Habitable Planets Research Articles

A Large and Variable Leading Tail of Helium in a Hot Saturn Undergoing Runaway Inflation

Atmospheric escape shapes the fate of exoplanets, with statistical evidence for transformative mass loss imprinted across the mass–radius–insolation distribution. Here, we present transit spectroscopy of the highly irradiated, low-gravity, inflated hot Saturn HAT-P-67 b. The Habitable Zone Planet Finder spectra show a detection of up to 10% absorption depth of the 10833 Å helium triplet. The 13.8 hr of on-sky integration time over 39 nights sample the entire planet orbit, uncovering excess helium absorption preceding the transit by up to 130 planetary radii in a large leading tail. This configuration can be understood as the escaping material overflowing its small Roche lobe and advecting most of the gas into the stellar—and not planetary—rest frame, consistent with the Doppler velocity structure seen in the helium line profiles. The prominent leading tail serves as direct evidence for dayside mass loss with a strong day-/nightside asymmetry. We see some transit-to-transit variability in the line profile, consistent with the interplay of stellar and planetary winds. We employ one-dimensional Parker wind models to estimate the mass-loss rate, finding values on the order of 2 × 1013 g s−1, with large uncertainties owing to the unknown X-ray and ultraviolet (XUV) flux of the F host star. The large mass loss in HAT-P-67 b represents a valuable example of an inflated hot Saturn, a class of planets recently identified to be rare, as their atmospheres are predicted to evaporate quickly. We contrast two physical mechanisms for runaway evaporation: ohmic dissipation and XUV irradiation, slightly favoring the latter.

Predicting the Dominant Formation Mechanism of Multiplanetary Systems

Most, if not all, Sun-like stars host one or more planets, making multiplanetary systems commonplace in our Galaxy. We utilize hundreds of multiplanet simulations to explore the origin of such systems, focusing on their orbital architecture. The first set of simulations assumes in situ assembly of planetary embryos, while the second explores planetary migration. After applying observational biases to the simulations, we compare them to 250+ observed multiplanetary systems, including 13 systems with planets in the habitable zone. For all of the systems, we calculate two of the so-called statistical measures: the mass concentration (S c ) and orbital spacing (S s ). After analytic and empirical analyses, we find that the measures are related to first order with a power law: Sc∼Ssβ . The in situ systems exhibit steeper power-law relations relative to the migration systems. We show that different formation scenarios cover different regions in the S s –S c diagram with some overlap. Furthermore, we discover that observed systems with S s < 30 are likely dominated by the migration scenario, while those with S s ≥ 30 are likely dominated by the in situ scenario. We apply these criteria to determine that a majority (62%) of observed multiplanetary systems formed via migration, whereas most systems with currently observed habitable planets formed via in situ assembly. This work provides methods of leveraging the statistical measures (S s and S c ) to disentangle the formation history of observed multiplanetary systems based on their present-day architectures.

Chapter 3: The Origins and Evolution of Planetary Systems.

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.

Earth as an Exoplanet. III. Using Empirical Thermal Emission Spectra as an Input for Atmospheric Retrieval of an Earth-twin Exoplanet

In this study, we treat Earth as an exoplanet and investigate our home planet by means of a potential future mid-infrared space mission called the Large Interferometer For Exoplanets (LIFE). We combine thermal spectra from an empirical data set of disk-integrated Earth observations with a noise model for LIFE to create mock observations. We apply a state-of-the-art atmospheric retrieval framework to characterize the planet, assess the potential for detecting the known bioindicators, and investigate the impact of viewing geometry and seasonality on the characterization. Our key findings reveal that we are observing a temperate habitable planet with significant abundances of CO2, H2O, O3, and CH4. Seasonal variations in the surface and equilibrium temperature, as well as in the Bond albedo, are detectable. Furthermore, the viewing geometry and the spatially and temporally unresolved nature of our observations only have a minor impact on the characterization. Additionally, Earth’s variable abundance profiles and patchy cloud coverage can bias retrieval results for the atmospheric structure and trace-gas abundances. Lastly, the limited extent of Earth’s seasonal variations in biosignature abundances makes the direct detection of its biosphere through atmospheric seasonality unlikely. Our results suggest that LIFE could correctly identify Earth as a planet where life could thrive, with detectable levels of bioindicators, a temperate climate, and surface conditions allowing liquid surface water. Even if atmospheric seasonality is not easily observed, our study demonstrates that next generation space missions can assess whether nearby temperate terrestrial exoplanets are habitable or even inhabited.

