ABSTRACT
We discuss how some of the first JWST datasets from the Early Release Science (ERS) program have showcased the transformative power of JWST to characterize extrasolar planets in the mid- and thermal-infrared for the first time. The results from this program have verified that JWST has the imaging sensitivity, spectral coverage, and spectral resolution to revolutionize our study of extrasolar planets. Specifically, I will describe how the stability and very low thermal background inherent within the observatory means that JWST coronagraphy will be sensitive to an entirely new class of planet: analogs of Saturn and Neptune in our own solar system, but at much wider orbital separations. Only JWST is sensitive to this last remaining region of uncharted exoplanet parameter space. I will also discuss how this ERS program has provided spectroscopy of planetary mass companions at completely new wavelengths providing unambiguous evidence of phenomena such as disequilibrium chemistry and the presence of silicates in the atmosphere. Lastly, I will discuss how the results from this ERS program are already providing valuable “lessons learned” that will educate our efforts with the Roman Space Telescope, and ultimately the Habitable Worlds Observatory with the goal of obtaining images and spectroscopy of terrestrial exoplanets.
1. INTRODUCTION
In the relatively short span of a quarter century, astronomers have transitioned from speculating about the prevalence of exoplanetary systems to discovering thousands, and it is now clear that most stars host planetary systems (e.g., Cassan et al. 2012; Dressing & Charbonneau 2013). The vast majority of these, however, have been identified only indirectly via the transit and radial velocity detection methods, and lie at close orbital separations from their host stars.
As the indirect transit and radial velocity detection methods are inherently much less sensitive to wide-separation planets with long orbital periods, the direct imaging technique (e.g., Bowler 2016) will be the only approach to fully define the outermost architectures of planetary systems (∼ 10 to hundreds of AU), and provide a more complete understanding of the true frequency of planetary mass companions to nearby stars (e.g., Nielsen et al. 2019; Vigan et al. 2021). In the last 15 years, imaging observations mostly at wavelengths of ≲2 μm have directly revealed ∼ 10-20, young (≲50 Myr), massive (≳1MJup) planets (e.g., Marois et al. 2008; Lagrange et al. 2010; Rameau et al. 2013; Chauvin et al. 2017; Macintosh et al. 2015; Bohn et al. 2020; Janson et al. 2021). Direct Imaging is also the only technique that will be capable of characterising exoplanets at orbital radii ≳0.5 AU, as transit transmission spectroscopy (e.g., Sing et al. 2016) requires multiple transits to achieve a strong signal for Earth-mass planets on Earth-like orbits around Sun-like stars (Morley et al. 2017) resulting in prohibitively long-time baselines. It is also the only technique projected to provide the in-depth characterization of such exo-Earths (e.g., The LUVOIR Team 2019; Quanz et al. 2021).
By spatially separating the light of the host star and the extremely faint planet, the direct imaging technique is also naturally suited to direct spectroscopy of planets themselves, allowing detailed characterization (e.g., Bowler et al. 2010; Macintosh et al. 2015; De Rosa et al. 2016; Chauvin et al. 2017; Currie et al. 2018). In addition to providing information on atmospheric properties and compositions (e.g., Hinkley et al. 2015; Kammerer et al. 2021), the direct imaging technique can provide powerful estimations of fundamental parameters, e.g. luminosity, effective temperature, and orbital properties (Gravity Collaboration et al. 2019). The characterization power of the direct imaging method becomes even more pronounced when combined with other exoplanet detection techniques, such as precise radial velocity monitoring or astrometry (e.g., Nowak et al. 2020; Lagrange et al. 2020; Wang et al. 2021a; Lacour et al. 2021; Hinkley et al. 2023), to more fully constrain parameters (e.g. planet mass) that are difficult to ascertain with one technique alone.
