Webb’s spectroscopic hunt for Earth-like planets

The The James Webb Space Telescope is actively exploring small, potentially habitable exoplanets, using transmission spectroscopy to analyze the composition of their atmospheres. The process is complicated by small signal sizes and the need for long observation times, making the detection of biological signatures of life a challenging task.

Exoplanets are common in our galaxy and some even orbit in the so-called habitable zone of their star. NASA’s James Webb Space Telescope has been busy observing several of these small, potentially habitable planets, and astronomers are now hard at work analyzing Webb’s data. NASA invited Dr. Knicole Colón and Christopher Stark, two Project Webb scientists at NASA’s Goddard Space Flight Center, to tell us more about the challenges of studying these other worlds.

Defining potentially habitable planets

“A potentially habitable planet is often defined as a planet similar in size to Earth that orbits in the ‘habitable zone’ of its star, the place where the planet could have a temperature at which liquid water could exist on its surface. We currently know of about 30 planets that may be small, rocky planets like Earth that orbit in the habitable zone.

“However, there is no guarantee that a planet orbiting in the habitable zone is actually habitable (could support life), let alone inhabited (currently supports life). At the time of writing, there is only one known habitable and inhabited planet – Earth!

Challenges of observing exoplanet atmospheres

“The potentially habitable worlds that Webb observes are all of them transiting exoplanets, which means their orbits are nearly oblique, so they pass in front of their host stars. Webb uses this orientation to perform transmission spectroscopy as the planet passes in front of its star. This orientation allows us to examine starlight filtered by the planets’ atmospheres to learn about their chemical composition.

“However, the amount of starlight blocked by the thin atmosphere of a small rocky planet is minuscule, typically much less than 0.02%. Just detecting the atmosphere around these small worlds is very challenging. Identifying the presence of water vapor that can boost the possibility of habitability is even harder. I’m looking biological signatures (biologically produced gases) is an extremely difficult but also exciting endeavor.

When exoplanet passes directly between its host star and the observer, we say that the planet is transiting in front of its host star. This transit dims the starlight by a measurable amount, and the starlight is also filtered through the exoplanet’s atmosphere, if it has one. This animation shows one planet and the corresponding change in light level during transit. Credit: NASA’s Jet Propulsion Laboratory

“There are currently only a handful of small, potentially habitable worlds that are considered accessible for atmospheric characterization with Webb, which includes the planets LHS 1140 ba and TRAPPIST-1 e.”

Technical challenges in detecting biological signatures

“Some recent theoretical work investigating the detectability of gaseous molecules in the atmosphere of the super-Earth-sized planet LHS 1140 bo highlights several challenges in the search for biological signatures. The work lists approximately 10–50 transits of the planet around its host star, equivalent to 40–200 hours of observing time with Webb, to attempt to detect potential biological signatures such as ammonia, phosphine, chloromethane and others. nitrogen dioxideat best a clear atmosphere without clouds.

The complexity of exoplanet observation plans

“Since Webb cannot observe LHS 1140 year-round due to the system’s position on the sky, it would take several years, if not nearly a decade, to collect 50 observations of the transit of LHS 1140b. Searching for biological signatures may require more than 50 transit observations if the planet’s atmosphere is cloudy.

“Most small exoplanets are known to have clouds or haze that dampen or obscure the signal being sought. Atmospheric signals of these biosignature gases also tend to overlap with other expected atmospheric signals (e.g. due to gaseous methane or carbon dioxide), so distinguishing between different signals is an additional challenge.

Hycean Planets: A New Path for Research

“A potential avenue in the search for biological signatures is the study of Hycean planets, a theoretical class of super-Earth-sized planets with a relatively thin hydrogen-rich atmosphere and a sizable ocean of liquid water. Super-Earth K2-18b 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 the atmosphere of K2-18 b, but not water. This means that the assumption that K2-18b is a Hycean world with a liquid water ocean remains based on theoretical models, with no direct observational evidence yet. The authors of the paper also suggested the possible presence of a potential biological signature of dimethyl sulfide in the atmosphere of K2-18b, but the signal of potential dimethyl sulfide is too weak to be reliably detected in the current data.

“The concept and study of the Hyckelian class of planets is very new, so alternative interpretations of the liquid water ocean scenario (and thus the potential for habitability) are still being explored. Webb’s upcoming NIRSpec and MIRI observations should shed further light on the nature of the potential Hycean planet K2-18 and the possible presence of dimethyl sulfide in its atmosphere.

Confounding factors in observational data

“One other confounding factor that makes Webb’s study of small, potentially habitable worlds challenging is that host stars may also show signs of water vapor. This was explored in recent Webb observations of a rocky exoplanet known as GJ 486b. We therefore have the additional problem of determining whether the water vapor detected by Webb actually comes from the planet’s atmosphere and not from its star.

Conclusion: The future of exoplanet studies

“Detecting biological signatures in the atmospheres of small, potentially habitable transiting planets orbiting cool stars is an extremely challenging task, usually requiring ideal conditions (e.g. cloudless atmosphere) or assuming that the environment of the early Earth (i.e. different from the modern Earth as we know), detection of signals significantly smaller than 200 ppm, a well-behaved star without significant starspot water vapor, and considerable telescope time to achieve a sufficient signal-to-noise ratio.

“It is also important to remember that the detection of a single biological signature does not constitute the discovery of life by any means. Discovering life on an exoplanet will likely require a large set of unambiguously detected biological signatures, data from many missions and observatories, and extensive atmospheric modeling efforts, a process likely to take years.

“The strength of Webb is that it has the sensitivity to detect and begin to characterize the atmospheres of a handful of the most promising potentially habitable planets orbiting cool stars. In particular, Webb has the ability to detect a number of molecules important to life, such as water vapor, methane and carbon dioxide. Our goal is to learn as much as possible about potentially habitable worlds, even if we cannot definitively identify habitable signatures with Webb.

“Webb’s observations, combined with studies of exoplanets by NASA’s upcoming Nancy Grace Roman Space Telescope, will ultimately lay the groundwork for the future Habitable Worlds Observatory, which will be NASA’s first mission designed to directly image and search for chemical signatures of life on Earth.” Earth-like planets around Sun-like stars.”

About the authors:

Knicole Colón is an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and serves as the James Webb Space Telescope’s deputy project scientist for Exoplanet Science.

Christopher Stark is an astrophysicist at the Exoplanet and Stellar Astrophysics Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and serves as deputy project scientist for the James Webb Space Telescope.