How to build a habitable planet: an interview with Charles H. Langmuir.

Earth is the only known habitable planet in the solar system. Understanding how Earth developed its unique habitability has been the frontier of Earth sciences and has become one of the main themes of current deep-space explorations. What are the decisive factors that led to a habitable planet? What is the role of solid Earth processes in the origin of life and in modulating the surface environment? Are Earth's habitability studies relevant to current challenges that human beings face? These questions have attracted the interest of both scientists and the public alike. NSR spoke to Prof. Charles H. Langmuir from Harvard University in the USA, who is a solid Earth geochemist who carries out research on diverse aspects of the plate tectonic geochemical cycle, including ocean ridges, convergent margins and intraplate volcanism. Prof. Langmuir is the author of the book How to Build a Habitable Planet (www.habitableplanet.org), one of the best Earth science books published in 2012.

Habitable planet to sustainable civilization: Global climate change with related clean energy transition reliant on declining critical metal resources

It took nearly four billion years after the birth of the Earth to make our planet habitable in terms of marine and terrestrial environment, resources, and life. This was made possible because of Earth’s unique tectonic system of recycling of critical elements through plate tectonics and subduction. However, with the spiraling growth in population and fast depletion of metallic resources, modern civilization is facing a severe energy crisis, in addition to the challenges to environment and life posed by anthropogenic activities and climate change. Here we present an outline of the evolutionary tectonic and geodynamic history of Earth from its birth into a habitable planet. We briefly address the current challenges that humans face in the fields of energy, environment, resources, and climate towards achieving sustainability of our modern civilization. We also evaluate the paradox of Net Zero targets, particularly in terms of the fast depletion of non-renewable critical metals required for alternate energy, as well as the threat of increasing ‘graveyards’ from green energy toxic waste enveloping the globe. We alert the unsound concepts being propagated, as viewed from a geological scale and geoscience perspective, on global warming and sea level changes. Our study emphasizes the need for a holistic logical and well-integrated scientific approach, rather than a one-dimensional climate science view, to effectively address the challenges of the planet’s sustainability.

Planetary Complexity Revealed by the Joint Differential Entropy of Eigencolors

We propose a measure, the joint differential entropy of eigencolors, for determining the spatial complexity of exoplanets using only spatially unresolved light-curve data. The measure can be used to search for habitable planets, based on the premise of a potential association between life and exoplanet complexity. We present an analysis using disk-integrated light curves from Earth, developed in previous studies, as a proxy for exoplanet data. We show that this quantity is distinct from previous measures of exoplanet complexity due to its sensitivity to spatial information that is masked by features with large mutual information between wavelengths, such as cloud cover. The measure has a natural upper limit and appears to avoid a strong bias toward specific planetary features. This makes it a novel and generalizable method, which, when combined with other methods, can broaden the available indicators of habitability.

Life on Earth can grow on extraterrestrial organic carbon

The universe is a vast store of organic abiotic carbon that could potentially drive heterotrophy on habitable planets. Meteorites are one of the transporters of this carbon to planetary surfaces. Meteoritic material was accumulating on early Earth when life emerged and proliferated. Yet it is not known if this organic carbon from space was accessible to life. In this research, an anaerobic microbial community was grown with the CM2 carbonaceous chondrite Aguas Zarcas as the sole carbon, energy and nutrient source. Using a reversed 13C-stable isotope labelling experiment in combination with optical photothermal infrared (O-PTIR) spectroscopy of single cells, this paper demonstrates the direct transfer of carbon from meteorite into microbial biomass. This implies that meteoritic organics could have been used as a carbon source on early Earth and other habitable planets, and supports the potential for a heterotrophic metabolism in early living systems.

