists at NASA’s Goddard Space Flight Center, explain the challenges of studying these distant worlds:
“A potentially habitable planet is often defined as a planet similar in size to Earth that orbits in the ‘habitable zone’ of its star.
This is a place where the planet could have temperatures that allow liquid water on its surface.
We currently know of about 30 planets that might be small, rocky worlds like Earth and that orbit in the habitable zone.
However, just because a planet is in the habitable zone doesn’t mean it can support life or that it currently has life. So far, Earth is the only known habitable and inhabited planet.
“The potentially habitable worlds Webb is observing are all transiting exoplanets. This means their orbits are aligned in such a way that they pass in front of their host stars from our point of view.
When this happens, Webb can perform transmission spectroscopy, examining the starlight that filters through the planets’ atmospheres to learn about their chemical compositions.
“However, detecting an atmosphere around these small rocky planets is very challenging. The amount of starlight blocked by the thin atmosphere of a small rocky planet is tiny, often less than 0.02%.
Identifying the presence of water vapor, which could indicate the possibility of habitability, is even harder. Searching for biosignatures, or signs of life like biologically produced gases, is extraordinarily difficult but also very exciting.
“When an exoplanet passes directly between its host star and us, we say that the planet is transiting.
This transit causes a measurable dip in the star’s light, and if the planet has an atmosphere, the starlight passes through it.
This helps us learn about the planet’s atmosphere. Webb’s observations of small, potentially habitable worlds are limited to just a few that are accessible for atmospheric characterization, such as LHS 1140 b and TRAPPIST-1 e.
“Recent theoretical work on LHS 1140 b, a super-Earth-size planet, highlights the challenges of detecting gaseous molecules in its atmosphere.
The study suggests it would take 10–50 transits of the planet around its star (equivalent to 40–200 hours of observing time with Webb) to attempt to detect potential biosignatures like ammonia, phosphine, chloromethane, and nitrous oxide. Given Webb’s limited viewing time for the LHS 1140 system, it could take multiple years, or nearly a decade, to collect enough data.
Additionally, if the planet’s atmosphere is cloudy, even more observations may be needed. Clouds or hazes in exoplanet atmospheres can obscure the signals we are trying to detect.
“A potential way to search for biosignatures is to study Hycean planets. These are theoretical super-Earth-size planets with a thin hydrogen-rich atmosphere and a substantial liquid water ocean. K2-18 b is a candidate for a potentially habitable Hycean planet based on data from Webb and other observatories.
“Recent studies using Webb’s NIRSpec and NIRISS instruments detected methane and carbon dioxide in K2-18 b’s atmosphere but not water. This means the idea that K2-18 b is a Hycean world with a liquid water ocean is still based on theoretical models, without direct observational evidence.
There is also a hint of a potential biosignature, dimethyl sulfide, in K2-18 b’s atmosphere, but the signal is too weak for a conclusive detection with current data.
“In conclusion, while the search for habitable and inhabited worlds is challenging, Webb’s observations are a significant step forward. By studying the atmospheres of these distant planets, we can learn more about their potential to support life and expand our understanding of the universe.”
