Planetary chemical composition and its origins in protoplanetary disks
In the 2020s, exoplanet studies are increasingly focussing on planetary chemical composition and its origins in protoplanetary disks. Some of the key goals are to link planetary elemental abundances to their formation location and migration history in a disk, and to establish the origins of habitable chemical compositions. Questions also abound concerning the potential chemical diversity of Earth-like worlds, or the origins and nature of various types of planets such as hot Jupiters or super-Earths. To tackle the above problems, we must study the composition, structure, and processes in planet-forming environments.
In my group, we mainly focus on studying protoplanetary disks, using our own observations and models (e.g. Kama et al. 2016; Keyte et al. 2022), but also contributing in large, international ALMA programmes which will provide a wealth of data on protoplanetary disk composition over the next few years. We are also working on projects relating host stars to their disks and planets (see e.g. Jermyn & Kama 2018; Kama et al. 2022), notably EXOHOST, and are heavily involved in the upcoming Ariel mission, which is led from UCL and will characterise the composition of ~1000 planets.
PhD projects are available in my group to work on planet formation science using revolutionary instruments such as the ALMA interferometer or JWST; on the physical-chemical modelling of planet formation processes and environments; and on the connections between planetary systems, their natal disks, and their host stars. You are welcome to contact me to discuss any details.
Contact: Dr. Mihkel Kama (m.kama AT ucl.ac.uk)
* Spectroscopy of Exoplanets
Thousands of exoplanets have been discovered in the recent years. These newly-discovered planets are generally unlike those in our Solar System. Some of the rocky super-Earths are evaporating with complex atmospheric compositions. These planets have a lot in common with the young Earth; the massive amounts of water in their atmospheres can melt rocks and put their constituents into the atmosphere. Similar processes are expected in the atmospheres of the post-impact planets. A prerequisite for advances to be made is the availability of the fundamental atomic and molecular data necessary for interpreting new observations. The unusual conditions found on most known exoplanets, involving elevated temperatures and high fluxes of stellar radiation, means the required data are missing and not readily measurable in the laboratory.
This PhD project will aim to produce comprehensive molecular opacities specific to lava-planets using advanced molecular quantum mechanics allied to high-performance computing in response to the modern challenges of exoplanetary models and retrievals. The results will be incorporated into exoplanet models developed at UCL and made available to the scientific community via the ExoMol database. These models will enable the interpretation of present and future spectroscopic studies of rocky super-Earths. Exactly these types of hot solid planets will be the likely targets of NASA's JWST or ESA's Ariel.
The ExoMol team at UCL is the world leader in providing spectroscopic data for the characterisation and modelling of exoplanets and other hot atmospheres, see http://www.exomol.com. The ERC funded ExoMolHD project is dedicated to providing high quality spectroscopic data. We have an open position to work on one of the following tasks:
1. Generation of molecular line lists for astronomical studies;
2. Poviding precise wavelengths for key molecules applicable for use in high resolution spectroscopic studies performed by telescopes using high resolution instruments;
3. Predicting accurate spectroscopic data on key isotopically-substituted species;
4. Providing temperature-dependent pressure shifts and pressure broadening parameters;
5. Computing photodissociation cross sections and photolysis rates both in and outside thermodynamic equilibrium;
6. Developing appropriate database structures, including detailed opacities, k-tables and precomputed atmospheric models.
We will act to ensure the widest possible utilisation of the data.
* Pandemonium in the planetary graveyard
Defying the notion of the silent graveyard, planetary systems refuse to go quietly into the long night. Instead, a significant fraction show one or more signs of dynamical reanimation, with strong indications of general mayhem during the final stages of stellar evolution. These rejuvenated planetary systems manifest as irregular and complex transit events, transient optical emission features, and variable infrared fluxes from dust production and destruction. Ultimately, all leave their detailed chemical signatures on the surface of the white dwarf stars they orbit, and provide powerful insight into the masses and geochemical structures of the planetary bodies. At UCL, we are leading the study of these evolved planetary systems in the infrared via their dusty debris disks, in the optical via transiting events, and in the ultraviolet where elemental abundances can be measured from the polluted stellar surfaces. The project will involve at least two observational approaches, including but not limited to: studies of available transit data, infrared data that track debris disk variability, and importantly bulk compositions for minor and major planetary bodies.
Contact: Prof Jay Farihi (j.farihi AT ucl.ac.uk)
Unveiling the nature of super-Earths with current and future observatories
Super-Earths, i.e. planets lighter than ten Earth masses, appear to be the most common planets in our galaxy. Being absent in our Solar Systems, their nature is rather mysterious: from their densities we gather there is a large variety of cases, ranging from big rocky planets to small Neptunes or more exotic types. The chemical composition and state of their atmospheres, can be used as a powerful diagnostic of the history, formation mechanisms and evolution of these planets. In the past fifteen years, the UCL exoplanet team led by Prof. Tinetti has worked at the forefront of the spectral/photometric measurements of exoplanet atmospheres and their interpretation, with molecular species being detected in the atmospheres of giant planets and super-Earths (e.g. extremely hot 55 Cnc-e and habitable-zone K2-18b). As part of the PhD, the student will have the opportunity to work in collaboration with Prof. Giovanna Tinetti, Dr. Angelos Tsiaras and Dr. Yuichi Ito on a number of aspects connected with the observations and modelling of super-Earths’ atmospheres with current and future observatories (HST, JWST) and dedicated space missions (ARIEL).