Eccentricity Distribution beyond the Snow Line and Implications for Planetary Habitability

A fundamental question in the study of planetary system demographics is: how common is the solar system architecture? The primary importance of this question lies in the potential of planetary systems to create habitable environments, and dissecting the various components of solar system evolution that contributed to a sustainable temperate surface for Earth. One important factor in that respect is volatile delivery to the inner system and the dependence on giant planets beyond the snow line as scattering agents, particularly as such cold giant planets are relatively rare. Here, we provide an investigation of the eccentricity distribution for giant planet populations both interior and exterior to their system snow lines. We show that the median eccentricity for cold giants is 0.23, compared with a far more circular orbital regime for inner planets. We further present the results of a dynamical simulation that explores the particle scattering potential for a Jupiter analog in comparison with a Jupiter whose eccentricity matches that of the median cold giant eccentricity. These simulations demonstrate that the capacity for such an eccentric cold giant system to scatter volatiles interior to the snow line is significantly increased compared with the Jupiter analog case, resulting in a far greater volume of Earth-crossing volatiles. Thus, many of the known systems with cold giant planets may harbor water worlds interior to the snow line.

Low Part-Per-Trillion, Humidity Resistant Detection of Nitric Oxide Using Microtoroid Optical Resonators.

The nitric oxide radical plays pivotal roles in physiological as well as atmospheric contexts. Although the detection of dissolved nitric oxide in vivo has been widely explored, highly sensitive (i.e., low part-per-trillion level), selective, and humidity-resistant detection of gaseous nitric oxide in air remains challenging. In the field, humidity can have dramatic effects on the accuracy and selectivity of gas sensors, confounding data, and leading to overestimation of gas concentration. Highly selective and humidity-resistant gaseous NO sensors based on laser-induced graphene were recently reported, displaying a limit of detection (LOD) of 8.3 ppb. Although highly sensitive (LOD = 590 ppq) single-wall carbon nanotube NO sensors have been reported, these sensors lack selectivity and humidity resistance. In this report, we disclose a highly sensitive (LOD = 2.34 ppt), selective, and humidity-resistant nitric oxide sensor based on a whispering-gallery mode microtoroid optical resonator. Excellent analyte selectivity was enabled via novel ferrocene-containing polymeric coatings synthesized via reversible addition-fragmentation chain-transfer polymerization. Utilizing a frequency locked optical whispering evanescent resonator system, the microtoroid's real-time resonance frequency shift response to nitric oxide was tracked with subfemtometer resolution. The lowest concentration experimentally detected was 6.4 ppt, which is the lowest reported to date. Additionally, the performance of the sensor remained consistent across different humidity environments. Lastly, the impact of the chemical composition and molecular weight of the novel ferrocene-containing polymeric coatings on sensing performance was evaluated. We anticipate that our results will have impact on a wide variety of fields where NO sensing is important such as medical diagnostics through exhaled breath, determination of planetary habitability, climate change, air quality monitoring, and treating cardiovascular and neurological disorders.

The benchmark M dwarf eclipsing binary CM Draconis with TESS: spots, flares, and ultra-precise parameters

ABSTRACT A gold standard for the study of M dwarfs is the eclipsing binary CM Draconis. It is rare because it is bright (Jmag = 8.5) and contains twin fully convective stars on an almost perfectly edge-on orbit. Both masses and radii were previously measured to better than 1 per cent precision, amongst the best known. We use 15 sectors of data from the Transiting Exoplanet Survey Satellite (TESS) to show that CM Draconis is the gift that keeps on giving. Our paper has three main components. First, we present updated parameters, with radii and masses constrained to previously unheard-of precisions of ≈0.06 and ≈0.12 per cent, respectively. Secondly, we discover strong and variable spot modulation, suggestive of spot clustering and an activity cycle of the order of ≈4 yr. Thirdly, we discover 163 flares. We find a relationship between the spot modulation and flare rate, with flares more likely to occur when the stars appear brighter. This may be due to a positive correlation between flares and the occurrence of bright spots (plages). The flare rate is surprisingly not reduced during eclipse, but one flare may show evidence of being occulted. We suggest that the flares may be preferentially polar, which has positive implications for the habitability of planets orbiting M dwarfs.

