The quest to find habitable rocky planets beyond our solar system has long captivated astronomers. With over 5,900 exoplanets discovered, a mere 217 have been confirmed as terrestrial. Extracting detailed atmospheric information from these rocky worlds, particularly those orbiting close to their stars, presents a significant challenge.
Now, the James Webb Space Telescope (JWST) is revolutionizing exoplanet research, shifting focus from discovery to characterization. While definitive atmospheric identification on rocky planets remains elusive, Webb’s initial data, albeit with uncertainties, offers unprecedented insights. “The full scope was unknown at first,” recalls Dr. Anya Sharma, a planetary scientist not involved in the study, remembering the initial excitement surrounding Webb’s launch.
A comprehensive analysis of Webb’s findings, conducted by researchers at the Max Planck Institute for Astronomy (MPIA) and Johns Hopkins University Applied Physics Laboratory (JHUAPL), proposes a new standard for atmospheric characterization: the “five-scale height challenge.”
Laura Kreidberg, MPIA professor and director of the Atmospheric Physics of Exoplanets (APEx) Department, spearheaded the study in collaboration with Kevin B. Stevenson, a research astronomer at JHUAPL, and the Consortium on Habitability and Atmospheres of M-dwarf Planets (CHAMPs). Their paper, “A first look at rocky exoplanets with JWST,” currently under review for publication in the Proceedings of the National Academy of Sciences, details their findings.
According to their research, Webb’s advanced instruments have achieved notable milestones, thanks to its sensitive infrared optics, coronagraphs, and spectrometers. Key achievements include:
- The most precise transmission spectra for rocky planets to date.
- Detection of heat emanating from approximately half a dozen rocky planets.
- Extending thermal emission measurements to cooler rocky planets, reaching temperatures as low as 100°C, a significant decrease from the previous 800°C limit.
These observations have spurred theoretical advancements in predicting the atmospheric properties of rocky planets, especially those orbiting M-type red dwarf stars, which are particularly important given that they constitute 80% of the Milky Way’s stellar population. These theoretical frameworks highlight the numerous factors influencing planetary atmospheres, like volatile delivery from comets, atmospheric loss processes, internal interactions, and even potential biological activity. Atmospheric loss is of particular concern, with the question of atmospheric retention remaining unanswered for many planets Webb is observing. Are these truely habitable environments or just celestial bodies? I have often wondered about the answers and sometimes, I feel small in the grand scheme of things.
The “cosmic shoreline” concept has become a prominent tool in assessing the likelihood of atmosphere retention. This framework suggests that planets with higher escape velocities (i.e., more massive planets) and lower levels of stellar irradiation are more likely to hold onto their atmospheres. However, understanding how stellar type and irradiation history affect this “shoreline” remains a crucial question. Older stars, particularly M-type stars, can undergo prolonged UV-bright phases, lasting up to 6 billion years, thus exposing nearby planets to damaging radiation and impacting atmosphere survivability, its lik ean oven roasting the planet.
To overcome these hurdles, the team proposes the “five-scale height challenge,” aiming for the measurement precision needed to detect subtle atmospheric features, like those found on Earth. As Kreidberg explained, “The five-scale height challenge is a goal to reach the measurement precision needed to detect Earth-like atmospheric features. The largest spectral feature in Earth’s atmosphere is carbon dioxide, and it spans about five scale heights… So far, the data is not precise enough to see a feature this small, so more observations are needed!”
JWST’s capabilities enable the detection of faint signals from elements like water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and carbon monoxide (CO). Future observations will focus on obtaining transmission and emission spectra from rocky exoplanets around M-type stars during transits and eclipses.
Kreidberg emphasizes that, “The five-scale height challenge sets a benchmark for how precise the data needs to be to detect an Earth-like atmosphere… This is going to require both more data and better modeling to remove noise from the star and JWST’s detectors.” A dedicated JWST program, GO 7073 (“Charting the Cosmic Shoreline“), has been established to study a selection of rocky planets deemed most likely to have atmospheres.
While JWST can’t analyze Earth analogs around sun-like stars, future missions such as the Habitable Worlds Observatory (HWO) will. Kreidberg’s team’s framework provides a stepping stone by refining our understanding of rocky exoplanet atmospheres. The dream of finding life beyond Earth pushes scientists to achieve their ambitions.
Kreidberg concludes: “[W]e have made great progress with JWST already in our understanding of which rocky planets could have atmospheres. This is a critical first step, well before we get to biosignatures. We need to learn to walk before we can run!” The idea of walking before running really shows that the end goal is to find life beyond Earth but the scientists are aware that they need to take baby steps. It is simialr to finding a job, you start by getting an entry level job and then you slowly move up.
As someone who followed the JWST project for years, from initial concepts to the triumphant first light, I find myself reflecting on the vastness of space and our place within it. This research, and future discoveries enabled by JWST, will undoubtedly reshape our understanding of planetary formation and the potential for life beyond our planet. As a user on X.com commented, “It’s only a matter of time before we find life on another planet!”. I think many peope will agree to this sentiment.