All these worlds are not yours

There may be hundreds of millions of habitable worlds out there. But only one of them is home.

With the U.S. election hogging our collective attention for weeks, you might have missed the big announcement that came out of the world of astronomy recently: There are at least 300 million “potentially habitable” planets in our galaxy, rocky worlds that orbit stars like our Sun and might have liquid water on their surfaces. 

This mind boggling galactic real estate estimate is the culmination of years of hard work by scientists who study extrasolar planets, or exoplanets. Dozens of these researchers spent two years crunching the data collected by NASA’s Kepler space telescope, which scanned more than half a million stars for planetary signals between 2009 and 2018 and confirmed nearly 3,000 exoplanets. Focusing on planets similar in size to Earth that orbit stars similar in age and temperature to our Sun, the scientists made a galactic extrapolation. The results, which will be published in an upcoming edition of The Astronomical Journal, suggest there are hundreds of millions of potential Earth lookalikes even under the most conservative assumptions. While we do not yet know if any these worlds are actually habitable, this points to a lot of chances for habitable conditions, and life, to exist elsewhere.

It does not, however, suggest that we humans are going to be planting terrestrial flags across the galaxy anytime soon. 

Despite what so much science fiction, and some very solid space tourism propaganda from NASA, would have us believe, our descendants are unlikely to visit many (or any) of these cosmic cousins for a simple reason: Space is really, really big. And in a universe bound by the laws of physics—meaning no faster-than-light travel—we simply can’t get around it very fast.

While the new study casts humanity’s home world within a vast sisterhood of Goldilocks planets, it also highlights just how astronomically alone we are. The authors conclude that there are likely four auspicious exo-rocks within 30 light years of the Solar System, including one that’s about 20 light years away. If you, like me, grew up watching the crew of the USS Voyager make its 7-season, 70,000-light year trek home, 20 light years might seem well within reach. But unless we’re lucky enough to discover a wormhole in the Kuiper Belt, this is still a staggering distance to cross.

The Parker Solar Probe, which holds the record for fastest spacecraft relative to the Sun, is set to achieve a top speed of 430,000 miles per hour in 2024. Rounding up, that’s about one-one thousandth the speed of light. If humanity were to engineer a ship that could achieve this rip-roaring velocity without being sucked into the Sun and sustain that speed across 20 light years of interstellar emptiness, it would take the craft 20,000 years to reach its destination. Even with advanced fusion drives a la The Expanse that allow us to travel at a much speedier clip—say, one tenth the speed of light—we’re still talking about a 200 year journey that would take multiple generations to complete. There’s a reason humanity never left the Solar System in that scientifically grounded series and TV show, at least, not until something kind of supernatural happened.

We also have to consider what life would be like on multigenerational journey to a new world. Science fiction author Kim Stanley Robinson makes an impressive attempt to imagine this in his 2015 novel Aurora, which chronicles a group of interstellar colonists taking the 12 light year trip to an exomoon in the Tau Ceti system. Robinson envisions a fantastically advanced generation ship with dozens of self contained biomes capable of supporting thousands of people. And yet, by the time the ship is nearing its destination, entropy has begun to catch up with it. The infrastructure is deteriorating; ecosystems are falling prey to micronutrient deficiencies; bacteria are mutating faster than human immune systems can keep up. Many colonists are unhappy, to say the least, at having been consigned by their elders to live and die on a perilous journey with an unknown end. When they arrive at Tau Ceti and things start to go really south, some of those colonists very reasonably decide to return to Earth.

Artist’s concept of Kepler-452b, a world similar in size to Earth that lies in the “habitable zone” of a Sun-like star. Credit: NASA Ames/JPL-Caltech/T. Pyle

There’s a reason science fiction often envisions us conquering spacetime with fantastical propulsion (spore drive, anyone?) or alien wormhole networks: It’s a damn exciting idea. And it opens up world building possibilities that are off limits in a universe bound by light speed. We can, as authors like Ian Banks and shows like Star Trek have done, envision galaxy-spanning civilizations where dozens of species intermingle and resource scarcity is a thing of the past. But while such settings offer a unique space to reflect on what makes us human, tales like Aurora and The Expanse have a more humbling message: That we are cosmically provincial beings; delicate meat bags whose fate is intimately tied to that of the lone planet with all the right ingredients to support our existence. And it’s the latter type of storytelling that I think can help us appreciate why 300 million potentially Earth-like worlds really matter. 

It’s not because these worlds are ever going to be ours. It’s because their very existence is evidence that we are part of a grand cosmic tapestry filled with wonders beyond our physical grasp but not, necessarily, beyond our ability to understand. As Steve Bryson, an astronomer at NASA’s Ames Research Center and lead author of the new study told me in an email, estimating the number of potentially habitable exoplanets is important “primarily because of what it tells us about the Earth.”

“[I]s the Earth unique? Is it common?” Bryson said. “By observing exoplanets of all sizes and distances from their star, we gain insight into how the planets form. So we learn about why we’re here, and how we got here.”

Perhaps, by continuing to investigate our place in the cosmos, we’ll start to realize just how precious the small sliver of galactic real estate we call home really is.

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