Usually, when we think of alien civilizations and the extrasolar planets conducive to such a thing, we think in terms of space and distance. That is, habitable planets must be distributed through the universe at such and such density, based on our observations of that universe at this moment in time, therefore contact is not improbable.
Space and its notion of Euclidean distance are easy to grasp, but there is also time to contend with. Time is just as vast: alien civilizations may be packed densely into the universe, but stretched through time, we might as well be living in a cold, empty husk.
According to a study published this week in the Monthly Notices of the Royal Astronomical Society, courtesy of researchers at NASA's Space Telescope Science Institute in Baltimore, Earth is an early arrival to the cosmic banquet, with some 92 percent of potentially habitable planets yet to even coalesce, let alone cultivate biological lifeforms. The STScI analysis, authored by astronomers Molly Peeples and Peter Behroozi, is based on data collected by the Hubble Space Telescope and the planet-hunting Kepler space observatory.
While we are in the last batch of the planets currently in existence—having formed after 80 percent of them—the universe is hardly short on raw materials. The study's conclusion largely has to do with the prevalence of these raw materials in the form of dust clouds intermingling with the vast dark matter halos theorized to envelop visible galaxies.
Most of the rest of the 92 percent of habitable planets will emerge anywhere between 100 billion years in the future to one trillion years
Once the Milky Way finally collides and merges with the Andromeda galaxy in around four billion years, give or take, there should be some 3.9×1011 solar masses left over to form new stars and planets within the resulting combined dark matter halo.
"Repeating this calculation for the Universe as a whole, we note that only 8% of the currently available gas around galaxies (i.e., within dark matter halos) had been converted into stars at the Earth's formation time," Peeples and Behroozi write. "Even discounting any future gas accretion onto haloes, continued cooling of the existing gas would result in Earth having formed earlier than at least 92% of other similar planets."
It's possible to take these numbers and come up with a rather conservative estimate of the number of civilizations the universe is likely to produce in its lifetime. If we start with the extremely conservative prediction that Earth is the first civilization in the universe, it's likely that the universe will produce around 12 civilizations in all.
Obviously, that's a fairly restrained view, but it also happens to align with our seemingly quite solitary existence here in the Solar System. If we assume that the Milky Way, the entire yawning starscape, contains just a single other civilization, then we can up our estimate to Earth being around the 10 billionth civilization in the universe.
All of that said, we can add with a bit more certainty that, while the universe may kick out a very large number of civilizations as it kicks out more and more suitable planets, most of them will be far, far in the future. Given the current declining rate of planet formation in the Milky Way and likely beyond, it's probable that most of the rest of the 92 percent of planets will emerge anywhere between 100 billion years in the future to one trillion years.
"Hence, as the Universe's accelerating expansion is rapidly reducing the number of observable galaxies," the current study concludes, "most future planets formed in other galaxies will not be visible from the Milky Way."
An open-access pre-print version of the paper can be viewed at the arXiv server.