Going forward, direct imaging will ultimately provide direct, high-resolution (R∼100,000) spectra of exoplanet atmospheres (e.g., Snellen et al. 2015; Mawet et al. 2018; Vigan et al. 2018; Otten et al. 2021; Wang et al. 2021b). Obtaining photons directly from the atmosphere of the exoplanet itself will allow us to apply to exoplanetary atmospheres all of the spectroscopic techniques that have been applied to stars and brown dwarfs over the last century. Indeed, all of the detailed information that can be retrieved using high-resolution spectroscopy of stars and brown dwarfs (e.g. chemical abundances, compositions, thermodynamic conditions, Doppler tomography) will also be directly obtained for exoplanetary atmospheres, allowing much more precise interpretation of the spectra (Konopacky et al. 2013; Barman et al. 2015).
At the same time, the direct imaging technique has been especially prolific at imaging both very young primordial (protoplanetary) disks as well as dusty circumstellar debris disks. Direct images of these disk structures uniquely allow the study of the dynamical interactions between circumstellar disks, and the planets that are dynamically sculpting them (Choquet et al. 2016; Matthews et al. 2017; Avenhaus et al. 2018; Esposito et al. 2020). Upcoming space-based missions, the advent of the 30-40m telescopes, as well as updates to existing ground-based high contrast imaging platforms (e.g. MagAOX, GPI 2.0, SPHERE+, SCExAO; Males et al. 2018; Chilcote et al. 2020) will vastly improve exoplanet direct detection capabilities. Consequently, the development of the direct imaging technique will be a major priority for the broader exoplanet community going forward, and is a key mode for current and future space missions including JWST (Gardner et al. 2006).
1.1. JWST
With a combination of unprecedented sensitivity and wavelength coverage JWST has already demonstrated its potential for carrying out potentially transformative science related to the detection and characterization of exoplanetary systems (e.g. Hinkley et al. 2022; Carter et al. 2023). The exquisite wave front stability, as well as the capability to observe exoplanet atmospheres near the peak of their thermal emission at 3-5 μm and beyond, means that JWST has the extraordinary power for characterizing wide-separation exoplanets through direct imaging. The precise calibration of the observatory Point Spread Function (PSF) that is afforded by this spacecraft stability (e.g., Perrin et al. 2018), combined with sensitivity in the near- and mid-infrared that is in some cases hundreds of times greater than that for ground-based instruments means that JWST is already sensitive to an entirely new class of sub-Jupiter mass planets at wide orbital separations (e.g., Carter et al. 2021, 2023). This stability and wavelength coverage, combined with wide fields of view in JWST imagers, has allowed resolved imaging of circumstellar disks at wavelengths largely out-of-reach to ground-based observatories (Gáspár et al. 2023; Rebollido et al. 2024; Lawson et al. 2023). The broad wavelength coverage is also allowing the JWST spectrographs within NIRSpec and MIRI to cover multiple spectroscopic features of exoplanet atmospheres, giving precise measurements of atmospheric compositions and atmospheric chemistries.
In 2017, the Space Telescope Science Institute (STScI) awarded roughly 500 hours of Director’s Discretionary Time to 13 community-driven Early Release Science (ERS) programs with the goals of: 1) testing the observatory in the modes expected to be commonly used by that community; and 2) to make clear recommendations to the community on best-practices to be used in future cycles; and 3) to distribute a set of Science Enabling Products (SEPs). Our program “High-Contrast Imaging of Exoplanets and Exoplanetary Systems with JWST” (Hinkley et al. 2022), was ultimately awarded 75 hours of DDT time as program 1386 to utilize all four JWST instruments, and assess the performance of the observatory in these representative modes. In total, three targets were observed as part of this program: The primary target for coronagraphy, HIP65426b (Chauvin et al. 2017), is a wide (92 AU) separation 6-9 MJup exoplanet and is a nearly perfect target for testing the performance of the JWST NIRCam and MIRI coronagraphs given its angular separation from the host star of 0.8 arcsec. To test the spectroscopic capabilities of JWST, we observed VHS1256b (Gauza et al. 2015; Stone et al. 2016), a planetary mass companion with an angular separation of eight arcseconds from its host star making it ideal for obtaining clean NIRSpec and MIRI R∼few thousand spectra that is relatively free from contaminating starlight. Our third target was HD141569A, a circumstellar disk (Weinberger et al. 1999; Clampin et al. 2003) with multiple, distinct rings which is not discussed here in this document. Figure 1 shows a montage of images of these targets from previous publications.
