Exoplanets and Stellar Physics

D.Phil Projects in Exoplanet and Stellar research

Stellar activity in Radial Velocity surveys
Suzanne Aigrain

Stellar activity is a broad term that refers to time-variable phenomena due to the interplay of magnetism and convection in the surface layers of stars. Activity gives rise to variability on wide range of times scales and wavelengths, and these are a major nuisance for exoplanet searches, particularly using the radial velocity method. At the same time, the time-series of spectra obtained during Radial Velocity (RV) planet searches contain very valuable information on activity-related phenomena - rotation rates, activity cycle, effect of different types of surface structures on different spectral lines. In this project we will use the vast database of over 250000 spectra collected to date by the HARPS and HARPS-N RV spectrographs to explore the dependency of different activity indicators, and their variability, on stellar parameters. One initial goal of the project will be to measure activity cycle periods and stellar rotation periods from activity indicators such as the log R'_HK index, in order to calibrate the relationship between these two periodicities and the stellar mass and overall activity level. Another goal will be to look for correlations between the activity indicators and the RV variability on different timescales, and identify which indicators are likely to be most useful to correct activity-induced RV variations for different types of stars.

Circumbinary discs and planets
Katherine Blundell

In recent years the existence and significance of circumbinary discs, orbiting outside of pairs of binary stars in orbit around one another, has emerged. Not only are these purported to have significant dynamical back-reactions on their inner binary stars (and hence their evolution) but are in some cases likely to be the breeding ground of Tatooine-like circumbinary planets. The goal is to explore and understand the nature of binary star systems that host such circumbinary structures, using data from the Global Jet Watch telescopes (PI K Blundell; www.GlobalJetWatch.net).

Reflected light from exoplanet atmospheres: towards rocky worlds
Jayne Birkby

We seek a motivated and curiosity-driven student with a passion for exploring exoplanet atmospheres and surfaces. This project focuses on the light that exoplanets reflect, and will study them initially using observations from very high resolution spectrographs, such as ESPRESSO/VLT and HARPS. The goal is to measure the planet’s reflection spectrum and use atmospheric modelling to interpret the molecular signatures detected. The program will begin with gas giant exoplanets, and will move towards sub-Saturns and Neptunes. This observational technique is still in its infancy but demonstrating its potential is key to detecting signatures of oxygen in the atmospheres of nearby rocky worlds with the Extremely Large Telescopes coming online in the mid-2020s. The 4-yr project will begin with observations, but has significant scope to explore theoretical modelling and simulations as well, depending on the interests of the student. The student will become a key member of Dr Birkby’s ‘exoZoo’ team of PhDs and postdocs, supported by a European Research Council Starting Grant. The student will also be encouraged to connect with the respective groups of Prof Suzanne Aigrain, Prof Niranjan Thatte, Prof Pierrehumbert, Prof Vivien Parmentier, and Prof Patrick Irwin in Astrophysics and AOPP. Oxford offers a highly stimulating environment for exoplanet atmosphere research, with many opportunities to explore new avenues towards the characterizing of rocky worlds beyond our Solar system.

Exoplanet Atmospheres in High Resolution
Jayne Birkby

Observing exoplanets at high spectral resolution is a powerful technique for characterizing the composition, structure, and dynamics of their atmospheres, including their global winds patterns and rotation. In this program, the student will lead the analysis of high-resolution spectra from MEASURE, a large survey of exoplanet atmospheres with the 6.5-m MMT telescope and ARIES spectrograph. They will investigate the properties of different hot gas giant exoplanets, performing comparative exoplanetology to begin understanding how the planet formation process resulted in such incredible diversity in the exoplanet population. They will study the chemical and physical processes occurring in extreme planetary environments and use these to define classes of exoplanet. They will further have the opportunity to propose observations for CRIRES+/VLT for smaller planets like mini-Neptunes. They will work closely with a postdoc as part of PI Birkby’s ERC exoZoo project, to assist with the theoretical modelling of the planet atmospheres, and help devise techniques to combine these data with space-based observations to extract maximal information about the exoplanet’s atmosphere, properties, and formation history.

