Brown Dwarfs orbiting White Dwarfs are some of the most extreme irradiated atmospheric environments known. Modelling the 3D atmospheres of the Brown Dwarf is a challenging prospect due to their extremely high irradiance (~ 3 times a typical HJ) at UV wavelengths, high gravity (~2 magnitudes greater than HJs) and rapid rotation rates (~ 2 hours). We present some preliminary GCM modelling of the WD0137−349B system using the ExoFMS GCM with a simplified grey radiative-transfer scheme. We find a very different thermal and dynamical environment compared to typical HJ GCM simulations, suggesting these systems are an excellent test-bed to push GCM modelling in extreme environments.
In studies of the XUV-driven evaporation of exoplanet atmospheres, it is typically assumed that the time evolution of the unobservable EUV energy band matches that of the more well studied X-rays, and that this atmospheric evolution primarily occurs in the first 100 Myr. We combine the empirical relations for extrapolation to the EUV from X-rays with those for X-ray time evolution, and show that the decline of EUV emission is slower than that of X-ray emission. EUV photons may therefore be able to drive atmospheric escape at rates sufficient for substantial evolution of the planetary atmosphere for much longer than generally acknowledged. We additionally apply these findings to a small sample of planets discovered by the K2 mission in the open cluster Praesepe.
Analysing currently available observations of exoplanetary atmospheres often invoke large and correlated parameter spaces that can be difficult to map or constrain. This is true for both: the data analysis of observations as well as the theoretical modelling of their atmospheres. In many aspects, data mining and non-linearity challenges encountered in other data intensive fields are directly transferable to the field of extrasolar planets as well as planetary sciences.
In this talk, I will discuss the use of information entropy and deep learning to increase the efficiency of atmospheric retrieval algorithms and to exploit the sparsity of a low to mid-resolution exoplanet spectrum using information content informed optimal binning.
Large impacts can progressively transform the bulk atmospheres of rocky planets, by removing the original atmosphere and leaving behind an atmosphere comprised of the delivered volatiles, processed thermally by the impact itself and subsequently by photochemistry. I combine models of atmospheric erosion and delivery, and trace atmospheric evolution throughout and after the tail end of accretion. I discuss current and future planned experimental work to explore chemical signatures of impacts within these atmospheres.
Evidence for the survival of outer planetary systems to the white dwarf phase comes from observations of planetary material polluting the atmospheres of white dwarfs. These observations are unique in providing the composition of exo-planetary material. Infrared observations of dust very close to white dwarfs reveal how planetary material arrives in the atmospheres of white dwarfs. We expect the scattering of planetary bodies that leads to pollution to be a stochastic process, with the potential for variability on human timescales. Such variability has been found for the white dwarf WDJ0959-0200 among others, where a drop in K band flux of 20% was observed within one year. I present the results from a large scale near-infrared monitoring campaign of ~80% of all known dusty white dwarfs using UKIRT (WFCAM) over a baseline of 3 years. I address the following questions: How often do dust discs vary? In what way do they change? Are all discs capable of varying?
M-Dwarfs are the most common spectral type in our galaxy, making up around 70% of the nearby stellar population. These stars have also played host to a number of exciting exoplanet discoveries in recent years, such as the seven terrestrial planets discovered in orbit around TRAPPIST-1 and the detection of water vapour in the atmosphere of K2-18b. However, when compared to other spectral types of star, we see that M-Dwarfs are not as well understood. In particular, the relationships between Mass, Temperature and Radius are not as well defined as they are for F/G/K stars. To rectify this, we must study a wide sample of these stars and measure their properties in an attempt to refine theoretical models. I will present the work being done with the Next Generation Transit Survey (NGTS) to identify both planets and stellar/substellar companions to M-Dwarfs and explain how these observations could help us to better understand these stars. NGTS has already identified and published many interesting M-Dwarf systems, and future follow up and investigation of our extensive number of candidates may allow us to further our understanding of M-Dwarfs. This will result in an improvement in our ability to characterise M-dwarf planetary candidates discovered by both current and upcoming missions.
