Planetary systems are prevalent around main sequence stars. However, little is known about their fate once the star evolves off the main sequence. In this talk, I will present different evidence that planetary systems are present and active around white dwarfs. Similar to main-sequence stars, white dwarfs have debris disks made from dust and gas. These debris disk hosts often have a heavily polluted atmosphere – spectroscopic observations can uniquely reveal chemical compositions of extrasolar planetesimals, which are hard to measure with other techniques. Recent discoveries of transiting planetesimals and planets have opened up many new opportunities to characterize these extreme planetary systems.

The habitable zone planets around low-mass stars, such as TRAPPIST-1e (Gillon et al. 2017) or Proxima-b (Anglada-Escudé et al. 2016), are the only habitable planets whose atmosphere is being probed/will be probed in the near future (with the JWST or RISTRETTO for instance). Understanding their atmosphere and their climate is therefore paramount. 

However due to the low-luminosity of these stars, the planets located in the habitable zone are close-in. Close-in planets are submitted to a variety of star-planet interaction mechanisms: tidal interactions (e.g. Mathis, 2018) and magnetic interactions (e.g. Strugarek et al. 2017), which influence both the orbit and rotation of the planets, but also the direct interaction of the stellar light with the atmosphere which can drive atmospheric escape (Bolmont et al. 2017). These planets are a perfect laboratory for all these processes. 

I will mainly discuss the influence of tides on the dynamical (orbital and rotational) parameters which directly influence the climate and the habitability of these planets. I will also talk about the current improvements we are implementing to account for tidal evolution in a more realistic way (Bolmont et al. 2020) and how this can impact the rotation evolution in particular. 

Giant impacts are crucial in early solar systems; their outcome is the formation of large protolunar disks. Using molecular dynamics simulations, we study the behavior of a silicate fluid with bulk silicate Earth's composition under typical impact conditions. We find that the Earth's protolunar disk reached a supercritical state. Then we follow the chemical evolution of the disk during its cooling. Liquids and gases separate according to the liquid-vapor dome. As the liquid rains towards the center, the leftover gas forms a hot dense atmosphere. Oxidized phases like SiO, O, O2, MgO, and cations like Na and Mg dominate the gas phase. Our simulations uncover a plethora of other phases present in the system, with lifetimes that allow them to play a role in the chemical and isotopic exchanges.

Super-Earths span a wide range of bulk densities, indicating a diversity in interior conditions beyond that seen in the solar system. In particular, an emerging population of low-density super-Earths may be explained by volatile-rich interiors. Among these, low-density lava worlds have dayside temperatures high enough to evaporate their surfaces, providing a unique opportunity to probe their interior compositions and test for the presence of volatiles. In this talk, I will discuss the atmospheric observability of low-density lava worlds. I use a radiative-convective model to explore the atmospheric structures and emission spectra of these planets, focusing on three case studies with high observability metrics. By simulating JWST observations, I find that key volatile spectral features could be detected with a feasible observing time (~ 5 secondary eclipses). Detecting volatiles in these atmospheres would provide crucial independent evidence that volatile-rich interiors exist among the super-Earth population.

The snow line is the location in protoplanetary disks beyond which water ice condenses. Knowing where the snow line should be is fundamental for understanding how and where planets of different compositions form. The classical theory of solar system formation assumed that the solar nebula's snow line occurred around the current position of the asteroid belt. However, models have shown that the snow line's location is in fact highly uncertain and depends on the disk's heating and cooling processes. Its location can even change as planet formation proceeds because the solids in the disks, which are the building blocks of planets, can control the efficiencies of both disk heating and cooling. In this talk, I will present our recent efforts to model the coupled evolution of solids and the snow line in protoplanetary disks. In particular, I will highlight the possibility that the solar nebula's snow line could have migrated inward across Earth's current orbit during the early stages of solar system formation.

Landforms, shaped by interactions between environmental fluids and geologic surfaces, encode information about hydrology, climate, and the overall environment that may be preserved over geologic timescales. Thus, understanding the mechanics of geomorphic and sedimentary processes that shape the landscapes of planets is key to deciphering their respective paleoenvironmental records. To date, the majority of mechanistic models for surface processes were derived from observations of modern Earth, where life thrives, and from scaled-down experiments. Numerical models help to probe wider parameter spaces than can be achieved on our planet, but they only contain the physical rules that they were designed to honor in the first place. However, the foreign parameter spaces spanned by other planets may lead to phenomena that we do not realize need to be included in our models – the unknown unknowns. Even Earth would have looked alien to any of us before the advent of macroscopic life, with a different atmospheric composition and different surface sedimentary dynamics for example. As a result, the applicability of existing models for surface processes is often limited to those systems that most closely resemble modern terrestrial conditions, impeding our ability to reliably decipher the environmental records of other planets and the early Earth. Flipping this paradigm, planetary bodies in our Solar System and beyond span a range of sizes, environments, and compositions that allow us to approach comparative planetology as a full-scale experiment, where other bodies offer a unique opportunity to develop more robust models and expand their applicability. Knowledge gained from the exploration of other planets not only contributes to our fundamental understanding of surface processes, but at times can feed back into our understanding of the Earth. In this presentation, I will illustrate how a dialogue between the Earth and planetary sciences can increase our ability to interpret the landscapes and rocks with three examples from our own Solar System – the formation of large eolian ripples under the thin martian atmosphere, the dynamics and record of unvegetated meandering rivers on the early Earth and Mars, and the alien organic-sediment cycle of Saturn’s moon, Titan.

