The proposed Habitable Worlds Observatory is intended to observe the atmospheres of nearby terrestrial exoplanets with a resolution greater than that of any previous instrument. While these observations present a substantial opportunity for astrobiology, they also incur the risk of false positives and false negatives. Here, we explore the use of systems science (in the form of network theory and thermochemical kinetics) to mitigate these risks, and briefly describe the technical specifications HWO would require in order to use these methodologies.

New observational facilities are beginning to enable insights into the three-dimensional (3D) nature of exoplanets. Transmission spectroscopy is the most widely used method for characterizing transiting temperate exoplanet atmospheres, but because it only provides a glimpse of the planet’s limb and nightside for a typical orbit, its ability to probe 3D characteristics is still an active area of research. Here, we use the ROCKE-3D general circulation model to test the impact of synchronization state, a “low-order” 3D characteristic previously shown to drive differences in planetary phase curves, on the transmission spectrum of a representative super-Earth land planet across temperate-to-warm instellations ( S _p = 0.8, 1, 1.25, 1.66, 2, 2.5, 3, 4, and 4.56 S _⊕ ). We find that different synchronization states do display differences in their transmission spectra, primarily driven by clouds and humidity, and that the differences shrink or disappear in hotter regimes where water clouds are unable to condense (though our simulations do not consider haze formation). The small size of the feature differences and potential for degeneracy with other properties, like differing water content or atmospheric structure, mean that we do not specifically claim to have identified a single transmission diagnostic for synchronization state, but our results can be used for holistic spectrum interpretation and sample creation and suggest the need for more modeling in this area.

The search for life beyond Earth necessitates a rigorous and comprehensive examination of biosignatures, the types of observable imprints that life produces. These imprints and our ability to detect them with advanced instrumentation hold the key to our understanding of the presence and abundance of life in the universe. Biosignatures are the chemical or physical features associated with past or present life and may include the distribution of elements and molecules, alone or in combination, as well as changes in structural components or physical processes that would be distinct from an abiotic background. The scientific and technical strategies used to search for life on other planets include those that can be conducted in situ to planetary bodies and those that could be observed remotely. This chapter discusses numerous strategies that can be employed to look for biosignatures directly on other planetary bodies using robotic exploration including those that have been deployed to other planetary bodies, are currently being developed for flight, or will become a critical technology on future missions. Search strategies for remote observations using current and planned ground-based and space-based telescopes are also described. Evidence from spectral absorption, emission, or transmission features can be used to search for remote biosignatures and technosignatures. Improving our understanding of biosignatures, their production, transformation, and preservation on Earth can enhance our search efforts to detect life on other planets.

As JWST begins to return observations, it is more important than ever that exoplanet climate models can consistently and correctly predict the observability of exoplanets, retrieval of their data, and interpretation of planetary environments from that data. Model intercomparisons play a crucial role in this context, especially now when few data are available to validate model predictions. The CUISINES Working Group of NASA's Nexus for Exoplanet Systems Science supports a systematic approach to evaluating the performance of exoplanet models and provides here a framework for conducting community-organized exoplanet model intercomparison projects (exoMIPs). The CUISINES framework adapts Earth climate community practices specifically for the needs of the exoplanet researchers, encompassing a range of model types, planetary targets, and parameter space studies. It is intended to help researchers to work collectively, equitably, and openly toward common goals. The CUISINES framework rests on five principles: (1) define in advance what research question(s) the exoMIP is intended to address, (2) create an experimental design that maximizes community participation and advertise it widely, (3) plan a project timeline that allows all exoMIP members to participate fully, (4) generate data products from model output for direct comparison to observations, and (5) create a data management plan that is workable in the present and scalable for the future. Within the first years of its existence, CUISINES is already providing logistical support to 10 exoMIPs and will continue to host annual workshops for further community feedback and presentation of new exoMIP ideas.

The Astrobiology Primer 3.0 (ABP3.0) is a concise introduction to the field of astrobiology for students and others who are new to the field of astrobiology. It provides an entry into the broader materials in this supplementary issue of Astrobiology and an overview of the investigations and driving hypotheses that make up this interdisciplinary field. The content of this chapter was adapted from the other 10 articles in this supplementary issue and thus represents the contribution of all the authors who worked on these introductory articles. The content of this chapter is not exhaustive and represents the topics that the authors found to be the most important and compelling in a dynamic and changing field.

