Mirror Earth

Introduction

Chapter 1:
   The Man Who Looked for Blinking Stars

Chapter 2:
   The Man Who Looked for Wobbling Stars

Chapter 3:
   Hot Jupiters: Who Ordered Those?

Chapter 4:
   An Ancient Question

Chapter 5:
   The Dwarf-Star Strategy

Chapter 6:
   Imagining Alien Atmospheres

Chapter 7:
   Invasion of the Female Exoplaneteers

Chapter 8:
   Kepler Approved

Chapter 9:
   Waiting for Launch

Chapter 10:
Kepler Scooped

Chapter 11:
“A 100 Percent Chance of Life”

Chapter 12:
The Kepler Era Begins

Chapter 13:
Beyond Kepler

Chapter 14:
How Many Earths?

Chapter 15:
What Does “Habitable” Really Mean?

Chapter 16:
A World Made of Rock, at Last

Chapter 17:
Astronomers in Paradise

Chapter 18:
Sara's Birthday Party

Acknowledgments

Notes

Bibliography

A Note on the Author

By the Same Author

For Seymour Lemonick,
whose love and support
have been there since
before I can remember

“The earth goes around the Sun.
That's
what's going on!”

These were the last words my father ever said to me. He said them—almost shouted them—with a vehemence and conviction that startled me. The fact that he could say anything at all was itself a surprise. He'd been semicomatose for nearly a week, unable to initiate movement or speech on his own. He would sometimes respond to questions, but only in a hoarse whisper, so faint that it was impossible to understand what he was saying. That afternoon, I'd arrived at his room at the nursing wing of the retirement community he lived in, and said, “Dad, what's going on?” with the artificial heartiness you sometimes put on to convince yourself and others that everything is perfectly normal, even though it's anything but normal. He barked out that single phrase, and then didn't speak after that. Two days later, he died.

At the time, those final words didn't make a bit of sense to me, but when I began to write this book, I thought about them again, and remembered the stories my father used to tell me
when I was very young. He was a professor of physics at Princeton University and an extraordinarily popular teacher. When they hear my last name, gray-haired men still tell me how much they loved taking his classes a half century ago or more. He would present long-established ideas in physics—Newton's laws of gravity or Maxwell's equations of electromagnetism or Einstein's theories of relativity—as though he had just stumbled on these mind-blowing truths for the first time. He could barely contain his excitement, and his students couldn't help sharing it. The fact that the Earth goes around the Sun was a big deal when Copernicus first proclaimed it in the 1500s. And it still is!

I never took one of those classes. I got a different sort of physics education, delivered in the form of stories my father would tell late at night, as we drove home from visiting his family or my mother's in Philadelphia. He told stories about atoms and molecules and planets and stars, pitched at a level an eight-year-old could understand, but filled with the same excitement and wonder he shared with his students at Princeton. I remember the time he told the story of Halley's Comet—about how it returned every seventy-six years to light up the night sky, how it was passing by the year Mark Twain was born and again as he lay on his deathbed. It would be coming again, my father promised, in 1986, when I would be an unimaginable thirty-three years old. I couldn't wait.

Those late-night stories about the universe didn't inspire me to become a physicist—too much math! They did, however, inspire me to become a journalist who never wanted to write about anything but science. I was intrigued, not only by
what astronomers had learned already, but also by mysteries still unanswered. When my father first told me about the cosmos, astronomers didn't know about quasars, or black holes, or pulsars. They didn't know that most of the matter in the universe is not the atoms that make up stars, planets, and people, but rather a mysterious, invisible substance known as dark matter. They knew that the universe was expanding, but had no idea that the expansion is accelerating, driven by an equally mysterious force known as dark energy.

And they didn't know the answer to perhaps the oldest questions of all: Do planets orbit distant stars? Do any of them harbor life? Is the human species alone in the universe?

It wasn't that astronomers and physicists hadn't tried to answer these questions, but planets around other stars are excruciatingly hard to see. They lie tens of trillions, or even hundreds of trillions, of miles away. Stars appear tiny when you look up into the night sky, but planets are far tinier, and far dimmer. Finding planets around even the nearest stars turned out to be so difficult that hunting for distant worlds had become a fringe area of astronomy. Anyone who thought he or she could figure out how to do such a thing shouldn't be taken very seriously.

