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Register a new userHabitable Zones for Earth-mass Planets in Multiple Planetary Systems
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Ji Jianghui
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We perform numerical simulations to study the Habitable zones (HZs) and dynamical structure for Earth-mass planets in multiple planetary systems. For example, in the HD 69830 system, we extensively explore the planetary configuration of three Neptune-mass companions with one massive terrestrial planet residing in 0.07 AU $leq a leq$ 1.20 AU, to examine the asteroid structure in this system. We underline that there are stable zones of at least $10^5$ yr for low-mass terrestrial planets locating between 0.3 and 0.5 AU, and 0.8 and 1.2 AU with final eccentricities of $e < 0.20$. Moreover, we also find that the accumulation or depletion of the asteroid belt are also shaped by orbital resonances of the outer planets, for example, the asteroidal gaps at 2:1 and 3:2 mean motion resonances (MMRs) with Planet C, and 5:2 and 1:2 MMRs with Planet D. In a dynamical sense, the proper candidate regions for the existence of the potential terrestrial planets or HZs are 0.35 AU $< a < $ 0.50 AU, and 0.80 AU $< a < $ 1.00 AU for relatively low eccentricities, which makes sense to have the possible asteroidal structure in this system.
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As part of a national scientific network Pathways to Habitability the formation of planets and the delivery of water onto these planets is a key question as water is essential for the development of life. In the first part of the paper we summarize the state of the art of planet formation - which is still under debate in the astronomical community - before we show our results on this topic. The outcome of our numerical simulations depends a lot on the choice of the initial distribution of planetesimals and planetary embryos after gas disappeared in the protoplanetary disk. We also take into account that some of these planetesimals of sizes in the order of the mass of the Moon already contained water; the quantity depends on the distance from the Sun - close-by bodies are dry, but starting from a distance of about 2 AU they can contain substantial amounts of water. We assume that the gas giants and terrestrial planets are already formed when we check the collisions of the small bodies containing water (in the order of a few percent) with the terrestrial planets. We thus are able to give an estimate of the respective contribution to the actual water content (of some Earth-oceans) in the mantle, in the crust and on the surface of Earth. In the second part we discuss in more detail how the formation of larger bodies after a collision may happen as the outcome depends on parameters like collision velocity, impact angle, and the materials involved. We present results obtained by SPH (Smooth Particle Hydrodynamics) simulations. We briefly describe this method and show different scenarios with respect to the formed bodies, possible fragmentation and the water content before and after the collision. In an appendix we discuss detection methods for extrasolar planets (close to 2000 such objects have been discovered so far).
Ultracool dwarfs (UCD; $T_{rm eff}<sim3000~$K) cool to settle on the main sequence after $sim$1 Gyr. For brown dwarfs, this cooling never stops. Their habitable zone (HZ) thus sweeps inward at least during the first Gyr of their lives. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to entering the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry worlds. Here we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets recently found around the $0.08~M_odot$ dwarf TRAPPIST-1. Despite assumptions maximizing the FUV-photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amounts of water in the HZ of UCDs, with a sweet spot in the $0.04$-$0.06~M_odot$ range. We also studied the TRAPPIST-1 system using observed constraints on the XUV-flux. We find that TRAPPIST-1b and c may have lost as much as 15 Earth Oceans and planet d -- which might be inside the HZ -- may have lost less than 1 Earth Ocean. Depending on their initial water contents, they could have enough water to remain habitable. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.
The effects of multi-layered clouds in the atmospheres of Earth-like planets orbiting different types of stars are studied. The radiative effects of cloud particles are directly correlated with their wavelength-dependent optical properties. Therefore the incident stellar spectra may play an important role for the climatic effect of clouds. We discuss the influence of clouds with mean properties measured in the Earths atmosphere on the surface temperatures and Bond albedos of Earth-like planets orbiting different types of main sequence dwarf stars.
Several concepts have been brought forward to determine where terrestrial planets are likely to remain habitable in multi-stellar environments. Isophote-based habitable zones, for instance, rely on insolation geometry to predict habitability, whereas radiative habitable zones take the orbital motion of a potentially habitable planet into account. Dynamically informed habitable zones include gravitational perturbations on planetary orbits, and full scale, self consistent simulations promise detailed insights into the evolution of select terrestrial worlds. All of the above approaches agree that stellar multiplicity does not preclude habitability. Predictions on where to look for habitable worlds in such environments can differ between concepts. The aim of this article is to provide an overview of current approaches and present simple analytic estimates for the various types of habitable zones in binary star systems.
The carbon-silicate cycle regulates the atmospheric $CO_2$ content of terrestrial planets on geological timescales through a balance between the rates of $CO_2$ 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 $CO_2$ 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.
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