The first science products from this program have already been presented in two initial publications. Carter et al. (2023) showcase the first-ever direct images of an exoplanet with JWST, as well as the first images of an exoplanet at wavelengths longer than 5 μm. Miles et al. (2022) show the first direct spectrum of a planetary mass companion with JWST, and the first spectrum of such an object covering its full luminous range from 1 to ∼20μm, clearly showing evidence for disequilibrium chemistry, and the first definitive detection of silicate clouds in an atmosphere of a planetary mass companion. In our ERS program, we also obtained images of a circumstellar disk, but we defer the discussion of this dataset as the data processing is still ongoing.
Here we utilize these results to highlight some of the capabilities of JWST that we have learned about in the last 18 months since the arrival of the first data. In §2.1 we highlight how JWST will help to complete our understanding of the last remaining region of parameter space, and in §2.2 we highlight that JWST will help to provide precise chemical abundances in exoplanet atmospheres, which is the beginnings of our ability to connect precise abundances with a formation history; and in §2.3 we discuss how the lessons learned from current high performance space coronagraphy with JWST will be directly applicable to the Roman Space Telescope Mission in just a few years, as well as the Habitable Worlds Observatory (HWO) in the 2040s.
2. THREE DRIVING QUESTIONS IN EXOPLANETARY SCIENCE THAT JWST IS HELPING TO ADDRESS
In this section, we will organize our discussion around three key themes of: 1) the architectures of planetary systems, 2) the atmospheres of planetary mass companions; and 3) the power of an image of an extrasolar planet. Specifically, we pose the following three questions:
- Architectures: What are the demographics of exoplanets at all orbital separations?
- Atmospheres: Can JWST help link our measurements of atmospheric composition with a formation history?
- Terrestrial Planets: How will JWST inform us about how to directly image terrestrial planets in the 2040s?
2.1. Architectures: What are the demographics of exoplanets at all orbital separations?
As part of this ERS program, HIP65426b was observed using both the NIRCam round mask and MIRI four-quadrant phase-mask coronagraphs in a variety of NIRCam filters from 2-5μm as well as the 11.4 and 15.5μm MIRI filters. Figure 2 highlights that the suppression of the host star light requires a combination of hardware via a physical coronagraph, as well as software, to suppress the host starlight. These images were the first-ever direct images of an extrasolar planet with JWST, but also showcased the first-ever direct observations of an exoplanet at wavelengths longer than 5 μm.
While these coronagraphic observations were a landmark moment for the field of exoplanet direct imaging and high contrast coronagraphy, another ancillary value of this dataset is that it verified that JWST will have the sensitivity to sub-Jovian planets at wide separation. These results largely verify the prediction shown in Figure 3 from Carter et al. (2021) that JWST would have sensitivity to an entirely new class of planet, reaching analogs of our Saturn or Neptune at wide separations. And while the photometry at wavelengths ≳5 μm can help to pin down basic quantities such as effective temperature and surface gravity, much more powerful characterization can come from direct spectroscopy as discussed next in § 2.2.
2.2. Atmospheres: Can we begin to link our measurements of atmospheric composition with a formation history?
VHS 1256b (Gauza et al. 2015; Miles et al. 2018) is a wide separation (∼103 AU), substellar companion (19±5 MJup) to a young M7.5 binary star (Stone et al. 2016; Rich et al. 2016). While the VHS1256 system is not a member of any known kinematic young moving groups, Gauza et al. (2015) derive an age of 150-300 Myr, consistent with its low surface gravity. With a spectral type of L7, infrared parallax measurements (Dupuy et al. 2020) show that VHS 1256b shares a region in a colour-magnitude diagram with other planetary mass companions that are near the L/T transition, and close to the deuterium burning limit such as HR 8799b, HD 203030B, and 2MASS J22362452+4751425 b (Marois et al. 2008; Metchev & Hillenbrand 2006; Bowler et al. 2017).
