The IPLU Science Day was held on 7 February 2022 to bring together researchers from across the University to discuss their work on planetary science and life in the Universe. We were also joined by our guest speaker, Matteo Brogi from the University of Warwick who spoke about his work on exoplanet atmospheres.
Below is a list of talks and posters that were given on the day.
Beyond-Earth System Modelling (slides here)
Alexander Archibald, Yusuf Hamied Department of Chemistry
The UK has led major advances in understanding atmospheric composition climate interactions and through strategic investment by RCUK pioneered the development of the UK Earth System Model (UKESM). In this talk I will describe UKESM and discuss how it can be used as an in silico environment to perform experiments that couple changes in atmospheric composition and climate relevant to Early Earth and beyond. I will discuss the critical gaps that require breakthroughs on the path to develop a Beyond-Earth System Model.
Tapping into the Complexity of Exoplanet Atmospheres with Ground-Based High-Resolution Spectroscopy (slides here)
Matteo Brogi, University of Warwick
Planet Formation by Core Accretion (slides here)
Marc Brouwers, Institute of Astronomy
The collective observational effort over the last decade has revealed that exoplanets are both abundant and far more diverse than initially anticipated. In the current paradigm of core accretion, planetary formation is modelled to begin with the coagulation of dust into small pebbles and larger planetesimals that merge to form massive cores. Smaller cores become terrestrial planets like Earth and Venus, while larger cores evolve into various types of gaseous planets. In this talk, I will highlight surprising processes and open questions within the core accretion paradigm. I will show that the heat from accretion can sublimate solids before they reach a planetary core, which implies that even small, Earth-mass planets are expected to form with hot, complex interiors. Furthermore, I will discuss the importance of cooling for planetary evolution and show how the expectation of efficient planetary cooling seems contradictory with the existence of planets like Uranus and Neptune.
Planets or Asteroids? A Geochemical Method to Constrain the Masses of White Dwarf Pollutants
Andy Buchan, Institute of Astronomy
Polluted white dwarfs that have accreted planetary material provide a unique opportunity to probe the interiors of exoplanetary bodies. The compositions of such bodies encode information about their formation histories, including the geological process of core-mantle differentiation, with wide ranging implications for habitability. However, the nature of the bodies which pollute white dwarfs is not well understood: are they small asteroids, minor planets, or even terrestrial planets? In our work, we present a novel method to infer pollutant masses from detections of Ni, Cr and Si. These elements exhibit variable preference for metal and silicate during core-mantle differentiation, depending on the conditions under which it occurs. This alters their relative abundance in the core and mantle of differentiated bodies, and in turn the composition of any fragments derived from these bodies. The pressure inside the body is a key parameter, and depends on the body’s mass. By modelling core-mantle differentiation self-consistently using data from metal-silicate partitioning experiments, we place statistical constraints on the differentiation pressures (and hence masses) of bodies which pollute white dwarfs. We find 3 systems whose abundances are best explained by the accretion of fragments of small parent bodies, and 2 systems which imply accretion of fragments of Earth-sized bodies. This provides evidence for the presence of core-mantle differentiated bodies of a range of masses in exoplanetary systems.
Mitigating Stellar Noise: Modelling Stellar Oscillations and Granulation in Radial Velocity Time Series (slides here)
Zhao Guo, Department of Applied Mathematics and Theoretical Physics
Tens of thousands of solar-like oscillating stars have been observed by space missions. Their photometric variability in the Fourier domain can be parameterised by a sum of two super-Lorentizian functions for granulation and a Gaussian-shaped power excess for oscillation. The photometric granulation/oscillation parameters scale with stellar parameters and they can also make predictions for corresponding parameters in radial velocity measurements. Based on scaling relations, we simulate realistic radial velocity time series and examine how the root-mean-square scatter of radial velocity measurements varies with stellar parameters and different observation strategies such as the length of integration time and gaps in the time series. Using stars with extensive spectroscopic observations from the spectrographs (SONG and HARPS), we measure the granulation amplitude and timescale from the power spectrum of the radial velocity time series. In conclusion, we show that the photometric scaling relations from Kepler photometry and the scaling relationship to Doppler observations can be very useful for predicting the photometric and radial velocity stellar variabilities due to stellar granulation and oscillation.