Using deep learning to model complex chemistries of exoplanet atmospheres
To determine the make-up of an exoplanet atmosphere, we solve the Bayesian inverse problem by iteratively fitting atmospheric models to the data to determine the statistically most likely distribution of atmospheric model parameters, such as water abundance (e.g. Al-Rafaie et al. 2021). Current atmospheric retrieval frameworks approximate the complex 3D nature of planetary atmospheres through 1D radiative transfer models coupled with heuristic or equilibrium chemical schemes. These approximations are sufficient in effectively describing Hubble data but break down profoundly when applied to the more complex JWST and Ariel data. For chemistry, the fundamental reactions, mixing through the atmosphere and its evolution over thousands of years, must be accounted for to solve the next-generation inverse problem. Current modelling of these disequilibrium schemes (e.g. Venot et al. 2012) are too slow or make approximations that oversimplify. Given that the Ariel mission will increase the number of spectroscopically characterised planets from 50 currently to 1000, the need for faster modelling techniques that maintain accuracy is apparent. This PhD project will focus on the fundamental issues of improving sampling speeds by approximating complex chemical models using neural networks (NNs) and improving the NNs' explainability. Simply put, we want to design a neural network that can both rapidly simulate non-linear chemical networks and explain why it chose that particular solution. The PhD candidate will build on initial work by the UCL Exoplanet group (e.g. Yip et al. 2021) to extend the atmospheric modelling framework to modelling complex disequilibrium/photochemical atmospheric chemistry pathways.
Contact: Dr Ingo Waldmann (ingo.waldmann AT ucl.ac.uk)
The orbits of charged particles in planetary magnetospheres, and the dynamics of the orbits of stars in galaxies, are the result of how the ‘test particles’ within each system - individual ions / electrons or individual stars - respond to either gravitational or electromagnetic fields. This project is principally a project in planetary plasma physics, but has an 'interdisciplinary element' based on exploring analysis techniques used in other areas of astrophysics, and seeing how they can be applied to model spacecraft magnetic field data which probe the plasma sheet regions of the planets Saturn and Jupiter. The main 'strands' of the project would be: (i) To develop a general magnetic field model for a planetary magnetospheric plasma sheet, by testing a variety of different multi-parameter fitting techniques; (ii) A minor work component related to the less 'fully formed' idea of exploring the role of action integrals (a central concept in dynamical systems) in modelling the trajectories of charged particles in magnetic fields - and whether this could lead to alternative, useful ways to represent such trajectories in a global magnetosphere.
Further update: The main part of the project described above has nowbeen taken on by one of our new PhD cohort.
However, similar 'giant planet' projects in the areas of magnetosphere-ionosphere coupling and magnetopause dynamics may be available.
Contact: Prof Nick Achilleos (nicholas.achilleos AT ucl.ac.uk)
* The Planetary X-ray Revolution
For decades, the X-ray waveband has been significantly underutilised in the study of planetary systems, yet it provides unique and essential insights into the composition of planetary bodies and their moons (important for habitability and formation), planetary aurora and magnetic fields (critical for protecting planets from the stellar winds of their partner star), and the energetic environment surrounding planets.
UCL are the world-leaders in the X-ray study of the outer planets systems. To address the shortages of planetary X-ray studies, the NASA and ESA flagship observatories (Chandra and XMM-Newton) have awarded our group the largest dataset of planetary X-ray observations ever acquired. This includes: the first X-ray observation of an interstellar comet; 1000+ hours of X-ray observations of Jupiter and its moons, coordinated with the Juno spacecraft (including the first ever spatially resolved Jupiter XUV observation) and 120 hours of X-ray observations of Uranus planned contemporaneous with JWST and Hubble (dwarfing our previous few-hour observation that was reported in the international media last year). Much of this data is untouched and rich with potential for new and transformative discoveries and our group has a strong track record for publishing in Nature and Science journals.
The observations above were acquired with Earth-orbit X-ray observatories. However, recent developments in X-ray instrumentation mean that lightweight X-ray instruments can now be flown to the planets. This planetary X-ray revolution begins in 2025 with the ESA-JAXA BepiColombo mission to Mercury and the ESA-CAS SMILE mission for Earth observation. We are currently working with partners at NASA, ESA and CAS to define and lead the first X-ray instrumentation to the outer planets and their moons. Students will have the opportunity to take key roles in developing the science for future NASA/ESA/CAS mission concepts and proposals (currently: LEM; COMPASS and Tianwen-4 spacecraft proposals), and with the paradigm-shifts in understanding that they will enable.
Contact: Dr. William Dunn (w.dunn AT ucl.ac.uk)