M dwarf (dM) stars are prime targets for exoplanet studies, but their magnetic activity and physical parameters are not well constrained yet. They begin their lives rapidly rotating and with high levels of activity, but as time goes on, they slow down via magnetically channeled winds, whilst their activity decreases. This rotation-activity relation leads to an age-activity relationship, which we aim to constrain for ages up to 10 Gyr. With this purpose, we are conducting a survey of dM + white dwarf (WD) wide binary systems identified in Gaia DR2, to use WDs as age calibrators. We also aim to investigate the possibility of prolonged rotation and activity in dMs due to tidal interaction with close-in planets, an effect previously observed for Sun-like stars with hot Jupiters by Poppenhaeger and Wolk (2014), that would biased the calibration of an age-activity relationship. We present some basic theoretical estimations on the strength of tidal forces exerted by planets in M dwarf hosts and on the angular momentum exchanged between these systems.
Stellar flares are explosive phenomena which may seriously affect the habitability of planets around them. This is particularly true for ultracool dwarfs (e.g. TRAPPIST-1) which host Earth-sized planets in their close proximity "habitable zone". These stars can flare with energies greater than the largest Solar flares, yet the maximum energies and occurrence rates of these explosive events remain unconstrained. In this talk I will present results from our search for ultracool dwarf flares using the Next Generation Transit Survey. I will show how our work is revealing the true flaring behaviour of these systems, including the detection of a super-Carrington event flare from an L2.5 dwarf and the impact these flares may have on nearby exoplanets.
It is well known that the presence of giant planets around a star is dependant on its metallicity. In this talk, I will present the results of my investigation into the giant planet-metallicity correlation for a subset of giant planets - a homogeneous, unbiased set of 217 hot Jupiters taken from nearly 15 years of wide-field ground-based surveys. I compared the host star metallicity to that of field stars using the Besançon Galaxy model, allowing for a metallicity measurement offset between the two sets. I will discuss my results, how they relate to past literature, and put them in context of current hot Jupiter formation theories.
Convergent migration involving multiple planets embedded in a viscous protoplanetary disc is expected to produce a chain of planets in mean motion resonances, but the multiplanet systems observed by the Kepler spacecraft are generally not in resonance. Under equivalent conditions, where in a viscous disc convergent migration will form a long-term stable system of planets in a chain of mean motion resonances, migration in an inviscid disc often produces a system which is highly dynamically unstable. This difference is due to the dependence of disc-planet interactions on the disc turbulent viscosity, as a series of our recent two and three dimensional simulation campaigns have explored. In particular, if planets are massive enough to significantly perturb the disc surface density and drive vortex formation, the smooth capture of planets into mean motion resonances is disrupted. As planets pile up in close orbits, not protected by resonances, close encounters increase the probability of planet-planet collisions, even while the gas disc is still present. While inviscid discs often produce unstable non-resonant systems, stable, closely packed, non-resonant systems can also be formed.
We carry out three-dimensional SPH simulations to study whether planets can survive in self-gravitating protoplanetary discs. These discs are modelled with a cooling prescription that mimics a real disc which is only gravitationally unstable in the outer regions. We do this by modelling the cooling using a simplified method such that the cooling time in the outer parts of the disc is shorter than in the inner regions, as expected in real discs. We find that giant planets (greater than Saturn mass) initially migrate inwards very rapidly, but do slow down in the inner gravitationally stable regions of the disc. A similar trend is also seen with low mass planets (a few Earth masses). This is in contrast to previous studies where the cooling was modelled in a more simplified manner where regardless of mass, the planets were unable to slow down their inward migration. This shows the important effect the thermodynamics has on planet migration. In a broader context, these results show that planets that form in the early stages of the discs' evolution, when they are still quite massive and self-gravitating, can survive.