In a distant solar system named TRAPPIST-1, located 40 light years away from our own Sun, there exists a unique configuration of seven Earth-sized planets orbiting a frigid star.
Fresh data collected by the cutting-edge James Webb Space Telescope (JWST) has been dedicated to the closest planet to the star within the TRAPPIST-1 system—TRAPPIST-1 b. These recent observations have unveiled valuable insights into the intricate interplay between a star and the examination of exoplanets dwelling in the habitable zone, where the potential for liquid water on a planet’s surface persists.
The research team, which notably includes Ryan MacDonald, an esteemed astronomer from the University of Michigan and a NASA Sagan Fellow, has officially documented its findings in The Astrophysical Journal Letters.
MacDonald expressed, “Our meticulous observations have yet to reveal any indications of an atmosphere enveloping TRAPPIST-1 b. This leads us to consider multiple scenarios, ranging from the planet being a barren rocky world to possibly hosting high-altitude clouds or possessing a dense molecule like carbon dioxide, rendering the atmosphere elusive to our detection. Nevertheless, a pivotal discovery is that the star itself exerts the most substantial influence on our observations, a factor equally pertinent when examining other planets within this remarkable system.”
The lion’s share of the research endeavor has been concentrated on deciphering the ramifications of the star’s presence on the scrutiny of the TRAPPIST-1 system’s planets.
MacDonald concluded, “It’s imperative that we comprehend the star’s impact now, as failing to do so will significantly complicate our ability to decipher atmospheric clues when scrutinizing planets situated within the habitable zone—namely TRAPPIST-1 d, e, and f.”
A promising exoplanetary system
TRAPPIST-1, a star markedly smaller and cooler than our own Sun, has been a source of fascination among scientists and space enthusiasts ever since its seven Earth-sized exoplanets were uncovered in 2017. Situated roughly 40 light-years away from Earth, these planets, clustered closely around their star, boast three residing within the star’s habitable zone, sparking optimism about the possibility of habitable environments beyond our solar system.
Conducted under the leadership of Olivia Lim, affiliated with the Trottier Institute for Research on Exoplanets at the University of Montreal, this study employed a method known as transmission spectroscopy to glean valuable insights into the characteristics of TRAPPIST-1 b. This technique involves scrutinizing the star’s light as it passes through the exoplanet’s atmosphere during a transit, enabling astronomers to discern the distinctive spectral signatures left behind by the molecules and atoms present within that atmosphere.
Michael Meyer, a professor of astronomy at the University of Michigan, remarked, “These observations were executed using the NIRISS instrument on the James Webb Space Telescope, crafted through an international collaboration led by René Doyon at the University of Montreal, with the support of the Canadian Space Agency over nearly two decades. Being part of this collaboration was an honor, and witnessing results like these, which unveil the diverse characteristics of worlds orbiting neighboring stars, is exceptionally thrilling, thanks to the unique capabilities of NIRISS.”
Know thy star, know thy planet
The study’s pivotal discovery revolved around the substantial influence of stellar activity and contamination in the quest to decipher an exoplanet’s nature. Stellar contamination encompasses the effects induced by the star’s inherent features, including the presence of dark regions known as spots and bright regions referred to as faculae, which can distort measurements of the exoplanet’s atmosphere.
The research team uncovered compelling evidence affirming the pivotal role played by stellar contamination in shaping the transmission spectra of TRAPPIST-1 b, and by extension, in all likelihood, the other planets within the same system. The dynamic activity of the central star has the capacity to generate what can be termed “ghost signals,” misleading observers into believing they have detected specific molecules within the exoplanet’s atmosphere.
This revelation underscores the critical importance of factoring in stellar contamination when orchestrating forthcoming observations of exoplanetary systems. Such considerations take on even greater significance for systems akin to TRAPPIST-1, given that it is centered around a red dwarf star renowned for its heightened activity, characterized by starspots and frequent flare events.
Olivia Lim, a key figure in the study, expounded, “In addition to the contamination emanating from stellar spots and faculae, we also encountered a stellar flare—a capricious event causing the star to momentarily intensify in brightness for minutes to hours. This flare had a direct impact on our measurements of the light obstructed by the planet. Signatures of stellar activity like these are challenging to model, but it’s imperative to account for them to ensure accurate data interpretation.”
Notably, Ryan MacDonald played a pivotal role in modeling the star’s impact and in the quest to identify an atmosphere within the team’s observations. This involved running an extensive array of millions of models to explore the entire spectrum of properties encompassing cool starspots, active hot regions on the star, and potential planetary atmospheres, all aimed at elucidating the observations made possible by the James Webb Space Telescope.
No significant atmosphere on TRAPPIST-1 b
Among the seven intriguing planets within the TRAPPIST-1 system, TRAPPIST-1 b stands out as a challenging but vital candidate in the quest for Earth-sized exoplanets with atmospheres. Positioned closest to its star, this planet faces more rigorous conditions compared to its planetary siblings, enduring four times the solar radiation Earth receives and maintaining surface temperatures ranging from 120 to 220 degrees Celsius.
Remarkably, despite the formidable conditions, TRAPPIST-1 b, if equipped with an atmosphere, offers a unique advantage for detection and characterization. Its close proximity to the star results in a more pronounced signal during its transit, making it a prime target for observation, albeit a complex one.
In their pursuit of accurate findings, the research team adopted two distinct approaches to unravel the influence of stellar contamination and decipher the potential atmosphere of TRAPPIST-1 b. The first approach involved the removal of stellar contamination from the data prior to analysis. The second approach, led by Ryan MacDonald, entailed simultaneous modeling of both stellar contamination and the planetary atmosphere.
In both cases, the outcomes suggested that the spectra of TRAPPIST-1 b could be effectively accounted for by the modeled stellar contamination alone. This compellingly implies the absence of a substantial atmosphere on the planet. Such a conclusion, while significant, serves the invaluable purpose of guiding astronomers by identifying which atmospheric compositions do not align with the observed data.
Drawing from the wealth of observations collected by the James Webb Space Telescope (JWST), Olivia Lim and her team embarked on an exploration of diverse atmospheric models for TRAPPIST-1 b. This comprehensive analysis encompassed various conceivable compositions and scenarios. One notable revelation was the firm exclusion of cloud-free, hydrogen-rich atmospheres. Consequently, it appears that TRAPPIST-1 b lacks a discernible, extended atmosphere.
However, the data could not definitively rule out the presence of thinner atmospheres, such as those composed of pure water, carbon dioxide, or methane, nor an atmosphere akin to that of Titan, Saturn’s moon renowned for its substantial atmosphere. These findings, representing the first-ever spectrum of a TRAPPIST-1 planet, align consistently with earlier JWST observations of TRAPPIST-1 b’s dayside, as viewed in a single color with the MIRI instrument.
As astronomers venture forth in their exploration of rocky planets scattered across the cosmos, these revelations will serve as valuable inputs for future observation programs conducted with the JWST and other telescopes. In doing so, they will contribute significantly to our broader comprehension of exoplanetary atmospheres and the potential for habitability beyond our solar system.
Source: University of Michigan