Photochemistry of Venus-like Planets Orbiting K- and M-dwarf Stars

Compared to the diversity seen in exoplanets, Venus is a veritable astrophysical twin of the Earth; however, its global cloud layer truncates features in transmission spectroscopy, masking its non-Earth-like nature. Observational indicators that can distinguish an exo-Venus from an exo-Earth must therefore survive above the cloud layer. The above-cloud atmosphere is dominated by photochemistry, which depends on the spectrum of the host star and therefore changes between stellar systems. We explore the systematic changes in photochemistry above the clouds of Venus-like exoplanets orbiting K-dwarf or M-dwarf host stars, using a recently validated model of the full Venus atmosphere (0–115 km) and stellar spectra from the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) Treasury survey. SO2, OCS, and H2S are key gas species in Venus-like planets that are not present in Earth-like planets, and could therefore act as observational discriminants if their atmospheric abundances are high enough to be detected. We find that SO2, OCS, and H2S all survive above the cloud layer when irradiated by the coolest K dwarf and all seven M dwarfs, whereas these species are heavily photochemically depleted above the clouds of Venus. The production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. Less steady-state photochemical oxygen and ozone forms with decreasing stellar effective temperature, and the effect of chlorine-catalyzed reaction cycles diminish in favor of HO x and SO x catalyzed cycles. We conclude that trace sulfur gases will be prime observational indicators of Venus-like exoplanets around M-dwarf host stars, potentially capable of distinguishing an exo-Venus from an exo-Earth.

Exogeology from Polluted White Dwarfs

It is difficult to study the interiors of terrestrial planets in the Solar System and the problem is magnified for distant exoplanets. However, sometimes nature is helpful. Some planetary bodies are torn to fragments and consumed by the strong gravity close to the descendants of Sun-like stars, white dwarfs. We can deduce the general composition of the planet when we observe the spectroscopic signature of the white dwarf. Most planetary fragments that fall into white dwarfs appear to be rocky with a variable fraction of associated ice and carbon. These white dwarf planetary systems provide a unique opportunity to study the geology of exoplanetary systems.

Europlanet 2024 RI - Global Collaboration and Integration Development, NA1-T4: Strategic Plan 2020–2024

<p>Since its foundation in 2005, Europlanet has sought to reach out and engage with planetary scientists across the globe. Today, Europlanet has a global role to connect the international planetary community through the common aim of working together to explore and understand our Solar System and exoplanetary systems beyond. It is therefore both timely and necessary to put in place a framework for a community-led roadmap for global collaboration as part of Europlanet’s future development as both a Research Infrastructure and as a Society.</p> <p>The Strategic Plan for Europlanet 2024 Research Infrastructure’s Global Collaboration and Integration Development that represents the outcome of a dedicated effort to define how to expand and intensify the new relationship between Europlanet and African, but also between Europlanet and North and South American and Asian collaborators, will be presented.</p>

ExoPhot: a new approach for the study of photosynthesis viability in exoplanetary systems.

