III. Ecosmos: A Revolutionary Fertile, Habitable, Solar-Bioplanet, Incubator Lifescape

H. Stellar Planetary Systems: A Diverse Profusion of Galaxies, Solar Orrerys and Habitable Zones

Martin, Rebecca and Mario Livio. The Solar System as an Exoplanetary System. arXiv:1508.00931. With a wealth of satellite, observatory, and laboratory findings about a dynamic cosmos filled with stochastic orbital worlds to now draw upon, University of Nevada and Space Telescope Science Institute astronomers can assess the relative nature of our home environs. While not “extremely rare,” this sun and nine planet array is special for circular orbits, only one close-in planet, and an absence of disruptive “super-Earths.” See also papers by Giovanna Tinetti, Elke Pilat-Lohinger, and Allessandro Morbidelli for similar notice of a favorable, unique solar system, along with Tsvi Piran for even a favorable Milky Way galaxy.

In general, there are three aspects in which the Solar System differs most from other observed multi-planet systems. First, the low mean eccentricity of the planets in the Solar
System maybe somewhat special, although this may be accounted for by selection effects. Secondly, there is in the total lack of planets inside Mercury’s orbit. Massive planets migrating through the habitable zone can change the course of planet formation in that region. Overall, however, processes that could act to clear the inner part of the Solar Systems (such as giant planet migration), are believed to be operating within a non-negligible fraction of the exoplanet systems. Third, the lack of super-Earths in our Solar System is somewhat special and could have allowed the Earth to become habitable. (6-7)

Mason, John, ed. Exoplanets: Detection, Formation, Properties, Habitability. Berlin: Springer/Praxis Publishing, 2008. I recall my youth some 6 decades ago when any hope of finding other extrasolar planets was held to be quite remote. In the past few years a burst of discoveries fueled by advanced telescope and satellite instrumentation, along with computer enhancement, has found over 200 such objects from initial Jupiter giants to lately more Earth size neighbors.

Mayor, Michel and Pierre-Yves Frei. New Worlds in the Cosmos: The Discovery of the Exoplanets. Cambridge: Cambridge University Press, 2003. Progress in detecting large, Jupiter-type orbiting planets as of 2001.

Meadows, Victoria, et al. Community Report from the Biosignatures Standards of Evidence Workshop. arXiv:2210.14293. A major NASA, ASU plus document by over 80 astro-authorities such as Glada Arney, Laura Barge, Chris Kempes, Sanjoy Som and Sara Walker to scope out, get in front of, this vast enterprise of our Earthly search quest for other near and far animate presences. Many more contributors are cited to give this circa 2021 project a broad credibility. The comprehensive, graphic report goes on for 86 pages, with a long reference list

The search for life beyond the Earth is the premier goal of the NASA Astrobiology Program for the scientific missions that explore the environments of Solar System planets and exoplanets. However, the detection of extraterrestrial life is so challenging that many technical approaches will be needed to make a convincing claim. Current and upcoming research efforts aimed at past and extant life could avail a consensus framework to plan for, assess and discuss life detection. Given the importance of exolife searches to NASA, and its complex scientific challenges and conveyance, the astrobiology community needs to develop guidelines for biosignature findings, along with clear reporting protocols. (Executive Summary)

Menou, Kristen. Climate Stability of Habitable Earth-like Planets. Earth and Planetary Science Letters. 429/20, 2015. The University of Toronto astrophysicist adds to the latest research on exoworld chemical geologies and consequent atmospheres. A result is that the width of a solar habitable zone need be finessed by such variable properties so that inner and outer edges become less conducive, a central path is most preferred. See also Haqq-Misra and Turbet herein for more science.