Exoplanet projects are also available within the sub-department of Atmospheric, Oceanic and Planetary Physics. Students interested in these projects should apply to AOPP; applications which ask to be considered by both astrophysics and AOPP are encouraged. Note that these projects will be computationally intensive using Fortran, IDL and other software packages, so a physics/computing/mathematics degree is preferred.

Exoplanet Projects with supervisors at AOPP are listed here:

A framework to understand atmospheric mixing in Neptune size exoplanets. .
Vivien Parmentier & Raymond Pierrehumbert

The atmospheres of a handful of Neptune-size exoplanets have been observed with the Spitzer and Hubble space telescopes. They provide the first insights into a population of atmospheres that will be characterised by the James Webb Space Telescope in the coming decade. To date, atmospheric spectra of these small planets have revealed a diverse population. Some of these atmospheres seem entirely covered by a thick aerosol layer, muting out most molecular features (e.g. GJ1214b, Kreidberg et al. 2014). In the other ones, where stronger molecular features have been detected, the atmospheres seem driven far from chemical equilibrium (e.g. GJ436b Morley et al. 2017, WASP-107b, Kreidberg et al. 2018).
The presence of larger than anticipated atmospheric mixing could explain both the presence of aerosols in the upper atmosphere and the importance of disequilibrium chemistry. Recently, theoretical studies on atmospheric mixing driven by the large scale circulation in exoplanet have been performed, both using analytical models (Zhang & Showman 2018) and numerical global circulation models of specific planets (Parmentier 2013, Charnay 2015). However, how the strength of the vertical mixing depends on planetary parameters and thus how the current theory can be applied to specific planets is yet unclear.

Here we propose to calculate a grid of global circulation models of Neptune size, tidally locked exoplanets and use this grid as a guide toward a theoretical framework to understand atmospheric mixing (both vertical and horizontal) in these small worlds. The study aim to explain current observations and develop the framework to choose the best targets (including the coming TESS targets) for atmospheric characterisation by JWST. The student will use the SPARC/MITgcm model to study the case of Neptune size planets (which involves updating the opacities to higher metallicities), perform a grid of global circulation models including the atmospheric transport of passive tracers representing either aerosols or chemical products. The student will explore how rotation rate and metallicity affect the mixing of clouds, of photochemical products and of chemical species of interest. The model grid will have two aims. It will be compared to actual Hubble and Spitzer Space Telescope observations and used to prepare observing proposals for James Webb Space Telescope observations. It will also be used as guide to build a theoretical understanding of the main processes driving the atmospheric mixing in Neptune-size exoplanets and their dependence with planet parameters. This theory will follow Pierrehumbert’s line of work on the chaotic mixing of chemical species in atmospheres.The model could be used in particular to interpret the recent Spitzer phase curve observations of GJ436b (PI: Parmentier), a mini-Neptune in a slightly eccentric orbit, which dayside spectrum is indicative of disequilibrium chemistry (e.g. Morley 2017).

Atmospheric circulation of hot gaseous exoplanets.
Vivien Parmentier

Most exoplanets for which atmospheric characterization is possible are tidally locked, hot, gaseous exoplanets named hot Jupiters. Their dayside is always facing the star while their nightside is never illuminated. A strong atmospheric circulation with km/s winds transfer energy from the dayside to the nightside of the planet, shaping the large-scale temperature, chemical and cloud maps of the planet.
With current facilities and coming observatories, the 2D (height-longitude) map of the temperature, the chemical composition and the cloud coverage have been observed for a dozen planets. In the next five to ten years, around a hundred exo-planets atmospheres should be characterized at this level, allowing the determination of trends between intrinsic planet properties such as rotation rate, equilibrium temperature and observables, such as temperature distribution, cloud distribution, wind speed etc.
In order to prepare and interpret coming observations, a change of scale is necessary on the theory side. The student will use a combination of analytical models and an already existing grid of ~500 global circulation models to understand the theoretical trends between atmospheric properties and planet properties. How the wind speed scales with equilibrium temperature? How the dayside mean temperature scales with atmospheric metallicity? How the day/night temperature contrast scales with rotation period? How the mean vertical transport varies with insolation?
The theoretical trends will first be compared to current observation of the transmission, emission spectrum and phase curves of hot Jupiters. Then the student will apply for telescope time with current facilities (such as the Hubble Space Telescope) or coming ones (such the James Webb Space Telescope) to test the validity of the theoretical predictions.

