Abstracts

Oliver Shorttle (University of Cambridge) - Tracing the building blocks of habitable worlds from disks to planets

The elements carbon, water, and sulfur, are some of the most abundant in the universe, are major constituents of planetary atmospheres, and are essential constituents of all known life. Yet, their arrival on planets and distribution therein remains uncertain. In large part this is due to their volatile nature: each element may form species that readily enter the gas phase at the temperatures and pressures of planetary differentiation. Here, we combine observations from Earth with observations from protoplanetary disks. Combining geochemical and astrophysical observations in this way gives us new insight into how these elements were partitioned between gas and dust during accretion, and where they are ultimately distributed in planets.

Dan Bower (University of Bern) - Some like it hot: the evolution of hot and molten terrestrial interiors

A new frontier is opening in exoplanetary science that aims to characterise the atmospheres and interiors of rocky worlds that orbit distant stars. Due to detection biases, many of the rocky planets we have discovered are a few Earth masses and are subject to intense insolation from their host star. Hence the surface temperatures on such planets can be near or above the melting point of planetary materials, and a strong day-night temperature contrast is imposed if the planet is tidally-locked. For these planets, understanding the exchange of energy and mass between reservoirs---notably the interior and atmosphere---provides not only a means to interpret astrophysical data, but also an opportunity to peer into the earliest days of Earth evolution. In this broad-ranging talk, I will discuss how the "magma ocean" stage of planetary evolution can set the stage for the subsequent thermal evolution of a planet by establishing chemical stratification within the interior. For a partially molten planet, the properties of silicate materials as well as the atmospheric speciation can explain trends in the mass-radius relationship observed for rocky exoplanets. I will furthermore present geodynamic simulations of tidally-locked exoplanets constrained by atmospheric models and astrophysical observations, and elucidate how the interior dynamics of these intriguing planets are different from Earth.

Scott McLennan (Stony Brook University / sabbatical @ Oxford Earth Sciences) - The Nature of Planetary Crusts: Lessons for the Study of Exoplanets?

Planetary crusts can be conveniently classified into primary (related to initial planetary differentiation), secondary (related to partial melting of planetary mantles) and tertiary (related to magmatic re-processing of secondary crustal material). Over the past decade, planetary missions, especially to Mars, Mercury, the Moon and the differentiated asteroid Vesta, have provided transformational insights into the diversity, compositions and histories of planetary crusts. Experience from our own solar system thus indicates that planetary crusts, while dominated broadly by basalts (and their magmatic differentiates and intrusive equivalents), in detail are each unique with respect to bulk chemistry and mineralogy with no simple criteria for predicting their development. Despite recurring suggestions to the contrary, Earth remains the only body in our solar system that clearly possesses both a significant tertiary (continental) crust and the plate tectonic processes needed to give rise to such a crust. Recent attempts at estimating the crustal compositions of rocky exoplanets, for example, by using stellar compositions to constrain bulk planetary compositions and melting experiments to then estimate crustal compositions, rely on a number of critical assumptions but nevertheless appear to confirm the likelihood of great diversity in exoplanet crustal compositions (e.g., >2X variations in Si/Mg, Fe/Mg, Al/Mg, Ca/Mg). Such diversity should be considered when evaluating both theoretical models (e.g., mineralogical models, chemical weathering history) and observational constraints (e.g., direct spectroscopy of atmospheres and rocky surfaces) on exoplanet compositions and geological histories.

Chelsea Huang (MIT) - Planets from the First Year of TESS Mission

The Transiting Exoplanet Survey Satellite promises to discover small planets around the nearest and brightest stars. After the first year of observations, the TESS mission has recovered more than a thousand planetary candidates in the Full Frame Images around bright stars. I will present a review of some of the exciting discoveries from the first year of TESS's primary mission, especially those from the TESS Full Frame Images.