The carbon–silicate cycle regulates the atmospheric CO2 content of terrestrial planets on geological timescales through a balance between the rates of CO2 volcanic outgassing and planetary intake from rock weathering. It is thought to act as an efficient climatic thermostat on Earth and, by extension, on other habitable planets. If, however, the weathering rate increases with the atmospheric CO2 content, as expected on planets lacking land vascular plants, the carbon–silicate cycle feedback can become severely limited. Here we show that Earth-like planets receiving less sunlight than current Earth may no longer possess a stable warm climate but instead repeatedly cycle between unstable glaciated and deglaciated climatic states. This has implications for the search for life on exoplanets in the habitable zone of nearby stars. (Abstract)

The key new feature of our analysis is the lack of stable climates on Earth-like planets lacking land vascular plants, at low enough insolation levels. This suggests that a subset of Earth-like planets located in the outer regions of habitable zones may be preferentially found in a frozen, rather than deglaciated, state. A globally frozen state might be observationally inferred from the very high albedo and the correspondingly low water content of the planet’s atmosphere. According to these results, some Earth-like planets in the outer habitable zone would also be caught in a transiently warm state with surface liquid water present only infrequently. (23)

Messeri, Lisa. Placing Outer Space: An Earthly Ethnography of Other Worlds. Durham, NC: Duke University Press, 2016. In response to 21st century discoveries of a prolific cosmos that appears to innately seed itself with planetary objects, a University of Virginia professor of science, technology, and society describes her project to broach a conceptual vision and response. By any measure, a vital imaginary shift is in order to appreciate our individual and collective identity as roundly global in kind, verily that habitable bioworlds seem to be nature’s essential feature. As the note says, she imbedded in the planet search community with interviews and travel, which welcomed her endeavor. A prior guide is Gayatri Spivak’s (search) sense of “planetarity,” by this novel vista, our allegiance is not to tribes or nations, but as Earthlings whose wholly home is this precious biosphere. See also her Op-Ed: What’s So Special About Another Earth? in the NY Times (August 25, 2016) on the finding of our Proxima Centauri neighbor.

In Placing Outer Space Lisa Messeri traces how the place-making practices of planetary scientists transform the void of space into a cosmos filled with worlds that can be known and explored. Making planets into places is central to the daily practices and professional identities of the astronomers, geologists, and computer scientists Messeri studies. She takes readers to the Mars Desert Research Station and a NASA research center to discuss ways scientists experience and map Mars. At a Chilean observatory and in MIT's labs she describes how they discover exoplanets and envision what it would be like to inhabit them. Today’s planetary science reveals the universe as densely inhabited by evocative worlds, which in turn tells us more about Earth, ourselves, and our place in the universe.

Mettler, Jean-Noel, et al.. Earth as an Exoplanet. III. Using Empirical Thermal Emission Spectra as Input for Atmospheric Retrieval of an Earth-Twin Exoplanet. arXiv:2310.02634. In three entries since October 2000, ETH Zurich astrophysicists propose an approach to quantify and evaluate our own candidate habitable bioworld as some manner of baseline guide. Prior versions are Earth as an Exoplanet. II. Earth's Time-variable Thermal Emission and Its Atmospheric Seasonality of Bioindicators (2210.05414) and Earth as an Exoplanet: I. Time variable thermal emission using spatially resolved MODIS data. (2010.02589).

In these studies, we treat Earth as an exoplanet and investigate our home planet by means of a potential future mid-infrared (MIR) space mission called the Large Interferometer For Exoplanets (LIFE). Key findings include: Our world is a temperate habitable planet with abundant CO2, H2O, O3, and CH4; seasonal variations in temperature, and albedo are detectable; and Earth's variable H2O profile and patchy clouds lead to biased results for atmospheric make up. 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. (Abstract)

Millholland, Sarah and Joshua Winn. Split Peas in a Pod: Intra-System Uniformity of Super-Earths and Sub-Neptunes. arXiv:2110.01466. Princeton University astrophysicists contribute to growing perceptions that entire solar orrery planetary systems have an overall mathematic formative basis. In these cases, the presence of repetitive patterns which govern orbital arrangements are becoming evident. See also Generalized Peas-in-a-Pod: Extending Intra-System Mass Uniformity by Armaan Goyal and Songhu Wang at 2206.00053.