The 3D, high resolution spectrum of exoplanets.
Vivien Parmentier

In the past ten years high resolution spectrum of exoplanet atmospheres became available from large ground based telescopes. Instruments such as NIRSPEC at the Keck telescope (Hawaii) or CRIRES at the Very Large Telescope (Chile) have been able to resolve individual spectral lines of both atoms (such as sodium) and molecules (such as water and carbon monoxide). These observations not only allowed the detection and quantification of molecules but have provided the first direct measurement of an exoplanet atmospheric wind speed. With more than ten new instruments about to come to the sky in large telescope worldwide (among them ESPRESSO at the Very Large Telescope and CARMENES at the Calar Alto have begun observing while SPIRou at the Canada France-Hawaii Telescope should start soon), high resolution spectra of exoplanet will be measured routinely with high accuracy. Further ahead, high resolution instruments such as METIS on the European Extremely Large Telescope will become our best chance to detect biosignatures in other worlds.
Despite these exciting prospects, very little work has been done to interpret these observations. We learned from low resolution spectroscopy with space-based telescopes such as Hubble( or Spitzer or soon the James Webb Space Telescope) that exoplanetary atmospheres are complex - particularly in Hot Jupiters, the main targets of current observations. These planets have the size of Jupiter but orbit much close to their star than mercury. They are tidally-locked, with a dayside heated to several thousands of degrees while their nightside never sees the star. Strong often supersonic winds transfer energy from the dayside to the nightside. Whereas the dayside can be hot enough to break molecules apart, the nightside shows signs of clouds made of rocky material such as silicates.
High-resolution spectroscopic observations are currently interpreted using one-dimensional atmospheric models. Whereas these can be used to estimate atmospheric wind speeds and abundances from current observations, they can be biased towards the wrong results. New more precise information such as recent observations by the ESPRESSO instrument can resolve the planetary limb during a transit, showing clear signs of 3D asymetries.
For this project we propose to use a state-of-the-art 3-dimensional global circulation model of hot exoplanets (the SPARC/MITgcm) to investigate the effects of a 3D thermal and wind structure on high-resolution observations. The dynamical model can predict complex thermal and chemical structure, cloud spatial distribution and wind patterns for a wide range of exoplanets. These outputs will be used by NEMESIS, a state-of-the-art radiative transfer code developed in Oxford by Pat Irwin to calculate with a high precision the shape and position of the molecular lines observed with high-resolution spectrocscopy.
The student will determine the observational consequences of varying planetary parameters, such as equilibrium temperature, wind drag, gravity, cloud composition and re-interpret current observations. We will also investigate what best 3D line shapes parametrization should be used when retrieving wind speed and abundances from the observations which should greatly enhanced the detectability of atmospheric species in these planets. Observing proposals in collaboration with local Oxford observers (such as Suzanne Aigrain in the astrophysics sub-department) will be written to obtain time on major observatories around the world following the insights gained from the modelling work.

Understanding the spectrum of three-dimensional exo-atmospheres. .
Vivien Parmentier & Patrick Irwin

