DPhil (PhD) Projects in Planetary & Exoplanetary Physics 2021

Oxford’s Planetary Physics group specialises in the exploration of planetary atmospheres and surfaces, with expertise ranging from hardware development to spectral analysis; ground-based astronomy to spacecraft operations; and numerical simulations of planetary climate to laboratory experiments of geophysical fluid dynamics. Our interests extend from the environments found in our own Solar System to the extreme conditions found in Extrasolar Planets and Brown Dwarfs. An overview of our research can be found on the Planetary Science page, and an overview of exoplanet research at Oxford can be found on the Oxford Exoplanet page.

Several projects are available within the group in the coming year. They typically involve the measurement and modelling of visible and infrared spectra of planetary objects, using these data to understand and model their environmental conditions (atmospheric dynamics, composition and clouds; thermophysical properties and compositions of surfaces). Next year we shall be looking for students in the following areas:

  • The transit spectra of exoplanets
  • Re-constructing Primitive Meteoritic Materials in the Laboratory to support remote sensing of asteroids and comets
  • Ground and space-based telescope observations of the Ice Giants (Uranus and Neptune)
  • Spectroscopic monitoring of Jupiter’s atmospheric dynamics and chemistry to support planetary missions
  • Dynamical modelling of storms and eddies in the atmospheres of Jupiter and Saturn
  • Modelling the circulation of slowly rotating planets, such as Venus and Titan
  • Climate modeling of terrestrial exoplanets, and fluid or rocky Super Earths or Sub-Neptunes,including possible effects of condensible species.
  • Modeling escape of planetary atmospheres to space.
  • Metrology and the InSight seismometer on Mars. When is it wind, when is it a Marsquake?
  • Infrared properties of asteroids and comets, from telescopes to returned samples

Project examples:

Exploring clouds and gaseous abundances in the atmospheres of Uranus and Neptune

Supervisor: Patrick Irwin (AOPP)
Project ref: AOPP/PGJI/1/2020

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.

V2 Photo.png

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).

This project will be computationally intensive using Fortran, IDL, python and others, so a physics/computing/mathematics degree is preferred.

Radiative fluxes in the atmosphere of Venus

Supervisor: Patrick Irwin (AOPP)
Project ref: AOPP/PGJI/2/2020. Additional supervisor Colin Wilson.

It’s always cloudy on Venus. Its main cloud deck extends from about 50 – 70 km altitude and is composed primarily of sulphuric acid, mixed with water, and there’s evidence for the existence of minor constituents in the main cloud deck and at other altitudes, of unknown composition. The clouds have a huge effect on both solar and thermal radiative fluxes in the atmosphere, playing a major part in greenhouse warming and in forcing the atmospheric circulation. Measuring these fluxes therefore is critical for understanding the dynamics of the atmosphere.

Radiative fluxes within the atmosphere have been measured by the Pioneer Venus, Venera, and VeGa descent probes, and will be measured from future descent probes and balloons in the atmosphere of Venus, like the ones proposed in the NASA Venus Flagship Mission Study.

In this studentship, modern radiative transfer codes and new cloud constraints from missions such as Venus Express will be applied to a reanalysis of Pioneer Venus and Venera/VeGa descent profiles, to retrieve cloud abundances, water vapour profiles and more as a function of altitude. Calculations to optimise spectral channels for future probe- or balloon-borne radiometers will be performed. Further calculations relevant to surface imaging and/or solar power availability could also be performed, depending on research priorities and mission opportunities as the project progresses.

This project will be computationally intensive using Fortran, IDL, python and others, so a physics/computing/mathematics degree is preferred

Exoplanet climate dynamics

Supervisor: Raymond Pierrehumbert (AOPP)
Project ref: AOPP/RTP/1/2020

The newly discovered exoplanets present possibilities for a diverse range of climate situations not encountered in our own solar system. The demands of this new subject challenge the limits of current modelling capabilities, and while they involve the same underlying physical components as are familiar from the Solar systems, these components are present in novel combinations. This project involves a range of modeling and theoretical activities aimed at understanding the new climates. There is a particular emphasis on identifying potentially observable consequences of various exoplanet climate phenomena. I am seeking up to two DPhil (PhD) students for work in the general area of exoplanet climate modeling, with a particular emphasis on smaller planets (super-Earth size and below) which can have a more rich diversity of atmospheric compositions than the hydrogen-dominated gas giants, and offer a range of additional phenomena associated with the possibility of a condensed rocky, icy or liquid surface. In all of our work, there is an emphasis on maintaining an appropriate balance between theoretical work or idealized modeling aimed at elucidating fundamental principles, and work with more comprehensive general circulation models.