Spectroscopy of point sources is highly efficient with JWST. Indeed, as pointed out in Hinkley et al. (2022), only 5.6 hours of observatory time were dedicated to spectroscopy of this object, with only 2.7 of these hours actually being dedicated to science observations. In Figure 4 we show the spectrum of VHS1256b as part of this ERS program, highlighting the very long wavelength coverage and multiple measurements of spectroscopic features due to various chemical species. The combination of a long wavelength range, which translates directly to measurement of multiple spectroscopic features, and good spectral resolutions of a few thousand, means that much more precise abundances can be measured for objects like this than ever before. Such abundance measurements are the beginnings of our ability to connect our measurements of the atmospheric composition of a substellar object with its formation location in a protoplanetary disk, but much more work is needed to understand the physics of chemical transport in a disk, as well as a more detailed understanding of the formation locations of circumstellar ice lines.
In addition to the multiple spectroscopic features shown in Figure 4, one of the most notable is the presence of a strong silicate feature at ∼9 μm. Spectral fitting is under way to determine if any information about the cloud structure for this object can be inferred (Whiteford et al. in prep). VHS1256b is a remarkable object because atmospheric measurements reveal that this object possesses three of the major phenomena that have been observed in several other substellar objects. Namely, VHS1256b clearly shows evidence for disequilibrium chemistry seen in other brown dwarfs and directly imaged planets (Skemer et al. 2012; Konopacky et al. 2013; Miles et al. 2020), a very high level of photometric variability (Bowler et al. 2020; Zhou et al. 2020), and the presence of silicates. All of these features point a young, turbulent, and dynamic atmosphere.
2.3. Terrestrial Planets: How can JWST inform us about directly imaging terrestrial planets in the coming decades
The observations gathered as part of ERS 1386 were really our beginnings of doing high-performance coronagraphy in space. The lessons learned on how to execute such observations in an optimal way from space will prove invaluable for the upcoming coronagraphic observations with the Roman Space Telescope. And while it is not expected that the Roman mission will directly detect an Earth-like planet, Roman will inform the community about the necessary protocols for carrying out high-performance coronagraphy in space with active wave front control, which is the necessary stepping stone for HWO. Thus, the lessons learned as part of ERS 1386 are really the beginning of a long-term, multi-decade, effort to directly characterize Earth-like planets in the habitable zones of their stars.
3. FINAL THOUGHTS: THE POWER OF AN IMAGE OF A PLANET
The direct detection and characterization of terrestrial planets in their habitable zones, with the possible detection of biosignatures, may indeed occur in our lifetime. Such an effort to gather the first images and spectroscopy of an Earth-like planet, whether with HWO or another mission, will trigger an irreversible change in our worldview. As stated in the words of the 2020 United States Decadal Survey: “The scientific goals of this mission, when achieved, have the potential to change the way that we as humans view our place in the Universe.”
However, it should be noted that we already have an image of an Earth-like planet with evidence for life: our own Earth. Indeed, with the goal of generating a “family portrait” of the members of our own solar system, on 14 February 1990 the voyager mission gathered on final snapshot of all the solar system planets as it was leaving our solar system. The power of this image inspired the writing of Carl Sagan in his book “Pale Blue Dot” (Sagan 1994), who wrote:
“It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we’ve ever known.”
Indeed, while we work for the next 20 years to gather images and spectroscopy of terrestrial exoplanets, we must not lose track of the fact that we currently face the greatest challenge of our lifetime: the climate crisis. Thus, while we search for other Earths, we should hold in equal importance the health of our own planet, the only home we’ve ever known.
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