The last decade has seen the discovery of thousands of exoplanets but, more excitingly, the characterization of their atmospheres has become possible. We now have spectra of dozens of planets, ranging from hot Jupiters, to warm Neptunes to temperate Earth-size worlds. In the hottest planets, which are easier to observe, molecules such as sodium, water, magnesium and carbon monoxide have already been detected. Quantifying their abundances remains the challenge of the coming decade.
The information we gather from exoplanet atmospheres only gives us a partial view of the atmosphere’s properties. We can measure physical quantities averaged over the limb of the planet during transit and over the dayside of the planet during secondary eclipse. For outstanding targets longitudinal information can be obtained by measuring the phase curve of the planet. Exoplanet atmospheres are, however, inherently three-dimensional objects. This is particularly true for the current sample of well-characterized close-in, hot and tidally locked planets dominated by the intense and inhomogeneous irradiation they receive. Even more challenging are the clouds that have been detected in most planets studied so far. They often hide an unknown fraction of the atmosphere, leading to the biggest uncertainty in the determination of molecular abundances, typically of the order of a factor a hundred to ten thousands.
In the next five years three groundbreaking missions will be launched that will revolutionize our view of exoplanet atmospheres. TESS and CHEOPS will find and characterize thousands of new exoplanets of which hundreds will be orbiting very bright stars and become golden targets for atmospheric characterization. The James Webb Space Telescope and possibly the ARIEL mission will obtain spectra of around a hundred of planets with a quality orders of magnitude better than currently available data.
In this project, the student will learn how to use the radiative transfer code NEMESIS in order to calculate the transmission spectrum, the eclipse spectrum and the phase curve of more than 500 hot Jupiter global circulation models exploring key parameters of exoplanets atmospheres (equilibrium temperature, rotation period, gravity, metallicity, atmospheric drag). As a first step, the student will compare the spectrum from the 3D models and equivalent 1D spectrum used traditionally to interpret an exoplanet’s atmospheric observations and highlight the biases produced by the 1D approach, particularly in the James Webb Space Telescope era. Then the student will incorporate the grid of calculated observables into the retrieval framework of NEMESIS, creating the first « 3D retrieval » framework able to interpret jointly the information gathered from transmission spectrum, emission spectrum and phase curve observations. Finally, the student will be reached how to use the global circulation model SPARC/MITgcm in order to refine or extend the grid of models as necessary, for example by adding the radiative feedback of cloud opacities directly inside the atmospheric circulation calculations. With a start date in 2020, the student would be well placed to analyse data collected during JWST’s first year of operations, and will analyse data from early release science (ERS) proposals and hopefully contribute to applications for further time.

Retrieval of Exoplanetary Phase Curve Spectra
Patrick Irwin (AOPP)

The discovery of exoplanets in 1995, just over twenty years ago, has ushered in a golden new age in scientific exploration. Over 5000 exoplanets have now been discovered and the study of these worlds has profound implications for our understanding of human evolution and our place in the universe. New missions, such as the James Webb Space Telescope (JWST), due for launch in 2021, will return better exoplanetary observations, including: transit spectra, where the planet passes either in front of or behind the host star; direct-imaging observations, where the planet is far enough from its star that its light can be cleanly discriminated; and phase curve spectra, where the emission and reflection spectrum of the planet is monitored at all phases in its orbit. Further into the future, potential dedicated exoplanet missions such as the ARIEL could enable greatly improved atmospheric characterisation through more precise transit and phase curve measurements, and missions such as WFIRST could record directly-imaged planetary reflection/emission spectra.

This is an exciting time of exoplanetary science and discovery and one of rapid development and advance. An area that we believe could be extremely important is phase curve observations. Here, the emitted and reflected spectra from close-orbiting tidally locked planets can be extracted at many points (or phases) around the planet’s orbit. Such observations can be used to extract longitudinal variations in the planet’s atmospheric temperature and composition. Up until now, the main method for doing this has been 1-D ‘retrieval’ models, where for each phase in the planet’s orbit the temperature and composition has been estimated with a single profile, assumed to be the same at all points on the visible disc. In the attached animated figure, we have computed from a GCM-based model what the exoplanet WASP-43b might look like at all phases to an observer able to spatially resolve the disc at a wavelength 4.5 microns (1st image of figure). If we take such a model, compute disc-averaged spectra, invert them with a 1-D retrieval model to retrieve temperature and composition, and then reconstruct the modelled spatially-resolved appearance of the planet (2nd image of figure) we can see that this approach is geometrically inaccurate and tends to smooth out longitudinal variations. To better analyse phase curve observations we have developed a novel 2.5-D retrieval technique as part of our NEMESIS radiative transfer and retrieval model that we believe represents a significant improvement over the 1-D approach. In our new model we retrieve the temperature and composition at all longitudes simultaneously, assuming that variations in latitude vary as (cos(latitude))^n, where n is a chosen coefficient (usually 0.25). Retrieving from the synthetic GCM-based phase curves with our 2.5-D model (3rd image of figure) we can see that we are able to improve our longitudinal resolution and obtain more realistic temperature variations.

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Figure 1. Modelled/retrieved appearance of WASP-43b during its orbit about its star at 4.5 microns to an observer able to spatially resolve the disc. 1) Modelled appearance from a GCM-based model. 2) Retrieved appearance with a 1-D retrieval model. 3) Retrieved appearance with our new 2.5-D retrieval model.