Two DPhil (PhD) students are sought for exoclimate projects in one or more of the following general areas: (1) Atmospheric escape and implications for habitability of M-star planets, (2) Baroclinic instability on tide-locked slowly rotating planets (3) Exchange of volatiles between planetary interiors and atmospheres, including effects of magma oceans on atmospheres and implications of the deep carbon cycle and CO2 outgassing on the outer edge of the habitable zone. (4) Transient phenomena in exoplanet atmospheres, their use in constraining planetary characteristics, and prospects for detection with future observational programs. (5) Spatially inhomogeneous chemistry of planetary atmospheres driven by mixing due to idealized large scale flow. In addition to these specific project areas, I am open to suggestions from students who wish to take the initiative in defining their own research direction within the general area of exoplanet climate and climate evolution.

There are also possibilities for DPhil students to work in collaboration with the ERC EXOCONDENSE project on topics related to effect of condensible substances on exoplanet climate dynamics, and on generalised moist convection in planetary atmospheres. Further information on EXOCONDENSE is available here Topics of interest include both generalized moist convection and its parameterization and condensation effects (including clouds) in planetary scale dynamics. Further information on EXOCONDENSE can be found at the EXOCONDENSE project page.

These projects require a thorough understanding of fundamental physics, including thermodynamics, mechanics and electromagnetic radiation, as well as a facility with analysis of mathematical models. Familiarity with physical chemistry is also desirable. Hence, a first degree in Physics, Mathematics or a related discipline is required. The projects involve considerable use of computational techniques, so basic familiarity with numerical analysis and familiarity with programming techniques in some computer language is required. The main programming languages used are Python and Fortran, but prior experience with these specific languages, while desirable, is not required.

A framework to understand atmospheric mixing in Neptune size exoplanets

Supervisor: Vivien Parmentier (AOPP)
Project ref: AOPP/VP/1/2020

The atmospheres of a handful of Neptune-size planets 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).

Students from both EU and non-EU countries are invited to apply.

Turbulence, convection and regional scale atmospheric dynamics on Titan

Supervisor: Vivien Parmentier (AOPP)
Project ref: AOPP/VP/2/2020. Additional advisors : Maxence LeFevre, Colin Wilson, Peter Read (AOPP), Aymeric Spiga (LMD)

Titan, the largest moon of Saturn, is an extraordinary world. It has a dense atmosphere composed largely of Nitrogen, with a hydrocarbon-based hydrological cycle featuring evaporation and precipitation, ricers and seas. Its inventory of complex hydrocarbon molecules and its subterranean water ocean make it an important target for the study of pre-biotic chemistry, to understand which environments & conditions do or do not lead to the development of life.

In 2027, NASA will launch a spacecraft to Titan, Saturn’s largest moon, carrying a nuclear-powered autonomous rotorcraft called Dragonfly. Equipped with sophisticated chemical, geophysical and meteorological instrumentation, Dragonfly will fly from site to site, characterising the environments it discovers. The Oxford-designed wind sensor it carries will serve a dual role: it will measure winds for the purpose of atmospheric dynamics & volatile transport, but also has an important role in flight safety, determining whether the atmosphere is calm enough for safe flight.

In this studentship, atmospheric modelling tools developed for Earth and Mars will be applied to Titan, to study winds in the lowest few kilometres of the atmosphere, the region in which Titan Dragonfly will carry out its mission. The studentship could focus on the kinds of small-scale turbulence associated with the sand dunes and crater rims amongst which Dragonfly will operate; or it focus on regional circulation patterns, important for volatile transport and/or cloud/haze formation, which Dragonfly’s meteorological suite will reveal. Models will be validated primarily using data from the Cassini/Huygens mission.

The work will be carried out in collaboration with other Titan modelling groups in the USA, France and Germany.

This project will be computationally intensive, using programming languages such as Fortran, IDL, MATLAB and others, so a physics/computing/mathematics background is preferred, and a first degree in Physics, Mathematics or a related discipline is required.

Atmospheric circulation of hot gaseous exoplanets

Supervisor: Vivien Parmentier (AOPP)

Project ref: AOPP/VP/3/2020

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.

This project will be computationally intensive using Fortran, IDL and others, so a physics/computing/mathematics degree is preferred.

Students from both EU and non-EU countries are invited to apply.

Further Astrophysics Exoplanetary Projects

Exoplanetary projects are also available at Astrophysics. Please visit their web site for a list of further projects. Applications to both Astrophysics and AOPP are encouraged.