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Register a new userHabitable Zone Boundaries for Circumbinary Planets
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Added by
Ravi Kumar Kopparapu
Publication date
2019
fields
Physics
and research's language is
English
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We use a one-dimensional (1-D) cloud-free climate model to estimate habitable zone (HZ) boundaries for terrestrial planets of masses 0.1 M$_{E}$ and 5 M$_{E}$ around circumbinary stars of various spectral type combinations. Specifically, we consider binary systems with host spectral types F-F, F-G, F-K, F-M, G-G, G-K, G-M, K-K, K-M and M-M. Scaling the background N2 atmospheric pressure with the radius of the planet, we find that the inner edge of the HZ moves inwards towards the star for 5ME compared to 0.1ME planets for all spectral types. This is because the water-vapor column depth is smaller for larger planets and higher temperatures are needed before water vapor completely dominates the outgoing longwave radiation. The outer edge of the HZ changes little due to competing effects of the albedo and greenhouse effect. While these results are broadly consistent with the trend of single star HZ results for different mass planets, there are significant differences between single star and binary star systems for the inner edge of the HZ. Interesting combinations of stellar pairs from our 1-D model results can be used to explore for in-depth climate studies with 3-D climate models. We identify a common HZ stellar flux domain for all circumbinary spectral types
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The discovery of many planets using the Kepler telescope includes ten planets orbiting eight binary stars. Three binaries, Kepler-16, Kepler-47, and Kepler-453, have at least one planet in the circumbinary habitable-zone (BHZ). We constrain the level of high-energy radiation and the plasma environment in the BHZ of these systems. With this aim, BHZ limits in these Kepler binaries are calculated as a function of time, and the habitability lifetimes are estimated for hypothetical terrestrial planets and/or moons within the BHZ. With the time-dependent BHZ limits established, a self-consistent model is developed describing the evolution of stellar activity and radiation properties as proxies for stellar aggression toward planetary atmospheres. Modeling binary stellar rotation evolution, including the effect of tidal interaction between stars in binaries is key to establishing the environment around these systems. We find that Kepler-16 and its binary analogs provide a plasma environment favorable for the survival of atmospheres of putative Mars-sized planets and exomoons. Tides have modified the rotation of the stars in Kepler-47 making its radiation environment less harsh in comparison to the solar system. This is a good example of the mechanism first proposed by Mason et al. Kepler-453 has an environment similar to that of the solar system with slightly better than Earth radiation conditions at the inner edge of the BHZ. These results can be reproduced and even reparametrized as stellar evolution and binary tidal models progress, using our online tool http://bhmcalc.net.
Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, Alpha Centauri. Based on 75-80% of the best quality images from 100 hours of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of Alpha Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around Alpha Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.
We show that collision-induced absorption allows molecular hydrogen to act as an incondensible greenhouse gas, and that bars or tens of bars of primordial H2-He mixtures can maintain surface temperatures above the freezing point of water well beyond the classical habitable zone defined for CO2 greenhouse atmospheres. Using a 1-D radiative-convective model we find that 40 bars of pure H2 on a 3 Earth-mass planet can maintain a surface temperature of 280K out to 1.5AU from an early-type M dwarf star and 10 AU from a G-type star. Neglecting the effects of clouds and of gaseous absorbers besides H2, the flux at the surface would be sufficient for photosynthesis by cyanobacteria (in the G star case) or anoxygenic phototrophs (in the M star case). We argue that primordial atmospheres of one to several hundred bars of H2-He are possible, and use a model of hydrogen escape to show that such atmospheres are likely to persist further than 1.5 AU from M stars, and 2 AU from G stars, assuming these planets have protecting magnetic fields. We predict that the microlensing planet OGLE-05-390L could have retained a H2-He atmosphere and be habitable at ~2.6 AU from its host M star.
We present occurrence rates for rocky planets in the habitable zones (HZ) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define $eta_oplus$ as the HZ occurrence of planets with radius between 0.5 and 1.5 $R_oplus$ orbiting stars with effective temperatures between 4800 K and 6300 K. We find that $eta_oplus$ for the conservative HZ is between $0.37^{+0.48}_{-0.21}$ (errors reflect 68% credible intervals) and $0.60^{+0.90}_{-0.36}$ planets per star, while the optimistic HZ occurrence is between $0.58^{+0.73}_{-0.33}$ and $0.88^{+1.28}_{-0.51}$ planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates using both a Poisson likelihood Bayesian analysis and Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with $95%$ confidence that, on average, the nearest HZ planet around G and K dwarfs is about 6 pc away, and there are about 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.
In this work is investigated the possibility of close-binary star systems having Earth-size planets within their habitable zones. First, we selected all known close-binary systems with confirmed planets (totaling 22 systems) to calculate the boundaries of their respective habitable zones (HZ). However, only eight systems had all the data necessary for the computation of the HZ. Then, we numerically explored the stability within the habitable zones for each one of the eight systems using test particles. From the results, we selected five systems that have stable regions inside the habitable zones (HZ), namely Kepler-34, 35, 38, 413 and 453. For these five cases of systems with stable regions in the HZ, we perform a series of numerical simulations for planet formation considering disks composed of planetary embryos and planetesimals, with two distinct density profiles, in addition to the stars and host planets of each system. We found that in the case of Kepler-34 and 453 systems no Earth-size planet is formed within the habitable zones. Although planets with Earth-like masses were formed in the Kepler-453, but they were outside the HZ. In contrast, for Kepler-35 and 38 systems, the results showed that potentially habitable planets are formed in all simulations. In the case of the Kepler-413 system, in just one simulation a terrestrial planet was formed within the habitable zone.
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