Exoplanet Census Suggests Earth Is Special after All:
A new tally proposes that roughly 700 quintillion terrestrial exoplanets are likely to exist across the observable universe—most vastly different from Earth
By Shannon Hall
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 (pdf) 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.
Take exoplanets as an example. NASA’s Kepler space telescope is arguably one of the world’s best planet hunters, but it uses a method so challenging that it is often compared with looking across thousands of kilometers to see a firefly buzzing around a brilliant searchlight. Because the telescope looks for subtle dimming in a star’s light from planets crossing in front of it, Kepler has an easier time spotting massive planets orbiting close to their stars. Thus, the catalogue of planets Kepler has found lean heavily toward these types, and smaller, farther-out planets are underrepresented, leaving our knowledge of planetary systems incomplete. Astronomers do use other techniques to search for smaller planets orbiting at farther distances, but these methods are still relatively new and have not yet found nearly as many worlds as Kepler. In addition, “everything we know about exoplanets is from a very small patch in our galaxy,” Zackrisson says, within which most stars are pretty similar to one another in terms of how many heavy elements they contain and other characteristics. The team had to extrapolate in order to guess how planets might form around stars with fewer heavy elements, such as those found in small galaxies or the early universe.
The scientists also have similar concerns about the galactic and cosmological inputs of their model but nonetheless they suspect that their final numbers are accurate to within an order of magnitude. With the estimated errors taken into account, the researchers conclude that Earth stands as a mild violation of the Copernican principle. Our pale blue dot might just be special after all. “It's not too much of a fluke that we could arise in a galaxy like the Milky Way, but nevertheless, it's just enough to make you think twice about it,” says Jay Olson from Boise State University, who was not involved in the study. Both he and Zackrisson think the Copernican principle could be saved by some unknown caveat to the findings. “Whenever you find something that sticks out…” Zackrisson says, “…that means that either we are the result of a very improbable lottery draw or we don’t understand how the lottery works.”
But Max Tegmark from the Massachusetts Institute of Technology, who also was not part of the research, thinks Earth is a colossal violation of the Copernican principle—not because of its location but because of its young age. “If you have these civilizations that had a 3.5-billion-year head start on us, why haven't they colonized our galaxy?” asks Tegmark. “To me, the most likely explanation is that if the planets are a dime a dozen, then highly intelligent life evolves only rarely.” So should we feel insignificant? Should we be reduced to near nothingness? Not at all, he says. “It might be that one day in the distant future much of our universe will be teeming with life because of what we did here.”
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