Scientific ideas about the potential existence of life elsewhere in the universe are predominantly informed by knowledge about life on Earth. Over the past ∼4 billion years, life on Earth has evolved into millions of unique species. Life now inhabits nearly every environmental niche on Earth that has been explored. Despite the wide variety of species and diverse biochemistry of modern life, many features, such as energy production mechanisms and nutrient requirements, are conserved across the Tree of Life. Such conserved features help define the operational parameters required by life and therefore help direct the exploration and evaluation of habitability in extraterrestrial environments. As new diversity in the Tree of Life continues to expand, so do the known limits of life on Earth and the range of environments considered habitable elsewhere. The metabolic processes used by organisms living on the edge of habitability provide insights into the types of environments that would be most suitable to hosting extraterrestrial life, crucial for planning and developing future astrobiology missions. This chapter will introduce readers to the breadth and limits of life on Earth and show how the study of life at the extremes can inform the broader field of astrobiology.

The materials that form the diverse chemicals and structures on Earth-from mountains to oceans and biological organisms-all originated in a universe dominated by hydrogen and helium. Over billions of years, the composition and structure of the galaxies and stars evolved, and the elements of life, CHONPS, were formed through nucleosynthesis in stellar cores. Climactic events such as supernovae and stellar collisions produced heavier elements and spread them throughout the cosmos, often to be incorporated into new, more metal-rich stars. Stars typically form in molecular clouds containing small amounts of dust through the collapse of a high-density core. The surrounding nebular material is then pulled into a protoplanetary disk, from which planets, moons, asteroids, and comets eventually accrete. During the accretion of planetary systems, turbulent mixing can expose matter to a variety of different thermal and radiative environments. Chemical and physical changes in planetary system materials occur before and throughout the process of accretion, though many factors such as distance from the star, impact history, and level of heating experienced combine to ultimately determine the final geophysical characteristics. In Earth's planetary system, called the Solar System, after the orbits of the planets had settled into their current configuration, large impacts became rare, and the composition of and relative positions of objects became largely fixed. Further evolution of the respective chemical and physical environments of the planets-geosphere, hydrosphere, and atmosphere-then became dependent on their local geochemistry, their atmospheric interactions with solar radiation, and smaller asteroid impacts. On Earth, the presence of land, air, and water, along with an abundance of important geophysical and geochemical phenomena, led to a habitable planet where conditions were right for life to thrive.

While the near-term Interstellar Probe mission will revolutionize our understanding of the astrosphere in which we live, it will provide only a snapshot of the history of the heliosphere.Trans-Neptunian objects (TNOs), with numerous options conveniently located along whichever Interstellar Probe trajectory would ultimately be chosen, provide an opportunity to examine the history of the heliosphere measurable in the colors, spectra, and geologies of their surfaces.Interstellar Probe with a planetary augmentation would enable a scientifically rich close flyby of a TNO along with "remote" observations.The spacecraft can function as an in situ observatory to study TNOs up to 2 -3 au away, an order of magnitude closer than they are to the Earth.

Haberle et al. (2019) documented the status of the NASA Ames Legacy Mars Global Climate Model (GCM) and presented a baseline water cycle simulation. Since publishing that manuscript, we have found an error in the Legacy Mars GCM radiation code related to how the infrared cloud opacities were computed. Here we document this code error, its correction, how the corrected radiation code affects the simulated water cycle from Haberle et al. (2019), and what is required to produce a new realistic baseline water cycle simulation. We show that we can recover many, but not all, of the key aspects of the Haberle et al. (2019) baseline simulation by implementing a small change to the contact parameter, which is a microphysical parameter that affects the ease at which nucleation occurs. The fact that such a small change to the contact parameter can yield very different water cycles illustrates how sensitive the climate system is to cloud microphysical processes and highlights the need for future numerical and experimental study.

Efforts to spectrally characterize the atmospheric compositions of temperate terrestrial exoplanets orbiting M dwarf stars with JWST are now underway. Key molecular targets of such searches include O _2 and CO, which are potential indicators of life. Recently, it was proposed that CO _2 photolysis generates abundant (≳0.1 bar) abiotic O _2 and CO in the atmospheres of habitable M dwarf planets with CO _2 -rich atmospheres, constituting a strong false positive for O _2 as a biosignature and further complicating efforts to use CO as a diagnostic of surface biology. Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under observation with JWST, would abiotically accumulate abundant O _2 and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O _2 and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O _2 remains a trace gas on habitable CO _2 -rich M dwarf planets, with earlier predictions of abundant O _2 and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO _2 photolysis peak on such worlds. Our work strengthens the case for O _2 as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O _2 production. However, observationally relevant false-positive potential remains, especially for O _2 's photochemical product O _3 , and further work is required to confidently understand O _2 and O _3 as biosignature gases on M dwarf planets.

