Introduction
The Mid-InfraRed Instrument (MIRI) is a joined US/EU project looking at the longest wavelength Universe accessible by the James Webb Space Telescope (JWST). With MIRI we can look at the thermal radiation from stars and planets. This unique wavelength range provides access to the spectroscopic fingerprint of many molecules and cloud particles not accessible by the other instruments on JWST. The European Guaranteed Time Observational program (GTO) program has been used to demonstrate the unique capabilities of MIRI to characterise the atmospheres of exoplanets and constrain their formation and climate conditions. Here several examples will be discussed showcasing these capabilities.
Clouds and climate on a warm Neptune mass planet
In January 2023 the JWST was pointed towards the star WASP-107 to observe the transit of the Neptune mass planet WASP-107b. WASP-107b is a relatively warm planet with a mass similar to that of Neptune but with a much larger size (more similar to that of Jupiter). It is therefore heavily inflated and carries the nickname ‘the cotton-candy planet’. The reason for its inflation is unknown, though there are speculations in the literature. This mystery makes it an ideal target for more in depth study of its atmosphere.
When the planet passes in front of its host star, the planet transit, some of the light from the central star filters through the upper layers of the planet atmosphere and imprints the spectroscopic signature of the molecules and cloud particles present in it. Figure 1 shows the resulting transit depth spectrum. Some interesting and unexpected results are seen in this spectrum.
First of all, very clearly there is the spectroscopic signature of sulfur dioxide. Under the conditions present in this atmosphere, this gas is only produced under the influence of significant ultraviolet radiation and even then it can only be formed if large amounts of sulfur and oxygen are available, more than would be expected for Galactic abundances of these elements.
Second, a very interesting result is the presence of a broad feature in the spectrum which can only be explained as coming from solid state silicate particles, more commonly known as grains of sand. These particles must float high in the atmosphere, creating a high-altitude sand cloud. The most likely explanation for the presence of these particles is that they were formed as condensation clouds. This system is similar to what we see in the Earth’s atmosphere with water instead of sand. Here we have water evaporating from the surface and condensing in the cold regions high up in the atmosphere forming water clouds. In the atmosphere of WASP-107b it is significantly warmer, creating this same type of condensation clouds but now forming from ‘silicate vapor’ creating sand clouds. These types of clouds have been anticipated by theorists to form in these objects, but it is the unique wavelength coverage of the MIRI instrument that now for the first time reveals them and allows us to study their properties. The fact that we see these sand clouds in the atmosphere of WASP-107b must mean that efficient atmospheric mixing is at play to avoid the sand cloud particles to sink below the detectable atmosphere.
The third interesting result is actually the absence of a feature expected to be seen. This is the spectroscopic signature of methane, CH4. At the irradiation temperature of WASP-107b methane is expected to be an important and detectable molecule. The fact that we do not see it in the spectrum can have two chemical explanations that need to work together to explain its absence. First, similar to the result from the sulfur dioxide, the absence of methane points towards an increased abundance of heavy elements (like carbon, oxygen, sulfur and other elements heavier than hydrogen and helium). For elemental mixtures with increased heavy element content, the methane abundance is reduced. However, this is not enough. In addition to this, we need the temperatures at which the chemistry is set to be significantly higher than the equilibrium temperature expected from its distance from the central star. This internal heat source has been speculated on also to explain the inflation of the planet.
Overall, a picture unfolds of a turbulent atmosphere with a relatively high, and yet unexplained, internal heat source (see Fig. 2). Similar to other hot gas giant planets observed with JWST it seems that the heavy element content of the planets is increased compared to Galactic abundances. All these aspects provide crucial puzzle pieces to unravel the formation and evolution of these types of planets. For more detailed information see Dyrek et al. (2024).