<p>NASA and ESA are making plans for the next generation of space telescopes, which should be able to detect biomarkers in the atmospheres of exoplanets in the classical habitable zones around their stars (i.e., the range of separations at which water would be in liquid state on the exoplanet surface). The launch of <em>James Space Webb Telescope</em> is scheduled for October 2021. The main questions are related with the type of organisms producing such possible biomarkers and with the related metabolism? Will autotrophs be the base of the exoplanet ecological pyramid, as on Earth? Will they be phototroph or chemotroph? Will they be photosynthetic? Oxygenic or anoxygenic? Which will their photosynthetic pigments be? ESA’s <em>LIFE</em> or any other new concept for which scientific requirements have not been defined yet might be able to not only detect biomarkers, but to shed light on the actual biochemistry of exoplanet ecosystems. Therefore, investigating the potential variety of photosynthetic systems in exoplanets, either real or to be discovered, is actually very timely, as the requirements of new such telescope concepts are not set yet.</p> <p>The conversion of solar energy to chemical energy through photosynthesis is considered one of the first metabolic routes on planet Earth. Although a low percentage of the solar radiation from our Sun is captured by photosynthesis, this metabolic route provides the energy to drive all the life on Earth. Cyanobacteria are thought to be the first photosynthetic microorganisms on Earth. Subsequent photosynthetic organisms acquired photosynthesis via cyanobacteria endosymbionts, that evolved into chloroplasts in plants (Tomioka & Sugiura 1983).</p> <p>At the same time, photosynthesis modified the atmosphere of the early Earth by producing oxygen as a by-product. The concentration in this gas was increased in the primitive atmosphere, transforming the metabolic possibilities for the rest of organisms and, nowadays, oxygen supports the whole aerobic organisms on the planet. The only requirements that photosynthesis has are the exposure to optical radiation from the corresponding star and the availability of water and carbon dioxide (as a carbon source), making photosynthesis a putative imperative metabolism to be present in any particular radiative planetary system.</p> <p>To deepen into this idea, ExoPhot aims to study the relation between photosynthetic systems on exoplanets around different types of stars (i.e. stellar spectral types) from an astrobiological and multidisciplinary point of view, by focusing on two aspects:</p> <ul> <li>Assess the photosynthetic fitness of a variety of photopigments (either real or hypothetical) as a function of star, exoplanet and atmospheric scenario.</li> <li>Delineate a range of stellar, exoplanet and atmospheric parameters for which photosynthetic activity might be feasible.</li> </ul> <p>To accomplish these goals, we will use state-of-the-art planetary and stellar models to retrieve the radiation signatures at the planet surface for a wide range of exoplanet, atmosphere and host star parameters, and will carry out a quantification of the overlap (convolution) between those spectra with the absorption spectra of photosynthetic pigments, both terrestrial and hypothetical (our own developments on computer-simulated primordial pigments). Here, at the EPSC2021 conference, we present our preliminary results and future work to be developed.</p> <p> </p> <p><em>Bibliography:</em></p> <p>Tomioka, N. & Sugiura, M. The complete nucleotide sequence of a 16S ribosomal RNA gene from a blue-green alga, Anacystis nidulans. <em>Molecular and General Genetics, </em>1983<em>, 191</em>, 46–50. https://doi.org/10.1007/BF00330888</p> <p> </p>

The evolution of the solar wind

How has the solar wind evolved to reach what it is today? In this review, I discuss the long-term evolution of the solar wind, including the evolution of observed properties that are intimately linked to the solar wind: rotation, magnetism and activity. Given that we cannot access data from the solar wind 4 billion years ago, this review relies on stellar data, in an effort to better place the Sun and the solar wind in a stellar context. I overview some clever detection methods of winds of solar-like stars, and derive from these an observed evolutionary sequence of solar wind mass-loss rates. I then link these observational properties (including, rotation, magnetism and activity) with stellar wind models. I conclude this review then by discussing implications of the evolution of the solar wind on the evolving Earth and other solar system planets. I argue that studying exoplanetary systems could open up new avenues for progress to be made in our understanding of the evolution of the solar wind.

“In-System” Fission-Events: An Insight into Puzzles of Exoplanets and Stars?

In expansion of our recent proposal that the solar system’s evolution occurred in two stages—during the first stage, the gaseous giants formed (via disk instability), and, during the second stage (caused by an encounter with a particular stellar-object leading to “in-system” fission-driven nucleogenesis), the terrestrial planets formed (via accretion)—we emphasize here that the mechanism of formation of such stellar-objects is generally universal and therefore encounters of such objects with stellar-systems may have occurred elsewhere across galaxies. If so, their aftereffects may perhaps be observed as puzzling features in the spectra of individual stars (such as idiosyncratic chemical enrichments) and/or in the structures of exoplanetary systems (such as unusually high planet densities or short orbital periods). This paper reviews and reinterprets astronomical data within the “fission-events framework”. Classification of stellar systems as “pristine” or “impacted” is offered.

Host-star and exoplanet compositions: a pilot study using a wide binary with a polluted white dwarf

ABSTRACT Planets and stars ultimately form out of the collapse of the same cloud of gas. Whilst planets, and planetary bodies, readily loose volatiles, a common hypothesis is that they retain the same refractory composition as their host star. This is true within the Solar system. The refractory composition of chondritic meteorites, Earth, and other rocky planetary bodies are consistent with solar, within the observational errors. This work aims to investigate whether this hypothesis holds for exoplanetary systems. If true, the internal structure of observed rocky exoplanets can be better constrained using their host star abundances. In this paper, we analyse the abundances of the K-dwarf, G200-40, and compare them to its polluted white dwarf companion, WD 1425+540. The white dwarf has accreted planetary material, most probably a Kuiper belt-like object, from an outer planetary system surviving the star’s evolution to the white dwarf phase. Given that binary pairs are chemically homogeneous, we use the binary companion, G200-40, as a proxy for the composition of the progenitor to WD 1425+540. We show that the elemental abundances of the companion star and the planetary material accreted by WD 1425+540 are consistent with the hypothesis that planet and host-stars have the same true abundances, taking into account the observational errors.