A team of astronomers has made a leap forward in our understanding of the intriguing TRAPPIST-1 exoplanetary system. Not only has their research shed light on the nature of TRAPPIST-1 b, the exoplanet orbiting closest to the system’s star, but it has also shown the importance of parent stars when studying exoplanets. The findings, published in Astrophysical Journal Letters, advances our understanding of the complex interplay between stellar activity and exoplanet characteristics.
TRAPPIST-1, a star much smaller and cooler than our Sun located approximately 40 light-years away from Earth, has captured the attention of scientists and space enthusiasts alike since the discovery of its seven Earth-sized exoplanets in 2016. These worlds, tightly packed around their star with three of them within its habitable zone, have fueled hopes of finding potentially habitable environments beyond our Solar System.
A research team, led by Olivia Lim from the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal (UdeM), employed the powerful James Webb Space Telescope (JWST) to observe the exoplanet TRAPPIST-1 b. These observations were collected as part of the largest Canadian-led General Observers (GO) program during the JWST’s first year of operations. This program also included observations of three other planets in the system, TRAPPIST-1 c, g and h. TRAPPIST-1 b was observed during two transits — the moment when the planet passes in front of its star — using the Canadian-made NIRISS instrument aboard the JWST.
“The star TRAPPIST-1 is a red dwarf, which poses some unique challenges: firstly it might have sterilized all of its planets billions of years ago, when they were young. This is why it is so important to study these planets with JWST: they could be habitable or they could be airless rocks. Secondly, red dwarf stars are more spotty and have more flares than stars like the Sun. This makes it extra hard to know whether we’re actually detecting a terrestrial exoplanet’s atmosphere, or just seeing stellar contamination in our data. We really need intensive observations of this star in order to make sense of the transit spectra we are obtaining with JWST,” said McGill Professor in the Department of Earth and Planetary Sciences and co-author on the study, Nicolas Cowan.
The study used the technique of transmission spectroscopy to gain important insights into the properties of the distant world. By analysing the central star’s light after it has passed through the exoplanet’s atmosphere during a transit, astronomers can see the unique fingerprint left behind by the molecules and atoms found within that atmosphere.
“This is just a small subset of many more observations of this unique planetary system yet to come and to be analysed,” adds René Doyon, Principal Investigator of the NIRISS instrument and co-author on the study. “These first observations highlight the power of NIRISS and the JWST in general to probe the thin atmospheres around rocky planets.”
Know thy star, know thy planet
The key finding of the study was the significant impact of stellar activity and contamination when trying to determine the nature of an exoplanet. Stellar contamination refers to the influence of the star's own features, such as dark spots and bright faculae, on the measurements of the exoplanet's atmosphere.
The team found compelling evidence that stellar contamination plays a crucial role in shaping the transmission spectra of TRAPPIST-1 b and, likely, the other planets in the system. The central star’s activity can create “ghost signals” that may fool the observer into thinking they have detected a particular molecule in the exoplanet’s atmosphere. This result underscores the importance of considering stellar contamination when planning future observations of all exoplanetary systems. This is especially true for systems like TRAPPIST-1, since the system is centred around a red dwarf star which can be particularly active with starspots and frequent flare events.
“In addition to the contamination from stellar spots and faculae, we saw a stellar flare, an unpredictable event during which the star looks brighter for several minutes to hours,” mentions Olivia Lim. “This flare affected our measurement of the amount of light blocked by the planet. Such signatures of stellar activity are difficult to model but we need to account for them to ensure that we interpret the data correctly.”
No significant atmosphere on TRAPPIST-1 b
While all seven of the TRAPPIST-1 planets have been tantalising candidates in the search of an Earth 2.0, an exoplanet like our Earth, TRAPPIST-1 b’s proximity to its star means it finds itself in harsher conditions than its siblings. It receives four times more radiation than the Earth does from the Sun and has a surface temperature between 120 and 220 degrees Celsius. However, if TRAPPIST-1 b were to have an atmosphere, it would be the easiest to detect and describe of all the targets in the system. Since TRAPPIST-1 b is the closest planet to its star and thus the hottest planet in the system, its transit creates a stronger signal. All these factors make TRAPPIST-1 b a crucial, yet challenging target of observation.
To account for the impact of stellar contamination, the team conducted two independent atmospheric retrievals — techniques to determine the kind of atmosphere present on TRAPPIST-1 b based on observations. In the first approach, stellar contamination was removed from the data before they were analysed. In the second approach, stellar contamination and the planetary atmosphere were modelled and fit simultaneously. In both cases, the results indicated that TRAPPIST-1 b's spectra could be well matched by the modelled stellar contamination alone. This suggested no evidence of a significant atmosphere on the planet. Such a result remains very valuable, as it tells astronomers which types of atmospheres are incompatible with the observed data.
Based on their collected JWST observations, Lim and her team explored a range of atmospheric models for TRAPPIST-1 b, examining various possible compositions and scenarios. They found that cloud-free, hydrogen-rich atmospheres were ruled out with high confidence. This means that there appears to be no clear, extended atmosphere around TRAPPIST-1 b. However, the data could not confidently exclude thinner atmospheres, such as those composed of pure water, carbon dioxide, or methane, nor an atmosphere similar to that of Titan, a moon of Saturn and the only moon in the Solar System with its own atmosphere. These results are generally consistent with previous (photometric, and not spectroscopic) JWST observations of TRAPPIST-1 b (from Greene, et al. and Ih, et al.) with the MIRI instrument. Furthermore, the study has proven that Canada’s NIRISS instrument is a highly performing, sensitive tool able to probe for atmospheres on Earth-sized exoplanets down to impressive levels.
The new insights gained from this study have deepened scientists’ understanding of the TRAPPIST-1 system and emphasised the need for more observations and comprehensive investigations that consider both stellar contamination and planetary atmospheres. As astronomers continue to explore the vast expanse of space, these findings will inform future observing programs on the JWST and other missions and contribute to our broader understanding of exoplanetary atmospheres and their potential habitability.
About this study
The paper “Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra” was published in Astrophysical Journal Letters on September 22, 2023. The lead author is Olivia Lim, Ph.D. student at the Trottier Institute for Research on Exoplanets at the Université de Montréal (UdeM). Other iREx researchers that contributed to this paper are Björn Benneke (UdeM), René Doyon (UdeM), Caroline Piaulet (UdeM), Étienne Artigau (UdeM), Louis-Philippe Coulombe (UdeM), Michael Radica (UdeM), Alexandrine L’Heureux (UdeM), Loïc Albert (UdeM), Salma Salhi (UdeM and University of Calgary), Pierre-Alexis Roy (UdeM), Marylou Fournier-Tondreau (UdeM), Jake Taylor (UdeM and University of Oxford), Neil Cook (UdeM), David Lafrenière (UdeM), Nicolas Cowan (McGill U), Jason Rowe (Bishop’s U), Lisa Dang (UdeM), and Antoine Darveau-Bernier (UdeM). Additional contributors are based out of the University of Michigan, MIT, Cornell University, STScI, and Johns Hopkins University.