TOI-1266c is a recently discovered super-Venus in the radius valley orbiting an early M dwarf. However, its notional bulk density (∼2.2 g cm −3 ) is consistent with a large volatile fraction, suggesting that it might have volatile reservoirs that have survived billions of years at more than twice Earth’s insolation. On the other hand, the upper mass limit paints a picture of a cool super-Mercury dominated by >50% iron core (∼9.2 g cm −3 ) that has tiptoed up to the collisional stripping limit and into the radius gap. Here we examine several hypothetical states for TOI-1266c using a combination of new and updated open-source atmospheric escape, radiative−convective, and photochemical models. We find that water-rich atmospheres with trace amounts of H 2 and CO 2 are potentially detectable (S/N > ∼ 5) in less than 20 hr of James Webb Space Telescope (JWST) observing time. We also find that water vapor spectral features are not substantially impacted by the presence of high-altitude water or ice clouds owing to the presence of a significant amount of water above the cloud deck, although further work with self-consistent cloud models is needed. Regardless of its mass, however, TOI-1266c represents a unique proving ground for several hypotheses related to the evolution of sub-Neptunes and Venus-like worlds, particularly those near the radius valley.

While the origin of lunar polar volatiles remains an open question, their most likely sources are volcanic outgassing or volatile-rich impactors. Both such sources are sporadic in nature and are characterized by release of large amounts of volatiles over a short period of time and long periods of repose between such events. If a sufficient amount of volatiles was generated in such a delivery event, a transient collisional atmosphere could form. Such an atmosphere, if it persists for a long enough time, would protect certain volatiles (like water) from photodissociation and escape to space and would promote their transport to the polar cold traps where they could be stored and preserved for billions of years. Hence, such transient atmospheres could have a significant impact on the distribution and abundance of volatiles currently observed on the Moon. Here we study such a hypothetical atmosphere that could have been formed due to volcanic outgassing during the peak of lunar volcanic activity at ~3.5 Ga and investigate its longevity, climatology and effect on volatile transport. We employ the ROCKE-3D [1] planetary climate model to simulate processes in a volcanically-induced lunar atmosphere. We use orbital and radiation parameters corresponding to conditions at 3.5 Ga (17.8 days rotation period and a solar constant 75% of the modern value). For most experiments we use zero obliquity, though we investigate the effect of non-zero obliquity on atmospheric stability and volatile transport. We assume a CO2-dominated atmosphere in accordance with predictions of our chemistry model [2]. For the atmospheric thickness we follow the argument of Head et al. [3] that due to long periods of repose between the volcanic events the atmosphere would not accumulate above the pressure of a few microbars, and thus we limit our parameter space to a range of 1 microbar to 1 mb surface pressures. To investigate the ability of such an atmosphere to transport volatiles we set up a typical volcanic eruption experiment [4] and follow the fate of the outgassed water. In most of our experiments the atmosphere was stable, though in some cases a small non-zero obliquity (a few degrees) was needed to prevent a collapse due to CO2 condensation at the poles. We found that even very thin atmospheres were efficiently transporting volatiles to the poles. The efficiency of transport sometimes was higher for thinner atmospheres, most likely due to a stronger circulation cell. We also found that water transport efficiency depended on initial conditions at the surface. A water-free dry surface suppressed re-evaporation, thus reducing the total flux of outgassed water to the poles. But even in the case of dry soil, water transport was efficient with 19% of outgassed water delivered to the poles in just a few months (for the 10 microbar atmosphere). References: [1] Way M. J. et al. (2017) ApJS, 231, 12. [2] Aleinov I. et al. (2019) GRL, 46, 5107–5116. [3] Head J. W. et al. (2020) GRL, 47, e2020GL089509. [4] Wilson L. and Head J. W. (2018) GRL, 45, 5852-5859.

Multiplanet systems orbiting M dwarfs provide valuable tests of theories of small-planet formation and evolution. K2-3 is an early M dwarf hosting three small exoplanets (1.5–2.0 R ⊕) at distances of 0.07–0.20 au. We measure the high-energy spectrum of K2-3 with HST/COS and XMM-Newton and use empirically driven estimates of Lyα and extreme-ultraviolet flux. We use EXOFASTv2 to jointly fit radial velocity, transit, and spectral energy distribution data. This constrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b and c to 13% and 30%, respectively; K2-3d is not detected in radial velocity measurements. K2-3b and c are consistent with rocky cores surrounded by solar composition envelopes (mass fractions of 0.36−0.11+0.14% and 0.07−0.05+0.09% ), H2O envelopes ( 55−12+14% and 16−10+17% ), or a mixture of both. However, based on the high-energy output and estimated age of K2-3, it is unlikely that K2-3b and c retain solar composition atmospheres. We pass the planet parameters and high-energy stellar spectrum to atmospheric models. Dialing the high-energy spectrum up and down by a factor of 10 produces significant changes in trace molecule abundances, but not at a level detectable with transmission spectroscopy. Though the K2-3 planets span the small-planet radius valley, the observed system architecture cannot be readily explained by photoevaporation or core-powered mass loss. We instead propose that (1) the K2-3 planets are all volatile-rich, with K2-3d having a lower density than typical of super-Earths, and/or (2) the K2-3 planet architecture results from stochastic processes such as planet formation, planet migration, and impact erosion.

