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

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

Zackrisson, Erik, et al. Terrestrial Planets Across Space and Time. arXiv:1602.00690. As a sign of the robust maturity of exoworld studies, Uppsala University, Stockholm University, Carnegie Observatories, and Lund University astronomers achieve a spacescape inventory which includes galaxy formation, cosmological parameters, stellar mass functions, planetary accretion, and so on. See also The Quest for Cradles of Life by Pratika Dayal, et al at 1507.04345 for a “cosmobiological” survey of galaxies.

The study of cosmology, galaxy formation and exoplanetary systems has now advanced to a stage where a cosmic inventory of terrestrial planets may be attempted. By coupling semi-analytic models of galaxy formation to a recipe that relates the occurrence of planets to the mass and metallicity of their host stars, we trace the population of terrestrial planets around both solar-mass (FGK type) and lower-mass (M dwarf) stars throughout all of cosmic history. We find that the mean age of terrestrial planets in the local Universe is 8±1 Gyr and that the typical planet of this type is located in a spheroid-dominated galaxy with total stellar mass about twice that of the Milky Way. When looking at the inventory of planets throughout the whole observable Universe (i.e. in all galaxies on our past light cone) we argue for a total of ≈2×1019 and ≈7×1020 terrestrial planets around FGK and M stars, respectively. Due to the hierarchical formation of galaxies and lookback-time effects, the average terrestrial planet on our past light cone has an age of just 1.7±0.2 Gyr and is sitting in a galaxy with a stellar mass a factor of ≈2 lower than that of the Milky Way. These results are discussed in the context of cosmic habitability, the Copernican principle and the prospects of searches for extraterrestrial intelligence at cosmological distances. (Abstract)

This means, that if Earth had been randomly drawn from the cosmological population of TPs, the most likely outcome would seemingly have been for it not to be located in a Milky Way-type system. This, by itself, indicates a mild violation of the Copernican/mediocrity principle (mild, because our position isn’t extremely improbable), but the balance could potentially be shifted back in favour of disk-dominated galaxies by habitability arguments. This requires that there is a mechanism that reduces the fraction of habitable TPs in spheroid-dominated systems compared to disk-dominated ones. (7)

Zeng, Li, et al. Growth Model Interpretation of Planet Size Distribution. Proceedings of the National Academy of Sciences. 116/9723, 2019. A 16 member team based at Harvard including Dimitar Sasselov provide a good example of how later 2010s exoplanet studies, now an intense global activity (search Astro2020), have begun to identify topological features across an array from rocky asteroids to gas giants. As the Abstract alludes, it is noted again how chancy and rare the presence of just the right size and location might be so to hold a benign atmosphere without becoming all dry or wet. A concurrent entry As Planetary Discoveries Pile Up, a Gap Appears in the Pattern by Rebecca Boyle in Quanta Magazine (May 16, 2019) which links to a similar Astrophysical Journal paper.

The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H2–He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H2O-dominated ices/fluids (water worlds). Planets in the mass range of 10–15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. (Abstract excerpt)

Zhu, Wei and Subo Dong. Exoplanet Statistics and Theoretical Implications. arXiv:2103.02127. For a paper to appear in the 2021 Annual Review of Astronomy and Astrophysics, Tsinghua University, Beijing and Peking University researchers contribute to this new phase of collective Earthwise studies all about every quantifiable aspect of myriad exo-solar systems. At issue is how they formed by accretive processes, their present arrays by mass/radii forces, and so on. Into the 2020s, a grand initiative opens for our home world, if we can come to our senses in time, to continue the ecosmic question and answer project.

In the last few years, significant advances have been made in understanding the distributions of exoplanet populations and the architecture of planetary systems. We review the recent progress of planet statistics, with a focus on the inner <~ 1 AU region of the planetary system that has been fairly thoroughly surveyed by the Kepler mission. We also discuss the theoretical implications of these statistical results for planet formation and dynamical evolution. (Abstract)

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