Interior Controls on the Habitability of Rocky Planets

No matter how fascinating and exotic other terrestrial planets are revealed to be, nothing generates more excitement than announcements regarding their habitability. From the observation of Mars to present-day efforts toward Venus and the characterization of exoplanets, the search for life, or at least environments that could accommodate life, has been a major drive for space exploration. So far, we have found no other unquestionably habitable world besides Earth. The conditions of the habitability of terrestrial planets have proved elusive, as surface conditions depend on the complex interplay of many processes throughout the evolution of a planet. Here, we review how the interior of a rocky planet can drive the evolution of its surface conditions and atmosphere. Instead of listing criteria assumed to be critical for life, we discuss how the bulk-silicate planet can affect the onset, continuation, and cessation of habitability. We then consider how it can be observed, and current efforts toward this end.

Designing a Virtual Toolkit for Analysing Planetary Science Data in Virtual Reality.

To understand the surface evolution and potential habitability of other planets we must analyse their geology - the 3D structure and chemistry of the rocks that are exposed at the surface. Although rovers capture this 3D structure using stereo camera systems and other instruments, when we present this information to mission scientists for analysis it is generally confined to the 2D plane of a computer screen, and the spatial information is lost at the point when it is needed most. To address this problem, we design, develop, and evaluate a prototype Virtual Environment to present geological data in the 3D form in which it was originally captured, and users are supplied with a toolkit for measurement and annotation of data. We observed that users were inspired by the environment and felt more connected to it because they could move within the data; they valued the tools but did not trust the scale and therefore did not always trust the results. We conclude with recommendations for others working in this application area, and pose a series of questions for future research.

Ocean, atmosphere, and cloud quantity on the surface conditions of tidally-locked habitable zone planets

The study of tidally-locked planets has become an active field of research in recent years due to their interesting properties and abundance in the universe. As one hemisphere of these planets is always facing a star and the other is covered in perpetual darkness, habitable conditions on such extreme worlds were generally considered impossible. However, recent studies have shown that under specific planetary atmospheric and oceanic conditions, temperatures could stay mild enough for the planet to be habitable. In this paper, we created an energy balance model to determine the atmospheric and surface temperatures of tidally-locked worlds as a function of several planetary factors. We hypothesized that these factors—atmospheric and oceanic heat circulation and clouds—are crucial for habitable conditions to exist on these planets. We then assessed what specific set of values of these factors would make habitable conditions possible. We found that global heat transportation cycles act as a major determining factor on the planetary climate and, therefore, habitability. Apart from being in the habitable zone, Earth-like oceans and atmospheric coverage could also be essential for the habitability of tidally-locked planets, which establishes viable criteria in future searches for extraterrestrial life and livable worlds.

Understanding the Magnetic Activity of M Dwarfs: Optical and Near-Infrared Spectroscopic Studies