Dan Spencer (Earth Sciences) - Internal magma dynamics and heat transfer on Io and other lava-worlds

Tidal energy dissipated in silicate planets is typically assumed to be removed by convection. For Jupiter’s moon Io, however, this is inconsistent with the surface heat flux. We present a model of coupled magmatism and volcanism showing that removal of tidal heat from Io’s mantle occurs predominantly by magmatic segregation. This is quantitatively consistent with Io’s eruption rate, elastic thickness, surface heat fluxes, and electrical conductivity. We derive from our model a simple predictor of globally averaged volcanic eruption rate that uses measurements available for exoplanets. In contrast to the terrestrial planets, heat loss on lava-worlds is dominated by magmatism, reducing the drive for mantle convection and precluding plate tectonics.

Nora Eisner (AOPP) - Planet Hunters TESS: people-powered exoplanet discovery in TESS data

I will present results from the Planet Hunters TESS project, which harnesses the power of citizen science to find transit events in the TESS data by engaging tens of thousands of volunteers. While most planets in the transit survey data are found using purpose-built algorithms and software, some are unavoidably missed – in particular if the signals are somehow unusual. Results from the Kepler version of Planet Hunters showed that humans can outperform the automated detection pipelines for certain types of transits, especially single (long-period) transits, as well as aperiodic transits (circumbinary planets) and planets around rapidly rotating, active stars (young systems). I will discuss the benefits and challenges of using citizen science to find transit signals; how we make use of the vast numbers of user classifications; and how we evaluate the sensitivity of our planet search using simulated transit signals. I will also provide an overview of the planet candidates that we have found so far.

Colin Wilson (AOPP)

How did Venus evolve?

Venus is the only Earth-sized terrestrial planet besides our own that we can study in detail - yet there's still much we don't know about it. It’s often said that Venus was Earthlike, with liquid water oceans, before it developed a runaway greenhouse effect; but this is not well constrained, and it’s equally possible that any primordial water on Venus was lost in a steam atmosphere phase and that is has never had a habitable phase. Understanding how Venus and Earth diverged in their evolution is How can we constrain Venus evolution scenarios? Isotopic ratios provide some clues, but many are only measured to poor accuracy or not at all (e.g. Xe isotope ratios). Measurements of Venus today are improving our ability to model its super-rotating atmospheric circulation, and its surprisingly atmospheric low escape rates. We are developing the scientific case for a proposed ESA Venus orbiter called EnVision, which would characterize current and past geological activity on Venus. Envision will determine whether there is volcanic and tectonic activity on Venus today, using radar at spatial resolutions down to 5 m per pixel, thermal imaging to detect volcanic activity (in near-IR and microwave wavelengths), measurement of volcanogenic gas plumes above and below the clouds, and – experimentally – differential interferometric radar, to search for cm-scale motion of the surface. All proposals for investigations to be carried out using EnVision, or supporting modelling work, would be most welcome, to further our understanding of Venuses and exo-Venuses, of Earths and exo-Earths!

Titan Dragonfly

The most recently selected NASA New Frontiers mission is Dragonfly, a nuclear-powered autonomous rotorcraft which will explore Titan and its wildly complex prebiotic chemistry. I am designing the wind sensor for Titan Dragonfly; its wind measurements are not only useful for understanding atmospheric circulation and volatile cycles, but also for mission safety, ensuring that the lander does not take off in dangerously high winds. Dragonfly will measure the chemistry of different surface units, using a mass spectrometer with laser desorption and gas chromatography capability, and with a neutron-activated gamma ray spectrometer; it will constrain interior structure using seismic instrumentation; it will measure a range of meteorological parameters; and it will return thousands of images of Titan’s richly varied surface. Your ideas about investigations to be carried out using Dragonfly, or supporting modelling work would be most welcome! As the only UK science team member for Dragonfly, I would be happy to maximise the science benefit of this mission participation to Oxford’s planetary scientists…

