This research group focuses on the study of exoplanets and their host stars. Exoplanets is a fast-paced, discovery-driven field and Oxford is home to one of its largest research hubs in Europe. As most exoplanets are studied via indirect means, the study of exoplanets is closely linked with stellar physics.
Exoplanets are complex physical systems requiring expertise from a wide range of research fields. For links to related research in other departments, see the Oxford Network for Planets.
Formation and dynamics of planetary systems
Most of the extrasolar planetary systems detected so far have characteristics very different from that of the solar system. Although these observations have not put into question the basic processes of planet formation as they were understood on the basis of our solar system, it has become clear that dynamical interactions between planets and the protoplanetary disc in which they formed, and between planets themselves in multiple systems, have been essential in shaping these systems. In Oxford we study these interactions by means of analytical and numerical investigations.
Most exoplanets to date have been found by indirect means, via the impact that planets have on the light we receive from their host stars. The radial velocity (RV) method searches for the tiny "wobble" of the host star caused by the gravitational tug of orbiting planet(s), while the transit method searches for the "micro-eclipses" that occur when a fortuitously aligned planet passes in front of its host star as seen from the Earth. We use both methods to discover new exoplanets and measure their orbits, masses and radii. We have a strong involvement in space-based transit surveys (TESS and PLATO) and in next-generation radial velocity surveys (HARPS3/THE) aiming to detect habitable planets around nearby stars. We use advanced statistical and machine-learning methods to analyse data from transit and RV surveys and disentangle weak and rare planetary signals from dominant instrumental and astrophysical noise sources. We also run the Planethunters-TESS citizen science project.
We are also involved new high-contrast, high spatial resolution instruments for the world's largest telescopes, such as HARMONI on the E-ELT. These instruments will enable astronomers to directly image exoplanets around bright, nearby stars, and to take spectra of these planets in order to study their atmospheres.
We use a range of different techniques to measure spectra of exoplanets, and infer information about the structure, composition and dynamics of their atmospheres. If we separate the photons coming from a planet from those coming from its host star, we can measure its spectrum directly. This can be done using purpose-built high-contrast, high spatial resolution instruments on large telescopes, or using high-dispersion spectrographs, by exploiting the motion of the planet relative to the star (and to the Earth). If a planet transits, we can also observe transits and eclipses as a function of wavelength and construct emission or transmission spectra that way. These different types of observations allow us to probe very different types of planets, from ultra-hot lava planets to warm Neptune-sized objects or cool Jupiters. They also probe different aspects of the planets' atmospheres, including their structure (variation of temperature with pressure), content (atoms, molecules, clouds and hazes), and dynamics (winds and rotation).
Given observations of an exoplanet's spectrum, we then wish to infer the properties of its atmosphere. This is done by postulating a structure and composition, applying the laws of radiative transfer to generate a model spectrum, and comparing it to the data, then altering the parameters of the model to optimize the match between the two. This process is known as atmospheric retrieval, and planetary scientists have been doing it for decades for solar system planets.
Oxford is also a world-leading centre for planetary atmosphere modelling. Our simulations explore the vast and eclectic zoo of exoplanets, from uninhabitable terrestrial planets such as lava planets, through habitable-zone terrestrial planets, and up through sub-Neptunes and hot and ultra-hot Jupiters and brown dwarfs. Pioneering work on long-term atmospheric evolution and habitability is being done at the interface of astrophysical, atmospheric and geochemistry disciplines.
Stellar activity and rotation
As well as developing tools for detecting exoplanets, we are also interested in stellar variability caused by magnetic activity and convection. This is one of the most important noise sources limiting exoplanet detection via the transit and radial velocity methods, but it is also a rich source of information about stellar evolution and, consequently, the evolution of the planets which orbit those stars.