Abstract
With JWST a new window into exoplanet atmospheres is now wide open. This paper focuses on initial results from JWST Cycle 1 programs that aimed to perform “deep” observations of specific transiting exoplanet hosting systems to explore the physics and chemistry at work in these planetary atmospheres. The JWST Telescope Scientist Team (JWST-TST) used 133 hours of Cycle 1 guaranteed observing time to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS) on three archetypical planets: a hot Jupiter (WASP-17b), a warm Neptune (HAT-P-26b), and a temperate Earth (TRAPPIST-1e). Through both transmission and emission observations spanning 0.6-12 microns, the DREAMS survey has uncovered new and unexpected chemistry at work in WASP-17b and HAT-P-26b’s atmospheres and allowed us to explore their three-dimensional atmospheric structure. With DREAMS survey observations of the TRAPPIST-1 system, we have achieved precisions that will allow for detailed exploration of TRAPPIST-1e’s transmission spectrum in search of planetary atmospheric signatures. The JWST observations presented here will hopefully prove to have a lasting legacy in our understanding of exoplanets and their atmospheres.
1. Introduction
Exoplanets that transit their host star as viewed from earth, which constitute the majority of the currently known exoplanet population, represent our best opportunity for atmospheric characterization studies with observational facilities like JWST. These transiting exoplanets allow their atmospheres and/or surfaces to be probed through high-precision (<1%) relative spectrophotometric observations obtained throughout the planet’s orbit. Atmospheric transmission spectra obtained as the planet passes in front of the host star probe the chemical composition of the planet’s atmosphere. The dayside thermal structure and emission spectrum of the planet are probed with secondary eclipse observations. Emission measurements as a function of orbital phase can be used to map the longitudinal thermal structure of the planet since most transiting exoplanets are on short period orbits within the tidal locking radius of their host star. Before the launch of JWST, space-based observatories such as Spitzer and Hubble provided the stability and precision required to probe the atmospheres of transiting exoplanets, but were limited in the wavelengths of light they could access and the achievable spectral resolution. JWST will go far beyond both Spitzer and Hubble to achieve unprecedented stability, precision, and wavelength coverage and resolution to usher in a golden age of exoplanet atmospheric characterization.
2. The JWST-TST DREAMS Strategy
The JWST Telescope Scientist Team [1] (JWST-TST) is using Guaranteed Time Observer (GTO) time awarded by NASA in 2003 (PI M. Mountain) for studies in three different subject areas: (a) Transiting Exoplanet Spectroscopy (lead: N. Lewis); (b) Exoplanet and Debris Disk Coronagraphic Imaging (lead: M. Perrin); and (c) Local Group Proper Motion Science (lead: R. van der Marel). A common theme of these investigations is the desire to pursue and demonstrate science for the astronomical community at the limits of what is made possible by the exquisite optics and stability of JWST. The transiting exoplanet spectroscopy subject area used 133 hours of JWST cycle 1 observing time to perform the Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS) program on three archetypical planets: a hot Jupiter (WASP-17b, Mp ~ MJupiter, Teq > 1200 K), a warm Neptune (HAT-P-26b, Mp ~ MNeptune, Teq < 1200 K), and a temperate Earth (TRAPPIST-1e, Mp ~ MEarth, Teq ~ 300 K). The DREAMS survey takes advantage of JWST’s spectral coverage, spectral resolution and achievable precision to characterize these three exoplanets in exquisite detail. Exoplanet scientists have long envied the observed spectra available for stars, brown dwarfs, and planets within our own solar system. Although spectral differences do exist among members of a given population of stars or planets, high-fidelity observed spectra of representative members are invaluable for understanding the bulk properties of the population and guiding observations of other population member. Additionally, by observing both transmission and emission spectra for our hot Jupiter and warm Neptune targets, we have the opportunity to probe the three-dimensional atmospheric structure of these worlds and better understand their weather. The JWST-TST DREAMS observations will become ‘Rosetta Stones’ that will serve as benchmarks for further observations of planets within each representative population and a lasting legacy of the JWST mission.
3. New Windows into Hot Jupiter Atmospheric Chemistry, Clouds, and Weather
The hot Jupiter WASP-17b was among the best studied exoplanets in the era before JWST, but our knowledge of the physics and chemistry at work in its atmosphere was still limited by the wavelength coverage offered by Hubble and Spitzer. Figure 1 shows our transmission and emission spectra of WASP-17b before and after our 75 hours of JWST observations on this target. Before JWST, we knew that WASP-17b was likely to have an atmosphere rich in heavier elements (30x Solar Metallicity) and have aerosols present (Alderson et al. 2022). Our JWST transmission observations have confirmed the presence of Carbon Dioxide (CO2) in WASP-17b’s atmosphere, consistent with a metal-rich atmosphere, and revealed signatures of Quartz (SiO2) clouds in its atmosphere, first reported in Grant et al. (2023). Before JWST, we had very little information on the temperature and dayside chemistry of WASP-17b’s atmosphere. Our JWST emission observations have revealed a rich dayside spectrum from WASP-17b’s atmosphere with features that add new dimensions to our understanding of the weather on this distant world, in particular strong winds that transport heat from the dayside to the nightside.