The planets within compact multi-planet systems tend to have similar sizes, masses, and orbital period ratios, like "peas in a pod". The smaller "super-Earths" are consistent with being stripped rocky cores, while the larger "sub-Neptunes" likely have gaseous H/He envelopes. Given these consistencies, we test for intra-system uniformity these planetary categories. For example, the sub-Neptunes tend to be 1.7+0.6−0.3 times larger than the super-Earths in the same system.

Millholland, Sarah, et al. Kepler Multi-Planet Systems Exhibit Unexpected Intro-System Uniformity in Mass Radius. arXiv:1710.11152. We cite this entry by Yale University astronomers as an example of how sunny stars with orbital worlds are being found to have their own overall, interactive properties. Just as Caroline Dorn, et al, for example, are treating exoplanets as dynamic units, so entire solar systems seem to possess an integral nature.

The widespread prevalence of close-in, nearly coplanar super-Earth- and sub-Neptune-sized planets in multiple-planet systems was one of the most surprising results from the Kepler mission. By studying a uniform sample of Kepler "multis" with mass measurements from transit timing variations (TTVs), we show that a given planetary system tends to harbor a characteristic type of planet. That is, planets in a system have both masses and radii that are far more similar than if the system were assembled randomly from planets in the population. This finding has two important ramifications. First, the large intrinsic compositional scatter in the planet mass-radius relation is dominated by system-to-system variance rather than intra-system variance. Second, if provided enough properties of the star and primordial protoplanetary disk, there may be a substantial degree of predictability in the outcome of the planet formation process. (Abstract)

Mishra, Lokesh, et al. A Framework for the Architecture of Exoplanetary Systems. I. Four Classes of Planetary System Architecture. arXiv:2301.02374. This is an important, historic paper to appear in Astronomy & Astrophysics because after three decades of myriad revolutionary orbital world findings, University of Bern and Geneva Observatory astronomers including Sterphane Urdy, can report that entire solar orrerys seem to settle into distinct, inherent planetary arrangements. A companion posting at 2301.02373 with a II. Nature versus Nurture: Emergent Formation Pathways of Architecture Classes subtitle describes initial and later protoplanet, metalliticy and other aspects. In more regard, see a news Item Order from Chaos by Lee Billings in Scientific American (May 2023) with a graphic rendition of these salient placings. Along with a notice of constant ecosmic regularities, quite an epochal discovery itself, it is observed that our own system sequence from an inner Mercury to outer gas giants is an uncommon occurrence.

We present a novel, model-independent framework for studying the architecture of exoplanets at its whole system level, which allows us to study and classify its overall character. As a result, we propose that solar-planet systems be partitioned into four classes: similar, mixed, anti-ordered, and ordered. with regard to mass, radius, density, and core water fraction. Our work suggests that global similarities are the most common formation; internal composition of planets shows a strong link with their overall nature; and most anti-ordered systems are expected to be rich in wet planets, while most observed mass ordered systems are expected to have dry planets. We find, in good agreement with theory, that orbital arrays are similar, mixed, or anti-ordered. We also speculate on the role of system architectures in hosting habitable worlds (2301.02374 Abstract)

Morbidelli, Alessandro. Dynamical Evolution of Planetary Systems. Oswalt, Terry and Linda French, eds. Solar and Stellar Planetary Systems. Berlin: Springer, 2013. In this chapter, the Observatoire de la Cote d'Azur, Nice, France astronomer reviews a decade of research across Europe and worldwide due to realizations of a novel universe that is filled with orbital worlds. As the quotes extol, and 2015 citations by Giovanna Tinetti, Elke Pilat-Lohinger and others confirm, some general summations accrue. As the formation of stars and planets becomes understood, these phases proceed in a widely stochastic fashion of eccentric orbits, gas giants and super-Earths running into each other, and so on. In this scenario, it is just dawning that our own solar system exhibits a rare multi-billion year stability, planets lie in the same plane with circular orbits, with an orderly sequence. Here is an epochal discovery that is just dawning. See also Terrestrial Planet formation in the Presence of Migrating Super-Earths (1408.1215) and Chaotic Disintegration of the Inner Solar System (1411.5066), each with Morbidelli as coauthor.