Could Dynamic Topography Avert a Waterworld? (slides here)
Claire Guimond, Department of Earth Sciences
Small water budgets produce desert worlds and large water budgets produce water worlds, but there is a narrow range of water budgets that would grant a marbled surface to a rocky planet. A planet’s highest point can constrain this range in that it defines the minimum ocean volume to flood all land. Thus we take a first step in quantifying water world limits by estimating how minimum surface elevation differences scale with planetary bulk properties. Our model does not require the presence of plate tectonics, an assumption which has constricted the scope of previous studies on exoplanet land fractions. We focus on the amplitudes of dynamic topography created by rising and sinking mantle plumes---obtained directly from models of mantle convection---but also explore rough limits to topography by other means. Rocky planets several times more massive than Earth can support much less topographic variation due to their stronger surface gravity and hotter interiors; these planets’ increased surface area is not enough to make up for low topography, so ocean basin capacities decrease with mass. We find that dynamically-supported topography alone could maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately one ten-thousandth of their mass, in the most favourable thermal states. Finally, we discuss some preliminary theoretical constraints on the total water budgets of rocky worlds based on their interior water storage capacities.
Reduced Atmospheres of Post-Impact Worlds: The Early Earth (poster here)
Jonathan Itcovitz, Institute of Astronomy
Impacts have been suggested as promising scenarios through which reduced species important to prebiotic chemistry (e.g., CH4, NH3, HCN) can form on Earth-like planets. This scenario relies on the ability of reduced phases in the impactor (e.g., metallic iron) to reduce the planet’s H2 O inventory and produce H2. Previous studies have focused on the atmospheric response to impact. Here, we carry out simulations to demonstrate the importance of two effects, 1) the distribution of the impactor’s iron inventory between the target’s interior and atmosphere, and escaping the system, and 2) melt-atmosphere interactions in the post-impact system. We find that the most important factors in determining the equilibrium state of the system are how much reducing power escapes the system, and what fraction of iron that is accreted by the target interior is available to reduce the impact-generated melt phase. Based on our end-member cases, we suggest that H2 abundances in the post-impact atmosphere could be up to an order of magnitude lower than previously estimated. Such abundances could still be sufficiently high for species important to prebiotic chemistry to form, but sufficiently low that problematic greenhouse heating effects are suppressed. We also note that the reduced melt phase, and hence the re-solidified mantle, forms a substantial redox heterogeneity within the target mantle. Such a reduced reservoir could be long lived, detectable, and a source of reducing power that is exploitable to drive prebiotic chemistry at the surface long after the impact event.
Evolution by Natural Selection and the Implications for Predicting the Nature of Life in the Universe (slides here)
Arik Kershenbaum, Department of Zoology
A high-resolution Survey of Sodium Absorption in Transiting Exoplanets (slides here)
Adam Langeveld, Institute of Astronomy
The alkali metal sodium (Na) is one of the most commonly detected chemical species in the upper atmospheres of giant exoplanets. We report on a homogeneous survey of Na in a diverse sample of transiting exoplanets using high-resolution transmission spectroscopy. We confirm previous detections and assess multiple approaches for deriving Na line properties from high-resolution transmission spectra. The homogeneously measured sodium doublet line depths were used to constrain the atmospheric heights probed by Na observations across the sample. We report an empirical relationship describing the sodium line properties as a function of the planetary bulk properties. We also report the Na D2/D1 line ratios across our sample as well as constraints on day-night wind velocities. Our results suggest that the broad sample of exoplanets considered in our work share common underlying processes which govern atmospheric dynamics. Our study highlights a promising avenue for using high-resolution transmission spectroscopy to further our understanding of how atmospheric characteristics vary over a diverse sample of exoplanets.
Prebiotic Photoredox Synthesis from Carbon Dioxide and Sulfite (slides here)
Ziwei Liu, MRC Laboratory of Molecular Biology
Carbon dioxide (CO2) is the major carbonaceous component of many planetary atmospheres, which includes the Earth through- out its history. Carbon fixation chemistry –which reduces CO2 to organics, utilizing hydrogen as the stoichiometric reductant – usually requires high pressures and temperatures, and the yields of products of potential use to nascent biology are low. Here we demonstrate an efficient ultraviolet photoredox chemistry between CO2 and sulfite that generates organics and sulfate. The chemistry is initiated by electron photodetachment from sulfite to give sulfite radicals and hydrated electrons, which reduce CO2 to its radical anion. A network of reactions that generates citrate, malate, succinate and tartrate by irradiation of glycolate in the presence of sulfite was also revealed. The simplicity of this carboxysulfitic chemistry and the widespread occurrence and abundance of its feedstocks suggest that it could have readily taken place on the surfaces of rocky planets. The availability of the carboxylate products on early Earth could have driven the development of central carbon metabolism before the advent of biological CO2 fixation.
