Current papers suggest that Earth is ranked between 5-30% in terms of how early it is in habitable planets in our galaxy.
A recent paper suggests that Earth is very early in habitability, but does not provide a percentile ranking. The paper is below.
Astrobiology - A Model of Habitability Within the Milky Way Galaxy
We present a model of the galactic habitable zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favor the development of complex life. The Milky Way galaxy was modeled using a computational approach by populating stars and their planetary systems on an individual basis by employing Monte Carlo methods. We began 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 varied some of these properties and created four models to test the sensitivity of our assumptions. To assess habitability on the galactic scale, we modeled supernova rates, planet formation, and the time required for complex life to evolve. Our study has improved on other literature on the GHZ by populating stars on an individual basis and modeling Type II supernova (SNII) and Type Ia supernova (SNIa) sterilizations by selecting their progenitors from within this preexisting stellar population. Furthermore, we considered habitability on tidally locked and non–tidally locked planets separately and studied 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 toward the inner Galaxy, distributed within, and significantly above and below, the galactic midplane
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