Robert Graham (AOPP) - Energetic and Thermodynamic Limits on Continental Silicate Weathering Strongly Impact the Habitability of Wet, Rocky Worlds

The classical concept of the “liquid water habitable zone” as presently understood relies on the silicate weathering feedback to stabilize climate across a range of instellations. However, the representation of the silicate weathering feedback that is used in rocky exoplanet climate stability studies does not account for 1.) the thermodynamic limit on the concentration of silicate weathering products in runoff pointed out by Maher & Chamberlain, 2014 or 2.) the energetic limit on precipitation set by planetary instellation. In our 0-dimensional CO2- and energy-balance simulations, when limit 1.) is included, rocky planet climate loses its sensitivity to silicate dissolution kinetics, and is instead controlled by runoff. When limit 2.) is included along with limit 1.), a novel climate instability emerges at low instellations where the maximum precipitation is too low to weather silicates fast enough to balance CO2 outgassing. This imbalance leads to the runaway accumulation of CO2, potentially rendering many planets in the outer reaches of the classical habitable zone uninhabitable. Simulations of climates of planets whose weathering behavior includes limit 1.) are also more sensitive to land fraction and CO2 outgassing rate than those without limit 1.).

Oscar Barragan Villanueva (Astrophysics) - Radial velocity confirmation of K2-100b: a young, highly irradiated, and low density transiting hot Neptune

The tens of young transiting exoplanets discovered to date by K2, and those yet to be found by TESS, are valuable tests of planet formation and early evolution scenarios. However, their scientific impact so far has been limited because none of them have mass measurements. We present an exhaustive analysis of RV measurements for the star K2-100, a member of the Praesepe cluster for which K2 photometry revealed a close-in planet with a period of 1.67 days. We apply a multi-dimensional Gaussian-Processes approach to HARPS-N radial velocity and activity indicators in order to measure the Doppler signal of the planet in this active star. We detect a Doppler signal with a semi-amplitude of 10.6 +/- 3.2 m/s, consistent with the transit ephemeris, and corresponding to a mass of 22.8 +/- 7.0 Earth masses for the planet. This is the first mass measurement of a planet orbiting a star in a young active star. The radius of K2-100b, at 3.9 +/- 0.1 Earth radii, implies a significant volatile envelope. However, the planet receives is ~1900 times more heavily irradiated than the Earth, and photo-evaporation is expected to play a significant role in its evolution. As the first young transiting planet with both radius and mass measurements, K2-100b provides valuable insights into the physical mechanisms that shape early planet evolution. This result also demonstrates the feasibility of RV follow-up for transiting planets around active stars, given enough measurements, and bodes well for the young planets yet to be discovered by TESS.

Greg Colyer (AOPP) - Atmospheric dynamics of slowly rotating terrestrial planets

Motivated by Venus, we consider an idealized dynamical model of a terrestrial atmosphere. We start from the benchmark of Held and Suarez (1994), then extrapolate their annually-averaged forcing to higher surface pressures, and increase their relaxation times to take account of increased atmospheric mass. Although such an atmosphere is still shallow compared to the radius of the planet, unlike Earth's it is dynamically thick in the sense that the tropopause is many scale heights above the surface: this allows room for more vertical structure to develop in the large-scale tropospheric circulation. We compare our numerical results from the Isca code (Vallis et al. 2018) with simple theory based on the axisymmetric, inviscid, steady-state model of Held and Hou (1980), and a variant with continuous zonal wind. In between the Hadley-cell region and the polar region, which are the only two regions in these theories, we find a temporally fluctuating region similar to that found by Adam and Paldor (2009) in a shallow-water model. We also find multiple superrotating states, dependent on initial conditions, sustained on time scales much longer than those of the forcing or of the planetary rotation. The Venus-like, strongly superrotating cases, are associated with a more complex overturning circulation, featuring stacked cells similar to those seen by Lebonnois et al. (2016).