Exoplanet Architectures from Resolved Exo-Planetesimal Belt Observations
Josh Lovell, Institute of Astronomy
Whilst exoplanet population demographics are relatively well sampled in the inner regions of planetary systems, the demographics of outer planets are less well understood. This sets limits on our understanding of how readily planets form/migrate to outer regions within planetary systems, and thus how rare the architecture of the Solar System may be, intricately linked to the search for life around other stars. But planets are just one type of component in wider planetary systems which also harbour planetesimals – km-sized rocky and icy bodies such as asteroids and comets. Within our Solar System the Asteroid and Kuiper Belts are both understood to have influenced the evolution of our planetary architecture, and planet compositions and atmospheres, and Solar System planets are likewise understood to have influenced the morphology of these two belts. In this talk I will outline results from a recently accepted modelling paper (Lynch & Lovell, MNRAS, in press) and an observational study with new ALMA data that I have led (Lovell+22a, in preparation). Combined, these show how observations of exo-planetesimal belts at 10s of au can provide constraints on the demographics of planetary architectures (for example, through the presence of belt gaps, rings, asymmetries and sub-structures) and thus provide new insights into the rarity of exo-Solar Systems.
Impact Craters as a Planetary Habitat (slides here)
Auriol Rae, Department of Earth Sciences
Impact cratering is a ubiquitous planetary geological process. Impacts have had a profound influence, both directly and indirectly, on the development of life on Earth, and potentially the Solar System and beyond. In this talk, I will discuss the implications of impact cratering mechanics for the viability of craters as microbial habitats on Earth and other planets. To achieve this, I will present results from geological and geophysical observations, experimental rock mechanics, and numerical impact simulations. These results will be primarily focussed on the Chicxulub impact structure, known for its role in the extinction of the non-avian dinosaurs, as an exercise in comparative planetology.
Comet Fragmentation as a Source of Zodiacal Dust (slides here)
Jessica Rigley, Institute of Astronomy
Understanding the source of the zodiacal cloud is important to the search for life, as analogous dust clouds in exoplanetary systems will obscure the habitable zones of stars when looking for Earth-like planets. Models of the thermal emission of the zodiacal cloud and sporadic meteoroids suggest that the dominant source of interplanetary dust is Jupiter-family comets. However, comet sublimation is unable to sustain the quantity of dust presently in the inner solar system, suggesting comet fragmentations as a possible source. We present a model for the dust produced in comet fragmentations and its evolution. Using results from dynamical simulations, we follow individual comets as they evolve and undergo recurrent splitting events. The dust produced by these fragmentations is followed with a kinetic model which allows to model the size distribution and radial profile of dust resulting from comet fragmentation. With physically-motivated free parameters this model provides a good fit to zodiacal cloud observables, supporting comet fragmentation as a plausible source of dust. By modelling individual comets we are also able to explore the variability of cometary input to the zodiacal cloud. We show that large comets should be scattered into the inner solar system stochastically, leading to large variations in the brightness of the zodiacal light.
The Effect of the Streaming Instability on Protoplanetary Disc Dust Emission
Chiara Eleonora Scardoni, Institute of Astronomy
The core accretion theory suggests that solid planets form by growing the initial micron-sized dust grains in protoplanetary discs up to the size of a planet. When the grains reach the size of around 1 cm, however, the growing process faces a critical stage; in fact, due to the interaction with the disc’s gas component, cm-sized grains are expected to drift rapidly towards the central star, becoming unavailable to form km-sized planetesimals (and therefore planets). Streaming instability (SI) is often invoked as a potential solution to this problem, as it promotes rapid dust overdensity formation. In our recent study, the action of SI is simulated through 2D local simulations, to which we then apply a radiative transfer model to compute the emission at mm wavelengths of resulting dust clumps. Although the small size of these dust clumps makes them inaccessible by direct observations (and thus we cannot directly compare the computed emission to the data), it is possible to define and study observable quantities from which we can infer the presence of such tiny substructures; thus we analysed the clumps’ radiative properties in terms of two observable quantities: the optically thick fraction ff (computed in ALMA band 6) and the spectral index alpha (in bands 3-7). By comparing the distribution of simulations in the ff-alpha plane before/after the action of streaming instability to recent multiwavelength data in the Lupus star forming region, we found that the action of SI drives simulations towards the area of the plane occupied by the data. We also illustrated that the same result is valid when we consider integrated disc models, provided that the instability is acting over a region of the disc that dominates the mm flux. Our study therefore suggests that clump formation via SI is consistent with recent observations, confirming that it can be considered a good candidate to solve the radial drift barrier to planetesimal formation.