4. Trends Across Exoplanet Size and Temperature
The Warm Neptune HAT-P-26b had a strong detection of H2O with Hubble (Wakeford et al. 2017) that gave us some insights into the atmospheric composition of this interesting member of the exoplanet population before the launch of JWST. HAT-P-26b is of particular interest because its mass (~22 MEarth) is similar to that of Neptune in our own Solar System and its equilibrium temperature (~1000 K) places it at an interesting transition for the formation of atmospheric aerosols (Gao et al. 2021). With the JWST NIRSpec instrument, we were able to spectroscopically probe HAT-P-26b in the 3-5 micron wavelength region that for the first time revealed clear signatures of CO2 and the photochemically produced SO2 molecules in its atmosphere. In Figure 2, we compare the NIRSpec G395H transmission spectra of WASP-17b and HAT-P-26b from the DREAMS observations with similar observations of the warm Jupiter WASP-39b from the JWST ERS program (Alderson et al. 2023). Despite the fact that WASP-17b, HAT-P-26b and WASP-39b represent a broad range in planetary sizes and temperatures, their spectra show remarkable similarities, notably strong absorption features from H2O, SO2, CO2, and CO (Figure 2). JWST has shown that photochemistry, in particular sulfur-based photochemistry, is a key process shaping a broad range of exoplanet atmospheres.
5. A First Detailed Look at Temperate Terrestrial Exoplanet Atmospheres
The high precision (< 100 ppm) available with JWST observations of transiting exoplanets offers the opportunity to probe the potential atmospheres of earth-sized planets in the habitable zones of their stars. The TRAPPIST-1 system (Gillon et al. 2017) offers seven earth-sized planets orbiting a cool M-dwarf star, three of which reside in the system’s habitable zone. As part of the JWST-TST DREAMS programs we focused our observations on capturing multiple transits of the habitable zone planet TRAPPIST-1e with JWST’s NIRSpec Prism to obtain transmission spectra spanning 1 to 5 microns in wavelength. Before JWST, Hubble offered our only opportunity to spectroscopically probe the planets in the TRAPPIST-1 system (e.g. de Wit et al 2018). As shown in the top panel of Figure 3, Hubble provided limited coverage of each planetary transit in the TRAPPIST-1 system and with significant instrumental and observatory systematics that required careful correction due to its location in Low Earth Orbit (LEO). JWST, stationed 1 million miles from Earth at L2, has provided spectroscopic observations of the TRAPPIST-1 system that have continuous coverage of each transit event and minimal systematics (Figure 3, bottom panel). Our multiple JWST-TST DREAMS observations of TRAPPIST-1e’s near-infrared transmission spectrum will allow us to push to even better precisions (<10 ppm) to search for signatures of its planetary atmosphere, better understand the properties of its host star, and guide further JWST observations of this key target in the search for life beyond our solar system.
6. Conclusions
With JWST Cycle 1 observations now complete, our JWST-TST DREAMS team is working to carefully analyze and interpret our observations of WASP-17b, HAT-P-26b, and TRAPPIST-1e. This work includes performing multiple reductions of each observational data set to ensure consistent results between different approaches. We will also produce a suite of atmospheric models for each of these targets to compare with our transmission and emission spectra and perform atmospheric retrieval exercises that will place constraints on the abundance of key atmospheric species and the planet’s thermal structure. Additionally, we will leverage our transmission and emission spectra to create multidimensional maps of WASP-17b and HAT-P-26b’s atmosphere that will finally provide us with true “pictures” of these distant worlds (e.g. Challener & Rauscher 2022, MacDonald & Lewis 2022, and Grant & Wakeford 2023). With more than a dozen planned manuscripts, there will be a rich body of JWST-TST DREAMS literature that will fully explore the potential science that can be achieved with large amounts dedicated observational time on single exoplanetary targets.
Acknowledgements
Funding for this work was provided by NASA through grant 80NSSC20K0586. Based on observations with the NASA/ESA/CSA JWST, associated with programs JWST GTO 1353, 1312, and 1331 (PI: N. Lewis), obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-03127.
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