We present here preliminary results of stellar activities on three active M dwarfs having strong magnetic fields using optical and Near-Infrared (NIR) (0.38 μm − 2.5 μm) spectroscopic observational data using HFOSC on the 2-m HCT and TANSPEC on the 3.6-m DOT. The sample includes new observations of AD Leo, EV Lac, and Stkm2-809 M dwarfs, including published literature values of 89 M dwarfs. To understand the magnetic field activity on those dwarfs, we used the equivalent widths of spectral features Ca IRT triplet (0.850 μm, 0.854 μm, 0.866 μm), which are generally chromospheric and coronal indicators in M dwarfs, and investigate the correlation between those line strengths with Rossby number, which is defined as the ratio of rotation period (Prot) to convective turnover time (τconv). We find a strong correlation between the equivalent widths of Ca IRT b and Ca IRT c and Rossby number (R0). The correlation shows an increasing trend of equivalent widths of those lines with decreasing R0, and saturation of the equivalent widths of those at lower R0 ≤0.1. To estimate R0, we estimate the rotation periods of three observed M dwarfs in our sample from TESS data, and in the case of other M dwarfs, we used the literature values. Interestingly, in TESS light curves of AD Leo, EV Lac, and Stkm2-809 M dwarfs, we find several flare events. We estimated the bolometric flared energy in a range of 1034 to 1037 erg, which is in the superflare range (more than 1032 erg). We estimate the highest flared energy of 1.22 × 1037 erg for EV Lac in TESS sector 57. To produce such kind of high superflare event, we have estimated the required magnetic field strength of 10.53 kG, which will have an impact on the life habitability of planets around such M dwarfs.

Detecting Biosignatures in Nearby Rocky Exoplanets Using High-contrast Imaging and Medium-resolution Spectroscopy with the Extremely Large Telescope

In the upcoming decades, one of the primary objectives in exoplanet science is to search for habitable planets and signs of extraterrestrial life in the Universe. Signs of life can be indicated by thermal-dynamical imbalance in terrestrial planet atmospheres. O2 and CH4 in the modern Earth’s atmosphere are such signs, commonly termed biosignatures. These biosignatures in exoplanetary atmospheres can potentially be detectable through high-contrast imaging instruments on future extremely large telescopes. To quantify the signal-to-noise ratio (S/N) with extremely large telescopes, we select up to 10 nearby rocky planets and simulate medium-resolution (R ∼ 1000) direct imaging of these planets using the Mid-infrared ELT Imager and Spectrograph (ELT/METIS, 3–5.6 μm) and the High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (ELT/HARMONI, 0.5–2.45 μm). We calculate the S/N for the detection of biosignatures including CH4, O2, H2O, and CO2. Our results show that GJ 887 b has the highest detection of S/N for biosignatures, and Proxima Cen b exhibits the only detectable CO2 among the targets for ELT/METIS direct imaging. We also investigate the TRAPPIST-1 system, the archetype of nearby transiting rocky planet systems, and compare the biosignature detection of transit spectroscopy with JWST versus direct spectroscopy with ELT/HARMONI. Our findings indicate JWST is more suitable for detecting and characterizing the atmospheres of transiting planet systems such as TRAPPIST-1 that are relatively further away and have smaller angular separations than more nearby nontransiting planets.

Thriving on Mars: Potatoes as the Answer to Martian Challenges

Humans have committed themselves to the exploration of space and the quest for a habitable planet. Central to the development of a sustainable ecosystem that can provide essential resources like food and water is the implementation of space plant cultivation systems. In the pursuit of bio-regenerative life support systems for human space exploration, the inclusion of plants is imperative. This study focuses on assessing potential challenges associated with cultivating Potatoes (Solanum tuberosum) under simulated Martian conditions. Potatoes exhibit adaptability to extremely cold and dry environments, making them a promising candidate for extraterrestrial agriculture. The advantages of potatoes include their high yield, rich content of easily digestible carbohydrates, elevated protein levels, and simplicity of propagation. When induced to tuberize, potatoes can achieve a harvest index exceeding 80%, approximately double that of traditional grain crops. Notably, unlike certain crops such as soybeans and certain grains, potatoes do not necessitate elaborate processing procedures before consumption. The Potato, specifically varieties like Norland, Denali, and Russet Burbank, has garnered attention in research funded by the US National Aeronautics and Space Administration (NASA). Tests have revealed that these cultivars can successfully undergo tuberization even under constant bright light conditions, emphasizing their potential suitability for life support functions in space exploration contexts

Practicable Artificial Photosynthesis: Its Relevance, Fundamental Challenges and Opportunities