The apparent regularity of the motion of the giant planets of our solar system suggested for decades that said planets formed onto orbits similar to the current ones and that nothing dramatic ever happened during their lifetime. The discovery of extra-solar planets showed astonishingly that the orbital structure of our planetary system is not typical. Many giant extra-solar planets have orbits with semi major axes of ∼ 1 AU, and some have even smaller orbital radii, sometimes with orbital periods of just a few days. Moreover, most extra-solar planets have large eccentricities, up to values that only comets have in our solar system. Why such a big diversity between our solar system and the extra-solar systems, as well as among the extra-solar systems themselves?

In fact, the best explanation for the large orbital eccentricities of extra-solar planets is that the planets that are observed are the survivors of strong instability phases of original multi-planet systems on quasi-circular orbits. The main difference between the solar system and the extra-solar systems is in the magnitude of such an instability. In the extra-solar systems, encounters among giant planets had to be the norm. In our case, the two major planets (Jupiter and Saturn) never had close encounters with each other: they only encountered “minor” planets like Uranus and/or Neptune. This was probably just mere luck, as simulations show that Jupiter-Saturn encounters in principle could have occurred. (Abstract)

This chapter discussed the evolution of planetary systems. The emphasis has been put on the evolution of our solar system. Our team effort, over the last 10 years, has been to reconstruct the history of our system using computer simulations and taking advantage of all possible detailed constraints (the orbits of the planets, the architecture of the populations of small bodies, radioactive chronologies for terrestrial planet formation, crater records etc.). Although imperfect, I think that our view of the evolution of the solar system, since the completion of giant planets formation, has reached a quite satisfactory level of coherence. (40-41)

Morbidelli, Alessandro and Sean Raymond. Challenges in Planet Formation. arXiv:1610.07202. Universite de Nice Sophia-Antipoli, and CNRS, Laboratoire d'Astrophysique de Bordeau astrophysicists provide a latest update about how object worlds might have formed. As we report this active literature, an auspicious realization is that our own solar system is a rarest case (one in a thousand herein) with a relatively benign, long lived conducive order. A philosophical reflection ought to note how incredible it is that a global sapience can look back and reconstruct how this special planet and people came to be.

Over the past two decades, large strides have been made in the field of planet formation. Yet fundamental questions remain. Here we review our state of understanding of five fundamental bottlenecks in planet formation. These are: 1) the structure and evolution of protoplanetary disks; 2) the growth of the first planetesimals; 3) orbital migration driven by interactions between proto-planets and gaseous disk; 4) the origin of the Solar System's orbital architecture; and 5) the relationship between observed super-Earths and our own terrestrial planets. (Abstract)

The origin of planets is a vast, complex and still quite mysterious subject. Despite decades
of space exploration, ground based observations and detailed analyses of meteorites and cometary grains it is still not clear how the planets of the Solar System formed. The discovery of extrasolar planets has added confusion to the problem, bringing to light evidence that planetary systems are very diverse, that our Solar System is not a typical case and that categories of planets that don't exist in our system are common elsewhere (e.g. the super-Earth planets). (2)

Just like any individual person, the Solar System has its own history. The probability of any other planetary system following an identical blueprint is zero. But how typical was the Solar System's evolutionary path? Based on statistically-sound exoplanet observational surveys, the Sun-Jupiter system is special at roughly the level of one in a thousand. First, the Sun is an unusually massive star; the most common type of star are M dwarfs, with masses of 10-50% of the Sun's. Second, only 10% of Sun-like stars have gas giant planets with orbits shorter than a few to 10 AU. Third, only about 10% of giant exoplanets have orbits wider than 1 AU and eccentricities smaller than 0.1. Taken together, these constraints suggest the Sun-Jupiter system is a 0.1% case. The numbers quoted here are a simple order of magnitude, but they clearly illustrate that the Solar System is not a typical case in at least one regard: the presence and orbit of Jupiter. (20)

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