A Universe Full of Planets

The phrase “scientific revolution” tends to get overused. But there is one happening right now that could arguably alter the entire trajectory of human existence by profoundly realigning what we know of the complex, undulating road that has taken us from atoms adrift in the void to living, thinking beings in merely four billion years.

Using techniques of exquisite sensitivity and technological finesse, astronomers have spent the past two decades on an astonishing voyage of cosmic discovery. They have found that the universe is full of planets: Cold, small, and dark next to their large and glaring suns, these worlds have previously been hidden from us.

Yet they are a critical piece of a great puzzle. In many ways they represent the final order of unfinished business for the Copernican worldview. We’ve never been able to completely convince ourselves that we’re nothing special, not central to the cosmos, because our solar system has been the only planetary collective known.

Planets are tiny crumbs of matter — leftovers from epochs where gravity collapses and compresses vast interstellar nebulae into new stars. They’re fossil detritus, matter that gathers like droplets on a cooling windowpane, surviving as dust and gas around them evaporates and dissipates. To spot them represents a challenge that has been compared to looking across thousands of miles to see a firefly buzzing around a brilliant searchlight. But they can be spotted, both deliberately and serendipitously.

Like everything else, they exert a gravitational pull, tugging their parent stars into a gently wobbling motion that we can now detect, even if it’s barely more than the pace of a slow walker seen from hundreds of trillions of miles away.

Sometimes, by sheer random alignment, they appear to flit across the face of their suns. From our perspective the star’s light dims minutely, mere fractions of a percent, as the planet gets in the way. And if we patiently monitor millions of stars we will occasionally see one brighten and fade — a manifestation of Einstein’s physics as another star, and its planets, drifts between us and the stellar backdrop, serving as a gravitational lens, an interstellar magnifying glass.

The result of extraordinary efforts to watch for these subtle signatures is that we now have firm evidence for thousands of planets, around thousands of stars. We also know something about these worlds, their sizes, their orbits, often their ages. In a handful of cases, for planets of Jupiter’s or Neptune’s size, we have even measured the temperature of their upper atmospheres and sniffed at their gaseous chemistry, finding substances like sodium, methane and water.

It’s a rich bounty that has taken even scientists by surprise, but it’s only the beginning, because the numbers are pointing to something even more remarkable in our future.

No matter how conservative or optimistic we are, the statistics tell us that something like an astonishing one out of every seven stars must harbor a planet similar in size to the Earth, and at roughly the right orbital distance to allow for the possibility of a temperate surface environment. In other words, roughly 15 percent of all suns could, in principle, be hosting a place suitable for life as we know it.

Since our galaxy contains at least 200 billion stars, this implies a vast arena for the universe’s ubiquitous carbon chemistry to play in — a process that, as here on Earth, might lead to the complex machinery of life. Indeed, there is a 95-percent confidence — give or take a few percent — that one of these worlds could be within a mere 16 light years of us. That’s a stone’s throw, practically our galactic backyard.

And it’s really only the tip of the proverbial iceberg. Even in our own solar system we have our suspicions that environments inside ice-rich moons around Jupiter or Saturn could also represent life-amenable zones. We know too that the conditions humans find palatable are not the same as those for countless other species. The opportunities for life seem enormous.

This incredible abundance of planets in our galaxy is a portent of what’s to come; the first shot fired; the first statement of intent in a new revolution. It doesn’t yet tell us whether or not we are alone in the universe or, as in the Copernican outlook, merely one among billions. But it is already telling us something shockingly profound.

Because the universe makes so many planets, the chances of our finding others that we can actually study in detail increases enormously. Until now there was nothing we knew about the universe that made this inevitable. Planets could have been few and far between. Even if there were billions in total, they could have been scattered thinly across the cosmos. Thus, it was still a possibility that we’d forever be like the lonely ship’s spotter, vainly looking for land from the crow’s-nest.

Instead, the age-old dream of finding other life in the cosmos, whether silent microbes or noisy technological civilizations, has just taken a very sharp turn for the positive.

It really is true therefore that we stand on the verge of learning more about the nature of life, and of our own significance, than at any other time in human history.

It was — and is — a big deal for us to peer into the microscopic to understand the atomic and subatomic. It has also been momentous to learn that the entirety of existence, our universe, originated some 13.8 billion years ago. And yes, we reached a pinnacle of self-discovery by realizing that we are merely one expression of an awesomely complex and ancient system of molecular machinery, evolving and selecting itself across the ages.

But to discover whether or not we are alone, whether or not something akin to this Earth has happened somewhere else, and perhaps, just perhaps, whether or not there are other minds, on other worlds, thinking these same kinds of thoughts? That’s big, perhaps the biggest thing that could ever happen to a species.

It could have been impossible to ever address the question, but we now know that nature has provided us with a shot at finding out, that there are billions upon billions of places to search. That really is the start of a revolution.

Caleb Scharf is director of astrobiology at Columbia University in New York and author of Gravity’s Engines and The Copernicus Complex.

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