We validate the planetary nature of an ultra-short-period planet orbiting the M dwarf KOI-4777. We use a combination of space-based photometry from Kepler, high-precision, near-infrared Doppler spectroscopy from the Habitable-zone Planet Finder, and adaptive optics imaging to characterize this system. KOI-4777.01 is a Mars-sized exoplanet ( R p = 0.51 ± 0.03 R ⊕ ) orbiting the host star every 0.412 days (∼9.9 hr). This is the smallest validated ultra-short period planet known and we see no evidence for additional massive companions using our HPF RVs. We constrain the upper 3 σ mass to M p < 0.34 M ⊕ by assuming the planet is less dense than iron. Obtaining a mass measurement for KOI-4777.01 is beyond current instrumental capabilities.

Astrophysical measurements at distances outside of 50 AU from the Sun enable transformative science that are impossible to obtain from inner system platforms, such as a: Complete census of all interplanetary dust and ice permeating the solar system, and direct comparison of its distribution to exo-circumstellar disks; Stable time-domain astronomy of exoplanet transit lightcurves, exoplanet microlensing events, supernova explosions, and black hole mergers; and Measurement of all the light created by planets, stars, and galaxies over the entire history of the Universe. A single powerful instrument could be used to obtain these measurements, and to perform New Horizons-style flybys of dwarf planets & KBOs, potentially doubling the number of these kinds of worlds that have been visited and studied in great detail [1].Though likely transformative, opportunities to fly instrumentation capable of these measurements are rare, and a mission to the distant solar system that includes instrumentation expressly designed to perform astrophysical science has never been flown.The proposed Interstellar Probe Mission (ISP;(2)(3)(4)(5)) provides just such an opportunity for launch in 2030.In this White Paper, we describe an instrument concept to ride along on a Heliospheric Division Interstellar Probe Mission that would enable groundbreaking science via cross-divisional collaboration and cooperation within NASA (c.f.[6]).

TRANSPORT AND DEPOSITION OF POLAR VOLATILES. I. Aleinov1,2, M. J. Way2,6, K. Tsigaridis1,2, E. T. Wolf3, C. Harman4, G. Gronoff5,7, and C. W. Hamilton8, 1Center for Climate Systems Research, Columbia University, New York, NY 10025, USA (igor.aleinov@columbia.edu), 2NASA Goddard Institute for Space Studies, New York, NY, 10025, USA, 3University of Colorado, Boulder, USA, 4Space Sciences Division, NASA Ames Research Center, Moffett Field, CA, USA, 5Science Directorate, Chemistry and Dynamics Branch, NASA Langley Research Center, Hampton, VA, USA, 6Theoretical Astrophysics, Department of Physics & Astronomy, Uppsala University, Uppsala SE-75120, Sweden, 7SSAI, Hampton, VA, USA, 8Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

Plain language summaryLooking back at the solar system offers several unique scientific and social perspectives, largely captured in a piecemeal fashion during past missions.These observations and the hypotheses based on them can be verified with other techniques and missions, unlike for extrasolar planets, where no such corroborating data sets are reasonably expected.This makes analog data sets, specifically observations of the solar system at larger distances, a critical stepping stone towards reliably interpreting future exoplanet observations.Looking back at the solar system should be encouraged for future missions by explicitly listing it as a target of opportunity.

Earth has irrevocably shaped our view of the natures of other worlds. In turn, discoveries about planets other than our own have fundamentally altered how we think about the origin, evolution, and ultimate fate of the Earth. But how much can we really know about planets outside our solar system? Here we begin to answer that question by laying out how we know what we currently know about the planets in our solar system. This holistic framework is then extended to current and future characterizations of extrasolar planets. As the majority of information available to us is derived from a planet's atmosphere, the remainder of the text lays out some of the links between a planet's atmospheric composition, bulk composition, and formation history.

The field of exoplanetary science has emerged over the past two decades, rising up alongside traditional solar system planetary science. Both fields focus on understanding the processes which form and sculpt planets through time, yet there has been less scientific exchange between the two communities than is ideal. This white paper explores some of the institutional and cultural barriers which impede cross-discipline collaborations and suggests solutions that would foster greater collaboration. Some solutions require structural or policy changes within NASA itself, while others are directed towards other institutions, including academic publishers, that can also facilitate greater interdisciplinarity.