To the next exoplanet and beyond! The JWST, Hubble, and Spitzer close relationship

While it is commonly believed that the development of modern technologies and scientific facilities renders older technologies obsolete, this is not always the case. In astronomy, the launch of the James Webb Space Telescope (JWST), while achieving ground-breaking milestones, has not made previous observations made by the Hubble and Spitzer space telescopes irrelevant. Join us in this astrobito to learn more about how JWST, Hubble, and Spitzer are working together to strengthen our understanding of exoplanet atmospheres!

Paper details:

Title: The importance of optical wavelength data on atmospheric retrievals of exoplanet spectra

Authors: Charlotte Fairman, Hannah R. Wakeford, and Ryan J. MacDonald

First-author Institution: HH Wills Physics Laboratory, University of Bristol, Bristol, UK

Status: Accepted for publication in the Astronomical Journal.

On December 25, 2021, NASA launched the James Webb Space Telescope (JWST) from French Guiana, heralding a new era in our comprehension of exoplanets and beyond. In the nearly two years since JWST became fully operational, it has facilitated groundbreaking discoveries, from offering direct evidence of photochemical processes in exoplanet atmospheres to unveiling unprecedented images of the universe. However, researchers from the University of Bristol and the University of Minnesota have found that combining JWST's infrared observations with UV/Vis data from the Hubble and Spitzer telescopes results in even more comprehensive exoplanet atmospheric spectra. This integrated approach offers a more robust understanding of the chemical and physical processes underlying exoplanet atmospheres.

Why study exoplanet atmospheres?

Exoplanets are cool but far, far away. The closest exoplanet, Proxima Centauri b, is approximately 4.2 light-years away, a distance that renders any thought of sending a

probe or visiting impossible. Instead, astronomers must turn to indirect methods such as analysing the light that passes through exoplanet atmospheres to glean insights into these distant worlds.

By studying the light filtering through an exoplanet's atmosphere, astronomers can identify the molecules and chemical species present. This information provides a wealth of knowledge about the exoplanet, including its physical and chemical environment. For example, on Earth, the presence of ozone (O3) in our atmosphere reveals a great deal about our planet, from the photochemical reactions that produce it to the biological activity that supplies the oxygen necessary for its formation. Remember that ozone is produced in our atmosphere by the following UV-induced reaction: 2O2 → O3 + O.

The same principle applies to exoplanet atmospheres. By examining the molecular composition of the atmosphere of these distant worlds, astronomers can infer crucial details about the planet's conditions, such as potential volcanic activity, lightning, geysers, or other dynamic processes. But how do astronomers identify these molecules?

Molecular fingerprints

Molecules interact with light in various ways depending on the wavelength and energy of the light. When exposed to ultraviolet (UV) and visible (Vis) light, electrons within the molecules transition to higher energy levels. Exposure to infrared (IR) light, on the other hand, causes the molecules to vibrate in very specific ways. These interactions produce unique spectra, which are collections of bands showing the wavelengths at which molecules absorb light. Figure 1 shows the infrared spectrum for water. There are three main modes of vibrations for water when interacting with infrared light: bending, and symmetric and asymmetric stretches.

Figure 1. Infrared spectrum for water. Each band represents a different molecular vibration. Red circles correspond to oxygen while grey circles correspond to hydrogen.

The spectra created by these interactions are like fingerprints for each molecule, as they are unique and specific to each molecular structure. By comparing atmospheric observations from telescopes with known spectra of various molecules, astronomers can determine the chemical composition of exoplanet atmospheres and gain insights into the planet's underlying conditions.

Combining efforts

Telescopes are designed to observe different wavelengths of light, allowing astronomers to identify molecules in exoplanet atmospheres. For instance, JWST and Spitzer specialise in infrared light, while Hubble focuses on UV/Vis light (Figure 2).

Figure 2. Spectral ranges for Hubble, JWST, and Spitzer. Adapted from Science News.

Traditionally, the identification of chemical species in exoplanet atmospheres has been done using data from a single telescope, e.g., looking at infrared atmospheric spectra from JWST alone. However, using a database containing atmospheric spectra from 4 exoplanets, the paper discussed in this astrobito suggests that a more complete understanding of exoplanet atmospheres can be achieved by combining observations from different wavelengths. This multi-wavelength approach is particularly useful for constraining cloud and aerosol parameters and estimating the abundance of alkali elements, with the authors noting improvements in constraints by up to 30%.

Figure 3 shows the median and 1-sigma errors of the Na and K abundances, and the aerosol parameter (log(a)) for the 14 exoplanets analysed in this study. The orange lines correspond to observations made in the infrared only, while the blue and purple lines include UV/Vis and infrared data. The figure shows an overestimation of the abundance of alkali metals and an underestimation of the aerosol parameter when only infrared data are considered (orange lines). However, by including the UV/Vis data (blue and violet lines), these parameters are inferred with greater precision since the 1-sigma error is lower in the blue and violet lines than in the orange one.

Figure 3. Medians and 1-sigma errors for the abundances of Na and K, and the aerosol parameter log(a) in the 14 exoplanets analysed using infrared data (orange lines) and infrared and UV/Vis data together (lines blue and violet). Figure adapted from Figure 5 in the original publication.

Properly understanding cloud and aerosol parameters is key in the study of exoplanet atmospheres because they can obscure spectral features and influence observed chemical abundances. Additionally, accurate alkali metal abundances rely on UV/Vis data only. Unlike most chemical species, alkali metals often exist as single atoms, lacking bond vibrations when interacting with infrared light. Therefore, these metals are only observable through electronic transitions with UV/Vis light. This highlights the importance of combining observations across different wavelengths, as JWST alone covers only the beginning of the UV/Vis spectrum.

A trio of titans

The launch of JWST has undeniably marked a thrilling new chapter in our journey through space exploration. With its groundbreaking resolution, we can now explore exoplanets in ways we have never done before. Yet, Hubble and Spitzer remain essential pillars in contemporary astronomy research. If JWST is already making extraordinary breakthroughs in exoplanet atmospheres, imagine the leaps we can achieve by combining more observations from these three astronomical titans!

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