The formation of substellar objects
Connecting to the formation of planets an interesting type of object are the Brown Dwarfs. These are objects with a mass in-between that of a star and a planet. Classically there are two formation scenarios proposed for giant planets: core accretion (similar to lower mass planets) and gravitational instability (similar to star formation). Roughly speaking in the gravitational instability scenario all mass available in the gravitational influence sphere of the forming planet is accreted in one go. Therefore, the composition directly reflects the composition of the formation environment. In the core accretion scenario, the slow accretion of material allows for several effects separating specific elements. More specific, in this scenario the significant accretion of planetesimals will create an enrichment of the planet atmosphere with icy material, similar to the material found in comets.
The cold brown dwarf named WISE J182831.08+265037.8 (hereafter WISE J1828) was observed with the MIRI instrument in very much detail (see Barrado et al. 2023). In the resulting spectrum we can find the presence of ammonia, water vapor and methane. Although this is the first ever observation of such an object in this detail, the model predictions of the spectrum very closely resemble the observed spectrum. The close match we can make with our model atmospheres allows us to look at very small deviations. One of these subtleties is the isotope ratio of nitrogen. Icy material typically is enriched in the heavier isotopes of nitrogen. Therefore, comparing the isotope ratio of various objects allows us to trace the amount of icy material that was incorporated in it. In the Sun there is around 400 times as much 14N as there is 15N. Comets are generally significantly enhanced in the isotope 15N, dropping the ratio by roughly a factor of four. The value we observe in WISE J1828 is close to, or even higher than, the Solar value, indicating that no significant accretion of icy material has contributed to the formation of the atmosphere (see Fig. 2 in Barrado et al. 2023 for an overview of various objects). This is interpreted as being consistent with the picture that brown dwarf atmospheres form via gravitational instability, similar to stars. This detection shows the power of the MIRI instrument to detect these very low abundance molecules like the isotope of ammonia, to a level that they can serve as tools to unravel the formation mechanism of planetary bodies.
Imaging of a very young exoplanet system
The MIRI instrument has next to its spectroscopic capabilities also an imaging mode. In this mode the very large spatial resolution of JWST, thanks to its huge mirror size, can be used to separate the light of a planet from the overwhelming brightness of its central star. This is very challenging and works best for systems where the spatial separation between planet and star is large but the contrast in brightness between them is relatively small. The HR8799 system is such an ideal system for direct imaging of exoplanets. The system has four detected planets at large orbital separation. The planets are all more massive than Jupiter and even the closest in of these four planets is at a distance from its central star more than three times the distance of Jupiter to the Sun. The system is very young which causes the planets to be very bright at mid infrared wavelength as they are still radiating the leftover heat stored inside the planets by the planet formation process. The HR8799 system was imaged with MIRI at four different wavelengths (see Boccaletti et al. 2024). All four planets are clearly visible in the images. In addition, the dust disk is imaged. This disk is likely created from the debris left over from collisions of planetesimals in the system. By measuring the brightness of the planets at these long wavelengths, we can compute how efficient the cooling of these planets must have been since their formation which constrains the formation scenarios for this exotic system further.
What the future holds
With the JWST instruments we have opened up a window for exoplanet research far beyond the previous possibilities. The analysis of these observations is challenging and poses ever new questions challenging our understanding of how exoplanets form and evolve. The years before the launch of JWST have seen an explosion of theories of what we should be able to see. With these first data we scrape the surface of possibilities to constrain all these ideas with this revolutionary observatory. In the future our understanding will grow. Important questions that will be addressed when more observations will become available are: What types of weather and climate systems exist on hot, irradiated planets? What is the nature of the internal heat source in puffed up planets? What are the variations in planet formation and evolution mechanisms that create the observed planet diversity? What types of atmospheres can we see around low mass planets? And many, many more. For all these questions the JWST and its revolutionary MIRI instrument provide the right tools.
Acknowledgements: The work discussed in this paper is part of the European Guaranteed Time Observations for the MIRI instrument. As such the author wishes to acknowledge all institutes and persons that made this instrument possible on the technical, organisational and scientific side.
References
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