Rocky Worlds Discussions

Sierra Grant (Carnegie Earth & Planets Laboratory).

JWST is transforming our understanding of chemistry in the planet-forming regions of protoplanetary disks. I will present trends we have discovered from large observational samples, showing how disk composition changes with host mass: disks around Sun-like stars are dominated by water vapor, while disks around low-mass stars and brown dwarfs are carbon-rich. Using high-contrast imaging techniques, we demonstrate that this carbon-rich chemistry extends down into the planetary-mass regime. Finally, I will discuss how some of the faintest spectral features give us the best insight into conditions in these planet- and moon-forming regions.

Tom Wilson (University of Warwick).

Exoplanets are built from protoplanetary disk ingredients that are thought to be preserved during formation. In small, terrestrial worlds, refractory elements sink to dominate planetary cores and mantles with volatile components potentially producing ocean and atmospheric layers. Thus, natal disk abundance ratios are predicted to influence planetary internal structures. Additionally, varying disk chemistry could alter formation mechanisms via different disk dust-to-gas ratios and surface-densities at crucial radial distances, and overall pebble accretion rates. Post formation, disk composition plays a key role in planet migration due to differing disk drift rates, and planet evolution via varying atmospheric mass-loss properties as a function of metallicity. Tentative evidence of a star-planet composition link exists. If true, this would imply that planets orbiting stars of varying chemistry, in the Milky Way's thick disk, should form and evolve planets differently, also impacting habitability. In this talk, I will review the current state of the field of observationally characterising planetary interiors for worlds around compositionally-diverse stars, and highlight a recent discovery of the first four-planet system spanning the radius valley around a Galactic thick disk M-dwarf. LHS 1903 is orbited by an inner super-Earth and two sub-Neptunes, with a distant fourth world. The inner worlds adhere to planet formation theory, however, the outer planet breaks this trend being smaller, denser, and essentially gas-devoid, rocky body. This finding supports gas-depleted planet formation for M-dwarfs, long-period planets that could imply disk compositional or formation timing effects. I’ll end with a view towards how upcoming missions, such as PLATO, could test formation and evolution pathways by expanding our work to outer regions of Sun-like stars for the first time.

Sarah Rugheimer (University of Edinburgh).

JWST is the first mission able to observe rocky exoplanet atmospheres. Future missions such as the proposed LIFE (the Large Interferometer for Exoplanets) mission concept and HWO aim to directly detect of the atmospheres of planets orbiting FGKM stars. As we observe these targets, we need to consider their UV environments to understand these observations. The diversity of the planets themselves adds additional complexity, and their atmospheres will not be fixed in time. In this talk, I will discuss the importance of UV radiation for the detection of biosignatures and prebiotic signatures, along with mechanisms for false positives with future missions.

Takanori Kodama (Earth-Life Science Institute, Institute of Science Tokyo)

More than 5,000 exoplanets have been discovered to date, including many terrestrial planet candidates. Understanding whether these worlds can sustain habitable environments has become a central challenge at the intersection of astronomy, planetary science, physics, and biology. Traditionally, habitability has been discussed using the concept of the habitable zone, where liquid water can be maintained on a planetary surface. Recent advances in three-dimensional climate modeling have revealed that atmospheric circulation, clouds, oceans, and surface conditions play critical roles in shaping planetary climates, particularly for tidally locked terrestrial planets around M-dwarf stars. As future observations begin to probe these environments directly, a quantitative understanding of the individual components of planetary climate systems is becoming increasingly important. In this presentation, I will review recent progress in habitable climate research and discuss key challenges and future directions in the study of exoplanet climates and habitability.

Cayman T. Unterborn (Southwest Research Institute)

Classifying small exoplanets as rocky, volatile-rich, or iron-enriched is a prerequisite for efficient allocation of atmospheric characterization resources. The Nominally Rocky Planet Zone (NRPZ) provides a framework for this classification by comparing observed bulk densities against the expected density range for volatile-free rocky planets, calibrated from stellar abundance distributions. However, the efficacy of the NRPZ is critically dependent on the precision of the underlying mass and radius measurements. Here, we perform an analytical propagation of observational uncertainties through the NRPZ classification framework for 37 candidate rocky exoplanets with radii below 2 Earth radii orbiting stars with visual magnitudes less than 11. Of these 37 planets, we find 11 of them currently have mass and radius uncertainties to definitively classify them as Mini-Neptunes or Super-Mercuries. The remaining planets, however, require extreme reductions in current mass and radius uncertainties, together, to classify them as either leaning rocky or not rocky. We further quantify the number of orbits needed to be observed for these planets to achieve these lower uncertainties, finding an average of ~2000. We argue that, while possible, this level of intense observation is likely not feasible for many of these planets and that mass-radius models alone are not likely to allow us to classify all but those planets with outlier densities. We propose that only through observations that reveal the presence, or lack thereof, of a significant atmosphere through characterization via JWST or next-generation telescopes will we definitively classify many intermediate density exoplanets as rocky. I will conclude the talk with a comparison of these results to our best-resolved exoplanet system: TRAPPIST-1 and discuss that while many planets may not yield habitable, or even temperate planets, we show that they can still provide utility in our understanding of planets as a whole, and exactly what it takes to build Earth-like worlds.

Eleonora Alei (NASA Goddard Space Flight Center)

One of the primary goals of the astronomical community in the next few decades is to characterize temperate terrestrial exoplanets to search for life. To address this challenge, the US Astro2020 Decadal survey recommended the pursuit of a technical and scientific study for the Habitable Worlds Observatory (HWO), an ultraviolet/visible/near-infrared (UV/VIS/NIR) “high-contrast direct imaging mission with a target off-axis inscribed diameter of approximately 6 meters” which shares design and technology heritage with the previous concept studies HabEx (Habitable Exoplanet Observatory) and LUVOIR (Large UV/Optical/IR Surveyor). Similarly, European scientists focus on the development of the MIR space-based nulling interferometer LIFE (Large Interferometer For Exoplanets).

These observatories would focus on complementary regions of the electromagnetic spectrum: HWO will explore the reflected light spectrum regime in the UV/VIS/NIR, while LIFE would capture the planetary thermal emission in the MIR. Both wavelength ranges provide us with their specific set of preferred information and come with specific drawbacks. Yet, the scientific yield of synergistic observations in the UV/VIS/NIR+MIR range has the potential of being greater than the sum of its parts: having access to multiple spectral windows into the atmosphere of a potentially habitable planet could be transformative for the search of life in the universe.

Shang-Min (Shami) Tsai (ASIAA)

In this talk, I will discuss how atmospheric chemistry and formation pathways shape sub-Neptune atmospheres, and how observed spectra can be used to learn their interior structure and potential habitability. Using the temperate sub-Neptune K2-18 b as a case study, I will highlight several diagnostics that can be generalized to the broader population. I will examine how orbital dynamics and cloud formation modulate observable water signatures, and present an agnostic framework to test chemical consistency in order to distinguish between different interior structures. I will conclude with an outlook on key concepts and directions for future studies.

Ana-Catalina Plesa (DLR)

Over the past decades, large-scale numerical simulations of interior evolution have grown to become one of the most powerful approaches to model the interior of planetary bodies in our Solar System and beyond. Geodynamic models are used to investigate the evolution of the interior that ultimately affects the distribution of seismic velocities, surface heat flow, and partial melting of the mantle. Combined with constraints derived from planetary mission data and laboratory experiments, these models help us to improve our understanding of the history and present-day thermal state of planetary interiors.

Miki Nakajima (University of Rochester)

Lisa Dang (Université de Montréal)