[LINK] "Exoplanet Census Suggests Earth Is Special after All"

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Scientific American's Shannon Hall reports on this study, "Terrestrial Planets Across Space and Time".

“Terrestrial Planets Across Space and Time”.


More than 400 years ago Renaissance scientist Nicolaus Copernicus reduced us to near nothingness by showing that our planet is not the center of the solar system. With every subsequent scientific revolution, most other privileged positions in the universe humans might have held dear have been further degraded, revealing the cold truth that our species is the smallest of specks on a speck of a planet, cosmologically speaking. A new calculation of exoplanets suggests that Earth is just one out of a likely 700 million trillion terrestrial planets in the entire observable universe. But the average age of these planets—well above Earth’s age—and their typical locations—in galaxies vastly unlike the Milky Way—just might turn the Copernican principle on its head.

Astronomer Erik Zackrisson from Uppsala University and his colleagues created a cosmic compendium of all the terrestrial exoplanets likely to exist throughout the observable universe, based on the rocky worlds astronomers have found so far. In a powerful computer simulation, they first created their own mini universe containing models of the earliest galaxies. Then they unleashed the laws of physics—as close as scientists understand them—that describe how galaxies grow, how stars evolve and how planets come to be. Finally, they fast-forwarded through 13.8 billion years of cosmic history. Their results, published to the preprint server arXiv and submitted to The Astrophysical Journal, provide a tantalizing trove of probable exoplanet statistics that helps astronomers understand our place in the universe. “It’s kind of mind-boggling that we’re actually at a point where we can begin to do this,” says co-author Andrew Benson from the Carnegie Observatories in California. Until recently, he says, so few exoplanets were known that reasonable extrapolations to the rest of the universe were impossible. Still, his team’s findings are a preliminary guess at what the cosmos might hold. “It’s certainly the case that there are a lot of uncertainties in a calculation like this. Our knowledge of all of these pieces is imperfect,” he adds.

The abstract?

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. We estimate that hot Jupiters have depleted the population of terrestrial planets around FGK stars at Redshift z = 0 by no more than 10%, and predict that 1/3 of the terrestrial planets in the local Universe are orbiting stars in a metallicity range for which such planets have yet to be been detected. 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.