The single main resource for the survival of humankind on this planet is the sunlight and it is freely coming to every home like the life-supporting gas, oxygen, is coming to every individual. When society becomes master in harvesting such a precious, Omnipresent, carbon-neutral, clean, freely and abundantly available renewable energy resource sunlight in a suitable form of chemical energy such as, H2, O2, CO, methanol, gasoline (petrol), diesel, etc., with a minimum conversion efficiency of 10% by using carbon dioxide (CO2) and water (H2O) as energy storing materials, which is referred to as the “Practicable Artificial Photosynthesis (PAP)” process, it can be employed for commercial practice with economic viability to meet all the energy needs of the society without any back-up by electricity generated at thermal power plants by burning greenhouse gas CO2 releasing fossil fuels. In fact, the PAP process is the only option available today for humankind to make energy, environment, economy, and life sustainable on our only habitable planet, Earth. In this article, (i) how the PAP process can change the global energy economy and energy dynamics of the world while solving all the humanities top 10 problems for the next 50 years, (ii) how the entire CO2 gas generated as a waste across the globe at all the major outlets can be eventually converted into petrol (i.e., gasoline) given the fossil fuels cannot sustain our civilization’s economic growth, (iii) the effectiveness of existing CO2 sequestration vs. carbon capture and utilization (CCU) processes, (iv) nature’s response to anthropogenic CO2 gas rise in the atmosphere, (v) steps involved in the realization of a desirable PAP process, (vi) why silicon photovoltaic cell (SPVC) solar panels cannot solve the energy problems of our society considering the outcome of Germany’s Energiewende program, etc., have been meticulously presented and described in this article.

A Tectonic Manifesto

AbstractTectonics is the scientific discipline that studies the relationship between planetary convection and surface deformation and magmatism. In recent years global tectonic regimes and planetary evolution have become foci of astronomical and planetological research dealing in particular with potential habitability of planets orbiting other stars in our galaxy (exoplanets). This broadens the scientific impact of “tectonics” and makes it useful to outline a few guiding principles. For many, the term “tectonics” is synonymous with Plate Tectonics on Earth, but it should not be limited to only the planet we know best. Specifically, the concept should be generalized to encompass other rocky planets and moons in our Solar System and exoplanets. This article summarizes 15 general concepts about how such bodies are expected to behave.

Geochemical and Photochemical Constraints on S[IV] Concentrations in Natural Waters on Prebiotic Earth

AbstractAqueous S[IV] species (, ) derived from volcanogenic atmospheric SO2 are important to planetary habitability through their roles in proposed origins‐of‐life chemistry and influence on atmospheric sulfur haze formation, but the early cycling of S[IV] is poorly understood. Here, we combine new laboratory constraints on S[IV] disproportionation kinetics with a novel aqueous photochemistry model to estimate the concentrations of S[IV] in natural waters on prebiotic Earth. We show that S[IV] disproportionation is slow in pH ≥ 7 waters, with timescale T ≥ 1 year at room temperature, meaning that S[IV] was present in prebiotic natural waters. However, we also show that photolysis of S[IV] by UV light on prebiotic Earth limited [S[IV]] &lt; 100 µM in global‐mean steady‐state. Because of photolysis, [S[IV]] was much lower in natural waters compared to the concentrations generally invoked in laboratory simulations of origins‐of‐life chemistry (≥10 mM), meaning further work is needed to confirm whether laboratory S[IV]‐dependent prebiotic chemistries could have functioned in nature. [S[IV]] ≥ 1 µM in terrestrial waters for: (a) SO2 outgassing ≥20× modern, (b) pond depths &lt;10 cm, or (c) UV‐attenuating agents present in early waters or the prebiotic atmosphere. Marine S[IV] was sub‐saturated with respect to atmospheric SO2, meaning that atmospheric SO2 deposition was efficient and that, within the constraints of present knowledge, UV‐attenuating sulfur hazes could only have persisted on prebiotic Earth if sulfur emission rates were very high (≳100× modern). Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry toward understanding the emergence and maintenance of life on early Earth.