The timing of delivery and the types of bodies that contributed volatiles to the terrestrial planets remain highly debated. For example, it is unknown if differentiated bodies, such as that responsible for the Moon-forming giant impact, could have delivered substantial volatiles, or if smaller, undifferentiated objects were more likely vehicles of water delivery. Measurements of water contents of nominally anhydrous minerals and melt inclusions in ungrouped achondrite meteorites (mantles/crusts of differentiated planetesimals) from both the inner and outer portions of the early Solar System are extremely low. Furthermore, measurements of water in minerals and quenched melts in ureilites demonstrate efficient degassing of water from the ureilite parent body (UPB), even though the UPB did not have a global magma ocean. Our results demonstrate that partially melted planetesimals efficiently degassed prior to or during melting. This finding implies that water could only have been delivered to Earth via unmelted material.

The presence of liquid water on the surface is one of Earth’s defining characteristics, and it is usually considered to be critical for planetary habitability. Moreover, Earth not only has surface water but also has just the right amount of it to allow the subaerial exposure of continental crust, which is important for various geochemical and biogeochemical cycles. Unlike its sister planet Venus, Earth also has a right amount of carbon dioxide in the atmosphere to maintain moderate surface environments. When considering the making of a habitable planet, therefore, understanding the volatile budget and its spatial distribution within a planet is particularly important. I will discuss two recent developments on this issue, (1) how early mantle evolution could dictate the long-term evolution of surface environments, and (2) how high-energy planetary accretion processes could shape the volatile budget.

In the past few decades, the fields of planetary science and astronomy have seen tremendous growth by exploring new worlds both inside and outside the Solar System. Recently, planetary science and exoplanet communities have noted the need for cross-disciplinary research to advance both fields. I will talk about my journey as a planetary scientist studying Titan and how that inspires me to work on exoplanets from a unique perspective. I will first talk about our group’s research on understanding various physical and chemical processes on Titan through our cross-laboratory comparative characterization of Titan’s haze analogs. I will then cover two aspects of my exoplanet research inspired by my Titan works, 1) how a collaborative haze analog study can be beneficial to understand the exotic clouds/hazes on exoplanets; 2) how to decipher the nature of sub-Neptunes with atmospheric trace species. At the end of my talk, I hope you will get a taste of how solar system research can be beneficial to the exoplanet field.

Detecting and characterizing the secondary atmosphere of Earth-sized planets has long been an ambition in the exoplanet field and would represent a major step forward. The newly launched James Webb Space Telescope (JWST) is capable of probing the atmospheres of terrestrial-sized exoplanets orbiting cool M dwarf host stars, if such planets retain atmospheres. In this talk, I will describe our first transit observation of TRAPPIST-1c with NIRSpec/PRISM, which is part of our JWST program to attempt to detect an atmosphere of TRAPPIST-1c (remaining three transits are schedule in the fall of 2023).

Secondly, our group has secured resources to design, build, and install the Second Earth Initiative Spectrograph (2ES): a next-generation extreme-precision radial velocity spectrograph on the MPG/ESO 2.2m Telescope on La Silla, Chile dedicated to a >5-year observing period with the ambitious goal of discovering temperate terrestrial Earth-mass planets orbiting the brightest solar-type stars in the Southern Hemisphere. Such discoveries would be the targets of next-generation flagship missions to characterize their atmospheres. Our collaboration has secured access to the majority of the time on the MPG/ESO 2.2m telescope to observe a modest sample of quiet solar-type stars every night (weather and visibility permitting) providing a large and uniformly sampled high cadence extreme precision RV dataset. Providing continuous radial velocity precision over longer periods of time and mitigating the inevitable presence of stellar activity will be crucial in the attempt to detect the minute signals of temperate terrestrial planets.