In this project the student will take our newly-developed 2.5-D retrieval model, and use it to: A) make new and improved retrievals of temperature and composition from existing phase curve spectral observations; B) explore what new observations could be made with existing telescopes and potentially propose for observation time; and C) examine how such retrievals could be improved by phase curve observations from the James Webb Space Telescope and ARIEL and determine how these observations could be best configured to maximise their scientific return. Such a programme of study will place the student in an excellent position to process the resulting data from JWST. The student will also investigate how such observations might best be combined with transit observations and explore how the presence of clouds affects these observations and retrievals and how clouds might best be accounted for in our retrieval analyses.

Exploring clouds and gaseous abundances in the atmospheres of Uranus and Neptune
Patrick Irwin (AOPP)

Uranus and Neptune, known as the “Ice Giants” are amongst the most mysterious and poorly understood planets in our solar system. The poles of Uranus are tipped over by an extraordinary 98º (compared with an obliquity of 23.5º for the Earth) leading to enormous annual variations in solar forcing, with the poles annually receiving more sunlight per unit area than the equator! In contrast, Neptune’s obliquity of 29º appears much less anomalous. The Voyager 2 fly-bys in 1986 and 1989 provided our only close-up views of these worlds and revealed that Uranus is in almost perfect radiative balance with the Sun, while Neptune emits thermally more than 2.5 times the solar radiation it receives! Perhaps as a result of this imbalance, the atmospheric circulation of Uranus was found to be rather quiescent, while that of Neptune was extraordinarily dynamic and active.

More than a quarter of a century later, the spatial resolution of ground-based telescopes has been transformed by the development of adaptive optics. The activity of Uranus’s atmosphere has been seen to increase dramatically through its equinox in 2007, while Neptune’s atmosphere shows enormous changes in cloud activity. We have been monitoring and these developments with an extensive programme of near-infrared ground-based observations at the Gemini-North Telescope in Hawai’i and ESO’s Very Large Telescope in Chile. Near infrared reflectance spectroscopy enables us to determine the vertical and horizontal distribution of cloud and gaseous abundances in these atmospheres, using our world-leading NEMESIS retrieval code. Recent highlights include the first positive detection of hydrogen sulphide in the atmosphere of Uranus, probable detection in Neptune’s atmosphere and the first ground-based detection of methane variability in Neptune’s atmosphere.

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Figure 1. Uranus (left) and Neptune (right) as observed by the Hubble Space Telescope in 2018. The development of a polar ‘cap’ on Uranus can be seen, which is expected to thicken in the coming years, while a new dark spot can be seen towards the top left of the Neptune image.

As Uranus advances towards its northern summer solstice in 2030, its north pole is swinging towards the Sun and we are starting to see the development of a polar ‘cap’ of haze (Fig. 1). Meanwhile, Neptune has seen the appearance of several mysterious dark spots (similar to the Great Dark Spot seen in 1989 by Voyager 2) whose structure is almost entirely unknown. Continuing visible and near-infrared observations offer an opportunity to better constrain and thus better understand such atmospheric features. At longer wavelengths, in the thermal infrared spatially-resolved observations with VLT/VISIR, Subaru and NASA’s Spitzer Space Telescope have allowed a better understanding of temperature variations and upper atmospheric composition.

The aim of this project is to develop our ground-based visible and near-infrared observing programme and combine these new observations with existing thermal infrared determinations to build a self-consistent picture of the circulation and cloud-forming processes at work on the ice giants during a period of rapid change. This work will help pave the way for future atmospheric studies by ground-based telescopes and also the James Webb Space Telescope, due for launch in 2021. Further in the future, the Ice Giants are being seen as a key future target for planetary space missions with concepts being studied by both NASA and ESA. The successful candidate would be joining a well-established planetary data analysis group that is actively involved in observational astronomy and planetary missions (e.g., the recently completed Cassini mission at Saturn).