Peter Read (AOPP) - Modelling Solar System planetary atmospheres and their broader context

For nearly 30 years the Oxford GPFD group has been developing (in collaboration with others) reasonably comprehensive numerical general circulation models (GCMs) for many of the major planetary atmospheres in the Solar System. To date, work has focused on simulating the circulation and climates of Mars, Venus and the major gas giants, Jupiter and Saturn, also making extensive use of spacecraft and telescopic observations for validation. This talk will give a brief overview of the state of development of these models, illustrated with some recent results, and will discuss how we are attempting to put the simulated circulations into a wider parametric context (relevant to understanding exoplanets) using simplified GCMs.

Hauke Marquardt (Earth Sciences) - Experimental Exploration of Planetary Interiors

Patrick Irwin (AOPP) - Ice Giants to WASP-43b: NEMESIS retrievals of atmospheric properties

Xianyu Tan (AOPP) - Atmospheric circulation of rapidly-rotating, tidally-locked gas giants

Tidally locked gas giants, which exhibit a novel regime of day-night thermal forcing and extreme stellar irradiation, are typically in several-day orbits, implying a modest role for rotation in the atmospheric circulation. Nevertheless, there exist a class of gas-giant, highly irradiated objects -- brown dwarfs orbiting white dwarfs in extremely tight orbits -- whose orbital and hence rotation periods are as short as 1-2 hours. Phase curves and other observations have already been obtained for this class of objects, which raise fundamental questions about the role of rotation in controlling the circulation. So far, most modeling studies have investigated rotation periods exceeding a day, as appropriate for typical hot Jupiters. In this work we investigate atmospheric circulation of tidally locked atmospheres with decreasing rotation periods (increasing rotation rate) down to two hours. With decreasing rotation period, we show that the width of the equatorial eastward jet decreases, consistent with the narrowing of wave-mean-flow interacting region due to decrease of the equatorial deformation radius. The eastward-shifted equatorial hot spot offset decreases accordingly, and the off-equatorial westward- shifted hot areas become increasingly distinctive. At high latitudes, winds becomes weaker and closer to be rotationally dominated. The day-night temperature contrast becomes larger due to the stronger influence of rotation. At depth zonal structures develop. Our simulated atmospheres exhibit variability, presumably caused by baroclinic and barotropic instability. Unlike typical hot Jupiters, thermal phase curves of fast-rotating, drag-free models show a nearly alignment of peak flux to secondary eclipse. Our results have important implications for phase curve observations of fast-rotating, tidally locked atmospheres.

Sarah Rugheimer (AOPP) - Impact of UV Environment on Detecting Pre-biotic signatures in the Atmospheres of Early Earth-like Planets Around Other Stars

We examine the plausibility of detecting prebiotically interesting molecules, such as HCN, NH3, CH4, N2O, and C2H6 in an early-Earth type atmosphere around stars with different UV environments: various M dwarfs and a solar analogue. We model four atmospheres with varying amounts of H2 and CH4 along with CO2, H2O, and N2. These molecules would be interesting to detect in an exoplanet atmosphere since they are known to be useful for key prebiotic chemical pathways. HCN in particular is present at each of the initial photochemical reactions that produce lipids, amino acids and nucleosides, the three building blocks of life (Patel et al. 2015)

Nick Tosca (Earth Sciences) - Sedimentary constraints on early atmospheric evolution of Earth and Mars

Shang-Min Tsai (AOPP) - Photochemical modelling in 1D, 2D, and 3D

Norbert Zicher (Physics) - Hide and Seek: Finding planets around young active stars

Rowan Curtis (Physics) - Towards Understanding Heat Transfer on Airless Bodies using the Oxford Space Environment Goniometer

Jake Taylor (AOPP) - Understanding and Mitigating Biases when Studying Emission Spectra with JWST

Joost Wardenier (AOPP) - Exploring the Use of Condensation Physics in 1D retrieval models for Cloudy Exoplanet Atmospheres