Saturn's moon Enceladus possesses a Na-carbonate-rich subsurface ocean. According to the in-situ analysis of plume materials erupting from the subsurface ocean, the Cassini spacecraft has revealed that seawater pH is alkaline, near 10, and that hydrothermal activity exists in the rock core. In addition to Na-carbonate-rich plume particles, a new type of phosphate-rich particle was recently discovered. Phosphorus (P) is a CHNOPS element essential for life on Earth, but of these, it is the least abundant in the Earth's aqueous environments. Throughout Earth's history, phosphorus has been the rate-limiting element of biological production. Based on the analysis of phosphate-rich particles of Enceladus, phosphate concentrations in the subsurface ocean would be 1000 times higher than in the Earth's oceans.

To understand the causative mechanism of the enrichment of phosphate in Enceladus' seawater, we performed hydrothermal reaction experiments and geochemical modeling. We find that alkaline (pH ~10) and carbonate-rich aqueous environments are essential for the phosphate enrichment, where calcium phosphate minerals are thermodynamically unstable compared to calcium carbonate minerals, releasing phosphate into liquid phase. Such alkaline carbonate-rich aqueous environments are commonly achieved in icy ocean worlds beyond the CO2 snowline of the Solar System. Phosphate could have been also enriched in similar alkaline carbonate-rich aqueous environments on early Earth, where earliest life on Earth might have utilized phosphorus as components of its building materials.

Protoplanetary disks around young stars of less than 10 million years old are the birth cradles of planets. Observations with ALMA have provided us with a wealth of data on the gas and dust distribution in protoplanetary disks and the early signs of planet formation. Whereas gaps in disks provide evidence for the rapid formation of gas giants, rocky planet formation remains more challenging to observe directly at this stage. On the other hand, several new insights have been obtained in the context of dust evolution in disks, such as coagulation, fragmentation and transport of dust pebbles in the first few million years of the lifetime of the disk. In this talk I will present an overview of our current knowledge of protoplanetary disks and their potential of forming rocky planets.

Planetary habitability is tied to the history of volatile accretion, volatile loss, and the evolution of surficial environments. Habitability is particularly related to the record of the life essential volatile elements such as hydrogen, carbon, nitrogen and sulfur. Noble gases are another set of volatile elements, but given their inert nature, are neither relevant for pre-biotic chemistry, or for life itself. And yet, the noble gases are intricately linked to the question associated with the planetary habitability and the origin of life through the remarkable portrait that they paint of the processes associated with the formation and evolution of a habitable planet. For example, the noble gas isotopes record processes associated with volatile delivery, how volatile sources changed through time, volatile loss, the evolution of the early atmosphere, and volatile exchange between the surface and the interior of a planet. It is this process-based framework provided by the noble gases that can be utilized as constraints towards constructing models around planetary habitability. An especially powerful approach is to integrate noble gas observations and models with those from hydrogen, carbon, nitrogen and sulfur. In this presentation, I will discuss how to read the noble gas record, recent work that is shaping our understanding of the volatiles in the inner Solar System, and where the frontiers and challenges lie in building a generalized framework for understanding volatile evolution in rocky planets.

Determining how terrestrial planets accrete and retain volatile elements is fundamental for understanding planetary diversity, evolution, and habitability. One mechanism that could play an important role in determining planetary volatile budgets is the loss of existing atmospheres (and potentially oceans) driven by impacts. I will demonstrate how the pre-impact surface conditions and distribution of volatiles on colliding bodies can dictate the efficiency by which different volatiles are lost in impact events. In particular, I will show that planets that experience more giant impacts later in accretion are much more suspectable to atmospheric loss.

The key to finding life on an exoplanet lies within the atmospheres of extrasolar planets, which have just recently become accessible to chemical analysis. The only way to detect a true ‘biosignature’—a definitive marker for life—is to rule out all abiotic processes that could lead to a ‘false positive.’ Thick atmospheres enshroud the interiors of the most common types of rocky planets. These planets’ geologic evolution is inextricably tied to their atmospheres in ways we don’t yet fully understand. To reveal the intricacies of this connection and ultimately open the door to the reliable detection of life beyond Earth we need a diverse team, access to facilities, and resources that will catalyze new discoveries and advance a new scientific paradigm. To accurately use planetary atmospheres as a proxy for life, establishing a baseline for a planet is critical. Atmospheres are complex, both physically and chemically. They are modified not only by the stellar insolation they receive from above but by the rocky surface below. The interiors and atmospheres of rocky planets interact initially during the magma ocean stage, that could last for a billion years, and then more slowly due to weathering, volcanism, and subduction. Our AEThER team has begun a coordinated scientific effort employing experimental, theoretical, and empirical approaches to address challenging yet fundamental questions, including: What is the principal control on planetary atmospheres from accretion until the time life begins to modify the signal? How sensitive is atmospheric chemistry to interior chemistry? This talk will discuss our current findings as well as our future directions.