Ground-based observations of Jupiter (and Saturn) in support of the NASA Juno mission.
Patrick Irwin (AOPP)

The clouds of the Solar System’s Gas Giants (Jupiter and Saturn) are amongst the most beautiful and complex phenomena displayed by the planets. Their spatial distribution reveals the forces and energy exchange mechanisms (e.g., moist convection) shaping the banded appearance of the planetary weather layers; their vertical structure tells us about the composition and cloud microphysics; and their colour reveals the chemical alteration of aerosols in a giant planet atmosphere. While most clouds are white, the clouds of Jupiter and Saturn are coloured with various hues of yellow and red, but the nature of these colours, or ‘chromophores,’ has yet to be determined, and remains an enduring mystery.

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Figure 1. NASA/Juno’s orbit insertion trajectory and early orbits showing very close perijove passes necessary to probe Jupiter’s gravity field (to determine internal structure), and also avoid radiation damage from Jupiter’s intense radiation belts.

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Figure 2. Left: Colour-composite MUSE image constructed from data recorded on 9th April 2018 (101.C-0097), with the GRS clearly visible. Right: False-colour image, where red is 630 nm (weak atmospheric absorption), green is 490 nm (sensitive to blue-absorbing ‘chromophores’, and blue is 890 nm (strong methane absorption, sensitive only to high level hazes).

The NASA Juno mission arrived at Jupiter in July 2016 and entered into a series of elliptical polar orbits designed to probe Jupiter’s interior structure through measurement of its gravity and magnetic fields and remote sensing of its deep atmosphere. Juno’s highly elliptical orbit (Fig. 1) minimises the damaging effects of Jupiter’s extremely harsh radiation belts, but means that its UV, visible and near-IR observations are mostly of Jupiter’s poles, while microwave observations using the MWR instrument are mostly confined to narrow north-south swaths during perijove (closest approach) passes that lack the global spatial context necessary to interpret them properly. Hence, a global campaign is under way to provide Earth-based observational support for Juno, in which our group closely involved making observations with the MUSE instrument at ESO’s Very Large Telescope (VLT) in Chile. The MUSE (Multi Unit Spectroscopic Explorer) instrument provides an unprecedented opportunity to study the clouds, dynamics and composition of the giant planet atmospheres at a spatial and spectral resolution never before possible. MUSE measures spectral ‘cubes’ in which each pixel of the 300 x 300 field of view is a complete spectrum covering the range spectral 480 to 930 nm (Fig. 2). These ‘cubes’ allow us to map the spatial distribution of the clouds and colouring agents, estimate cloud top levels and determine spatial variations of ammonia abundance. While other ground-based and space-based instruments can provide partial coverage of these wavelengths (usually as images in discrete filters), only MUSE provides the unique combination of spatial and spectral coverage, which makes it a very powerful tool for studying clouds in giant planet atmospheres.

Analysis of existing MUSE observations has been used to model the distribution of cloud, chromophores and ammonia in Jupiter’s atmosphere (https://ora.ox.ac.uk/objects/uuid:a5856d5a-eba1-487b-82c5-0a31761ff218, https://ora.ox.ac.uk/objects/uuid:3ab6381e-3cdf-4975-b255-9f3077ede8b8), but Jupiter’s atmosphere continues to develop and the Juno mission is set to observe for several more years. Hence, continuing observations are vital and more are planned or remain waiting to be processed. At longer wavelengths, instruments such as VLT/VISIR provide thermal mapping that can be used to determine the vertical and spatial distribution of temperature and gaseous abundances. A set of MUSE observations was made within a few minutes of a set of VISIR observations on one night in 2018 and provides a golden opportunity to link cloud features to observed thermal anomalies. We envision that our VLT/MUSE programme could extend to further observations of Saturn also to compare and contrast these two worlds. In addition to these ground-based facilities, NASA’s James Webb Space Telescope (JWST), scheduled for launch in 2021, is planned to make many observations of the gas giants over a wide wavelength range that will need to be understood and interpreted.

The successful candidate would be joining a well-established planetary data analysis group that is actively involved in observational astronomy and planetary missions (e.g., the recently completed Cassini mission at Saturn). In this project, the student will analyse existing VLT/MUSE Jupiter observations and participate in proposing and reducing further measurements, gaining excellent experience in telescopic observational data analysis and gaining a deep insight into the atmospheric circulation and cloud formation on these worlds. The spectral cubes will be analysed with our sophisticated multiple-scattering radiative transfer model, NEMESIS. Given the timeline, it is possible that the student may also analyse JWST observations.