Our Galaxy's Habitable Zone

Where are the habitable planets in our galaxy and how numerous are they during the course of the galaxy's life? A new paper by researchers Michael G. Gowanlock, David R. Patton and Sabine M. McConnell seeks to provide answers to these questions.

We present a model of the Galactic Habitable Zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favour the development of complex life. The Milky Way galaxy is modelled using a computational approach by populating stars and their planetary systems on an individual basis using MonteCarlo methods. We begin with well-established properties of the disk of the Milky Way, such as the stellar number density distribution, the initial mass function, the star formation history, and the metallicity gradient as a function of radial position and time. We vary some of these properties, creating four models to test the sensitivity of our assumptions. To assess habitability on the Galactic scale, we model supernova rates, planet formation, and the time required for complex life to evolve. Our study improves on other literature on the GHZ by populating stars on an individual basis and by modelling SNII and SNIa sterilizations by selecting their progenitors from within this preexisting stellar population. Furthermore, we consider habitability on tidally locked and non-tidally locked planets separately, and study habitability as a function of height above and below the Galactic midplane. In the model that most accurately reproduces the properties of the Galaxy, the results indicate that an individual SNIa is ∼5.6× more lethal than an individual SNII on average. In addition, we predict that ∼1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy. Of those stars with a habitable planet, ∼75% of planets are predicted to be in a tidally locked configuration with their host star. The majority of these planets that may support complex life are found towards the inner Galaxy, distributed within, and significantly above and below, the Galactic midplane.


(Habitable planets [pdf])

In this Figure, Model 1 is plotted. Top Left panel: The number of planets that are habitable (tidally-locked and non-locked) as a function of radial distance and time. We trace the history of each habitable planet to determine in which periods they remain habitable. It is clear, given the assumptions made in our model, that the number of habitable planets is greatest in the inner Galaxy, at all epochs. Top right panel: The fraction of stars with a habitable planet as a function of time and radial distance. We trace the history of each habitable planet to determine in which periods they remain habitable. Given the assumptions made in the model, the inner region of the Galaxy exhibits the highest probability of having habitable planets at the present day. At R∼5-11 kpc, at the present day, the entire range has roughly the same probability of having habitable planets. Lower Left panel: The number of habitable planets integrated over all epochs as a function of radial distance and height above the midplane. We predict that the position in the Galaxy with the greatest number of habitable planets is located in and around the midplane in the inner Galaxy. Lower Right panel: The fraction of habitable planets integrated over all epochs as a function of radial distance and height above the midplane. The region with the greatest fraction of habitable planets exists above the midplane in the inner Galaxy. The high metallicity that produces a high planet formation rate and the lower stellar density that exists well above the midplane at this radial position permits a greater fraction of habitable planets to form.

When I read about this kind of research, I think about the science fiction authors who expanded our ideas about life in the galaxy, like E.E. 'Doc' Smith and Isaac Asimov. Your favorites?

From A Model of Habitability Within the Milky Way Galaxy by Michael G. Gowanlock, David R. Patton and Sabine M. McConnell.

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