Impact of Photoevaporation on the Composition of Planet Forming Discs (poster here)
Andrew Sellek, Institute of Astronomy
Thermal (photoevaporative) winds are generally more effective at removing gaseous material from protoplanetary discs than solid material (i.e. dust). This has the potential to increase the solids-to-gas ratio in the disc, though the radial drift of dust grains due to gas drag causes them to rapidly migrate inwards towards the star and so far has generally been seen to negate this effect. However, this same radial drift redistributes both refractory elements and those frozen out in ices through the disc, delivering the latter to locations where they may be protected from the wind. By coupling photoevaporation prescriptions to a simple chemical evolution model (Booth et al. 2017) we are exploring when this leads to winds that are depleted in metals, and whether this can, conversely, leave behind a disc with an altered composition.
Marine Phosphate Availability and the Chemical Origins of Life on Earth (slides here)
Nicholas Tosca, Department of Earth Sciences
Prebiotic systems chemistry suggests that high concentrations and persistent sources of phosphate were necessary to synthesise molecular building blocks and sustain primitive cellular systems. However, current understanding of mineral solubility limits phosphate to negligible concentrations in most natural waters, and the role of Fe2+, an important component on the early Earth, is poorly quantified. We determined the solubility of Fe(II)-phosphate in synthetic seawater solutions as a function of pH and ionic strength, and integrated these observations into a thermodynamic model that predicts phosphate concentrations across a range of aquatic conditions. Experiments and models show that Fe2+ significantly increases the solubility of all phosphate minerals in anoxic systems, in turn suggesting that prebiotic seawater featured phosphate concentrations ~3-4 orders of magnitude higher than currently estimated. These data release stringent environmental constraints on prebiotic synthesis and indicate that seawater readily met the phosphorus requirements of emergent cellular systems, early microbial life, and, in combination with new data from Archean rocks, fuelled primary production during the advent of oxygenic photosynthesis. Our results also show that phosphate-rich fluids could have developed on the ancient surface of Mars, providing exploration efforts with mineral signatures of past environments where biomolecular synthesis may have been feasible.
A Search for Longer-Period Exoplanets with TESS and CHEOPS (slides here)
Amy Tuson, Cavendish Laboratory
Longer-period planets are some of the most promising targets for the search for life. However, only 13% of exoplanets discovered by the Transiting Exoplanet Survey Satellite (TESS) have periods longer than 20 days. One way to probe longer-period planets with TESS is via duotransits - planet candidates with two observed transits separated by about two years. These detections do not have a unique period. Instead, they have a discrete set of period aliases which can be used to perform targeted follow-up with the CHaracterising ExOPlanets Satellite (CHEOPS). In this way, their periods can be recovered and their planetary origins confirmed. I will be presenting the first results of an automated pipeline that has been specially developed to search for TESS duotransits that are well-suited to CHEOPS follow-up. This work offers the exciting opportunity to discover small, longer-period transiting exoplanets, potentially orbiting in the habitable zones of their host stars and amenable to life as we know it.
The Shocked Meteorite Record: A Rosetta Stone for Solar System Dynamical History? (slides here)
Craig Walton, Department of Earth Sciences
Asteroid collision rates increase during solar system reorganisation, such as in the wake of Earth's Moon-forming impact, or giant planet migration. Evidence for these collisions is brought to Earth by meteorites, which can preserve impact-reset radioisotope mineral ages. However, as meteorites often preserve numerous mineral ages, their interpretation is controversial. Here, we combine analysis of phosphate U-Pb ages and mineral microtextures to construct a simplified collision history for the Chelyabinsk ordinary chondrite meteorite. We show that phosphates in this meteorite experienced both recent fracturing and partial Pb-loss at 1 +/- 56 Ma, suggesting recent separation from the Chelyabinsk parent body asteroid after a minor collision. Phosphate textural-age relationships also indicate complete early Pb-loss during the deformation and recrystallisation of phosphates at 4,479 +/- 12 Ma, revealing massive collisional reheating at this time. Our results support an interpretation of the wider chondritic phosphate texture-age record where energetic collisions widely affected the inner Solar System from 4.48-4.44 Ga. This event could reflect giant planet migration, or debris scattering in the wake of Earth's Moon-forming impact. The ability to resolve a temporal structure of early asteroid collisional rates should also unlock related problems, such as the flux of collision-generated and highly reactive CHNOPS-bearing asteroidal micrometeorites/dust to early Earth, which is not represented in cratering records.
