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| This infographic compares three classes of stars in our galaxy: Sunlike G stars, less massive and cooler K dwarfs, and even fainter, cooler reddish M dwarfs. The habitable zone varies for each star type. In our solar system, the habitable zone starts just beyond Venus’s orbit and nearly reaches Mars. |
Exoplanets are common in our galaxy, with some even residing in the so-called habitable zone of their star. NASA’s James Webb Space Telescope has been diligently observing a few of these small, potentially habitable planets, and astronomers are now hard at work analyzing the data collected by Webb. To shed light on the challenges of studying these distant worlds, we invite Drs. Knicole Colón and Christopher Stark, two Webb project scientists at NASA’s Goddard Space Flight Center, to share their insights.
“A potentially habitable planet is often defined as a planet similar in size to Earth that orbits within the ‘habitable zone’ of its star, a region where the planet could maintain temperatures suitable for liquid water to exist on its surface. Currently, we know of around 30 planets that might be small, rocky planets like Earth and that orbit within the habitable zone. However, it’s important to note that being in the habitable zone doesn’t guarantee that a planet is actually habitable (capable of supporting life) or inhabited (currently supporting life). As of now, Earth is the only known planet that is both habitable and inhabited.”
The potentially habitable worlds Webb is observing are all transiting exoplanets, meaning their orbits are nearly edge-on, allowing them to pass in front of their host stars. Webb uses this orientation to perform transmission spectroscopy when the planet transits its star. This method lets us analyze the starlight filtered through the planets’ atmospheres to determine their chemical compositions. However, detecting an atmosphere around these small rocky planets is very challenging, as the amount of starlight blocked by the thin atmosphere is typically less than 0.02%. Identifying water vapor, which could suggest habitability, is even harder. Searching for biosignatures (biologically produced gases) is extraordinarily difficult but also an exciting endeavor.
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| When an exoplanet passes directly between its host star and the observer, it is said to be transiting. This transit dims the star’s light by a measurable amount, and if the exoplanet has an atmosphere, the starlight is filtered through it. This animation shows a single planet and the corresponding change in light levels during the transit. |
There are currently only a handful of small, potentially habitable worlds accessible to atmospheric characterization with Webb, including the planets LHS 1140 b and TRAPPIST-1 e. Recent theoretical work exploring the detectability of gaseous molecules in the atmosphere of the super-Earth-size planet LHS 1140 b highlights several challenges in searching for biosignatures. This research suggests that approximately 10-50 transits of the planet around its host star, equivalent to 40-200 hours of observing time with Webb, would be needed to attempt detecting potential biosignatures, such as ammonia, phosphine, chloromethane, and nitrous oxide, in the best-case scenario of a clear, cloud-free atmosphere.
Given Webb’s limited viewing window of the LHS 1140 system due to its position in the sky, it could take multiple years, if not close to a decade, to collect 50 transit observations of LHS 1140 b. If the planet’s atmosphere is cloudy, even more observations may be required. Most small exoplanets are known to have clouds or hazes that obscure the signals being searched for. Additionally, the atmospheric signals of these biosignature gases often overlap with other expected atmospheric signals, such as those from methane or carbon dioxide, making it even more challenging to distinguish between them.
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| A simulated transmission spectrum of an Earth-like atmosphere shows wavelengths of sunlight absorbed by molecules like ozone (O3), water (H2O), carbon dioxide (CO2), and methane (CH4). On this graph, the y-axis shows the amount of light blocked by the Earth-like planet’s atmosphere, with brightness decreasing from bottom to top. Model transmission spectrum by Lisa Kaltenegger and Zifan Lin (2021, ApJL 909). |
A potential avenue in the search for biosignatures lies in studying Hycean planets, a theoretical class of super-Earth-size planets with a relatively thin hydrogen-rich atmosphere and a substantial liquid water ocean. The super-Earth K2-18 b is a candidate for a potentially habitable Hycean planet based on current data from Webb and other observatories. Recently published work used NIRSpec and NIRISS to detect methane and carbon dioxide in K2-18 b’s atmosphere, but not water. This suggests that K2-18 b might be a Hycean world with a liquid water ocean, although this remains speculative without direct observational evidence. The authors also hinted at the possible presence of the potential biosignature dimethyl sulfide, but the signal is too weak for a conclusive detection with current data. As the study of Hycean planets is new, alternative interpretations to the liquid water ocean scenario are still being explored. Upcoming Webb observations with the NIRSpec and MIRI instruments should provide more insight into K2-18 b and the potential presence of dimethyl sulfide.
Another challenge in studying small, potentially habitable worlds with Webb is that their host stars can also exhibit signs of water vapor. This was observed in Webb’s recent study of the rocky exoplanet GJ 486 b, highlighting the difficulty in determining if detected water vapor originates from the planet’s atmosphere or its star.
Detecting biosignatures in the atmospheres of small, potentially habitable transiting planets orbiting cool stars is extremely challenging. It typically requires ideal conditions, such as cloud-free atmospheres, detection of signals significantly smaller than 200 parts per million, well-behaved stars without significant water vapor in star spots, and extensive telescope time to achieve sufficient signal-to-noise ratios. Moreover, detecting a single biosignature does not constitute the discovery of life. Confirming life on an exoplanet will likely require a large set of unambiguously detected biosignatures, data from multiple missions and observatories, and extensive atmospheric modeling, a process that could take years.
Webb’s power lies in its sensitivity to detect and begin characterizing the atmospheres of a few of the most promising potentially habitable planets orbiting cool stars. It can detect a range of molecules important for life, such as water vapor, methane, and carbon dioxide. Our goal is to learn as much as we can about these worlds, even if we cannot definitively identify habitable signatures with Webb. Combined with exoplanet studies by NASA’s upcoming Nancy Grace Roman Space Telescope, Webb observations will lay the foundation for the future Habitable Worlds Observatory, NASA’s first mission purpose-built to directly image and search for chemical traces of life on Earth-like planets around Sun-like stars.
