As far as we know, there’s only one place in the universe capable of hosting life: Earth. This fact might suggest that Earth is unique or simply indicate our ignorance, so to quantify the probability of having planets like Earth, a team of scientists constructed a powerful computer simulation that looks at planetary formation across the universe from the Big Bang to today.
Astronomer Erik Zackrisson from Uppsala University and his colleagues discovered some unusual results in the simulation. There are about 700 quintillion terrestrial exoplanets in the visible universe and they are on average a few billion years older than Earth. Our planet is also in the minority when it comes to location. Three in four planets are found in large spheroidal (elliptical) galaxies because they have older stars and they are richer in heavier elements (which make up planets) than spiral galaxies like the Milky Way.
As there are a similar amount of ellitpical galaxies to spiral galaxies, one might expect to calculate a similar amount of terrestrial planets in both, but this paper (submitted to the Astrophysical Journal and available on Arxiv) suggests that is not the case.
Statistically speaking, Earth seems to occupy a somewhat peculiar position. This indicates a slight violation of the Copernican principle (Earth is not at the center of the universe, i.e. we are not special), so while it's far from impossible for Earth to form in the Milky Way, it’s unusual enough to raise a few eyebrows.
Talking to Scientific American, lead author Erik Zackrisson said: “Whenever you find something that sticks out that means that either we are the result of a very improbable lottery draw or we don’t understand how the lottery works.”
While the results are significant, the simulation is not without its limits. The team based the model on the currently observed population of planets in our local neighborhood. To simulate the universe, they used the most precise value of cosmological parameters, well-tested models of galaxy evolution, and an estimate of how and when stars forms, which carry their own uncertainties.
Everything considered, the researchers are confident that the final numbers are correct within an order of magnitude. It will be interesting to see how the estimates change as more rocky planets are discovered.