Can Stable Isotopes Trace Missing Magma Oceans? (slides here)
Helen Williams, Department of Earth Sciences
The differentiation of the Earth ~ 4.4 billion years ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation and the generation of mineralogically distinct reservoirs in the planet's silicate interior (the mantle). However, the magma ocean model remains difficult to validate on Earth due to the tectonic resurfacing of our planet over the last ~ 3 billion years and the scarcity of geochemical tracers of high-pressure, lower mantle (>720 km below the Earth's surface) mineralogy. However, the stable iron (Fe) isotope compositions of ancient mafic rocks can be used to reconstruct the mineralogical evolution of their mantle source regions. We present Fe isotope data for 3.72 billion year old volcanic rocks from the Isua Supracrustal Belt (Greenland). The Fe isotope signatures of these ancient rocks are elevated relative to modern equivalents and define striking correlations with fluid-immobile trace elements and tungsten (W) isotope anomalies (excesses in 182-W, indicative of hafnium-tungsten fractionation by processes such as planetary core formation or silicate differentiation during the first~ 45 million years of solar system history). Phase equilibria models demonstrate that these features can be explained by melting of a high-pressure magma ocean crystal cumulate (Mg-Fe perovskite, bridgmanite) component in the Earth's upper mantle. Crystal and melt residues from early magma ocean differentiation may still survive in the Earth's interior today, as evidenced by the iron and tungsten isotope heterogeneity of modern oceanic basalts. It remains to be determined whether the crystal residues of early magma ocean stages are preserved in the mantles of other terrestrial planets and planetesimals in our solar system.
Planetesimal Belts in Wide Binaries: The Eccentric Kozai Mechanism as a Possible Origin of Transiting Exocomets (slides here)
Steven Young, Institute of Astronomy
Planetary systems have been found to be a common occurrence across the galaxy and consist of both planets and belts of planetesimals. The orbits of planetesimals in such discs can be perturbed by the presence of nearby massive bodies such as a distant companion star. These companions, if sufficiently inclined and eccentric, can excite planetesimals to extremely high eccentricities: an effect known as the Eccentric Kozai Mechanism. These planetesimals can then pass very close to their host star, possibly producing observable transits. This is one explanation for the deep, aperiodic dips in the light curve of Boyajian’s star which has recently been confirmed to have a wide binary companion. I will present a Monte Carlo model of the Kepler field to investigate how often this mechanism would be expected to produce such a signature and hence how important distant stellar companions are to the evolution of planetary systems.
What Dust Sizes Tell Us About Planet-Forming Disc Evolution? (slides here)
Francesco Zagaria, Institute of Astronomy
Proto-planetary discs (PPDs) are the birthplace of planets: knowledge of their evolution mechanism is key to understand how the planet-formation potential changes with time and to explain the properties of the currently detected exoplanets. Traditionally, PPDs have been thought to evolve viscously: angular momentum redistribution allows for accretion and determines outward disc spreading. Recently, it was hypothesised, instead, that accretion is due to angular momentum removal by MHD winds, implying that no disc spreading is expected. We run several 1D gas and dust simulations of viscous and MHD models with the aim of assessing which evolutionary mechanism is the dominant one. To do so, we compute disc dust sizes and compare them with ALMA observations. We show that viscous and MHD angular momentum transport determine very different dust disc sizes. In particular, MHD models are compact, as expected from the bulk of the data, while in viscous models dust sizes increase with time. However, current observations lack enough sensitivity to discriminate between the two evolutionary scenarios. We show that deeper ALMA observation could be helpful in assessing the dominant evolution mechanisms of planet-forming discs.
Evolution of Life in the Universe (poster here)
Department of Zoology