All of that changed in 1995, however, when Swiss astronomers found the very first planet orbiting a Sun-like star. In 1996, American astronomers found several more. The Americans, especially, had staked their careers on finding planets (the Swiss were working on other things as well), and they hadn't gotten a lot of respect. But once this small handful of pioneers had shown it was possible to find planets, their colleagues quickly changed their attitudes, and started looking
too. Over the next decade and a half, the number of known worlds beyond our own solar system would climb into the tens, then the hundreds, then to more than a thousand.

But the ultimate goal of finding a Mirror Earth—a planet of about the same size as our home planet, with the right mix of land and ocean and temperature so that life might have established a foothold and gone on to thrive—remained just out of reach.

It won't be out of reach for long, though. By early 2012, it was clear that a Mirror Earth was finally within astronomers' grasp. It likely would be only months, rather than years, before astronomers would be able to take my father's dying words one step further by declaring: “A Mirror Earth goes around a Sunlike star—and here's where it is!”

Five billion years ago, the Milky Way didn't look much different than it does today. It was, and remains, an enormous pinwheel some five hundred quadrillion miles across, made up of hundreds of billions of stars, rotating once every two hundred million years or so. Between the stars lay immense, swirling clouds of gas and dust, and every so often, one of these clouds would begin to collapse under its own gravity. The collapse might be triggered by a shock wave from a supernova—a star ending its life in a titanic explosion—or it might be caused by the blast of radiation from a hot, blue supergiant star, or simply by the gravity of a star lumbering by.

Once it started, the cloud kept falling in on itself, getting smaller and denser, spinning faster and faster and, because of the spin, flattening itself out like a pancake. When it was about
five billion miles across, one one thousandth of its original size, and one hundred million miles or so thick, the cloud was dense enough and spinning so fast that gravity could no longer force it to shrink any further. Particles of iron, nickel, and silicates—molecules made of oxygen and silicon—collided and stuck together and began to grow larger.

But by now, the intense pressure in the densest part of the cloud—the very core—was heating it up to temperatures of millions of degrees. The searing heat ripped electrons away from atoms and forced atomic nuclei to overcome their natural repulsion, releasing enormous energy. The core of the pancake burst into life as a newly formed star—a gigantic, self-perpetuating thermonuclear furnace that would burn for the next ten billion years. The star would one day be known as the Sun. Once the Sun flared into life, it had a quick and profound effect on the pancake that still swirled around it. The heat and light and intense magnetic fields emanating from the young star would collectively have swept most of the remaining gases out toward the edges of the pancake, leaving the rocky and metallic solids behind.

The grains grew into pebbles, and on up in size until they were what planetary scientists call planetesimals—rocky, metallic objects big enough for their gravity to pull them into a roughly spherical shape. The largest asteroids that remain in the solar system, including Ceres and Vesta, resemble those early planetesimals. In those early days, however, there were hundreds of thousands of them whipping around the Sun, slamming together, breaking apart, re-forming, and ultimately growing larger and larger, with fewer and fewer planetesimals
to wreak havoc on one another. Close to the Sun, the chaos would end with four major rocky planets: Mercury, Venus, Earth, and Mars. (Earth's relatively huge Moon, scientists believe, was created when one final Mars-size object slammed into Earth and ripped off a chunk).

Rocky planets formed in the outer solar system as well. They were up to ten times as massive as the Earth, with ten times the gravity, and they began to suck in gas—mostly hydrogen, but also an enormous amount of water vapor and nitrogen and carbon compounds—forming the thick, deep atmospheres of present-day Jupiter, Saturn, Uranus, and Neptune. The stuff that was left over, out beyond the orbit of Neptune, formed into the comets.

This is the picture astronomers put together during the 1960s, '70s, and '80s to explain how the solar system formed, and while they weren't certain of every step of the process, they were pretty confident that it all held together. That being the case, it made sense that other solar systems, if they existed, would form in more or less the same way. There would be local variations, of course, from one solar system to the next. The formation process couldn't have unfolded in
exactly
the same way every time. But just as every normally developed human is born with two legs at the bottom, two arms sticking out the sides, and a head on top, astronomers expected most solar systems to have rocky planets close in and gas-giant planets farther out.

Then came 1995. In the fall of that year, a Swiss observer named Michel Mayor announced that he'd discovered a planet orbiting a Sun-like star in the constellation Pegasus. This was
a very big deal: It was the first such planet ever found, after thousands of years of speculation about whether such worlds might exist, and after decades of searching by prominent astronomers. But the planet was all wrong. It was impossible. It was a world more massive than Jupiter, but sitting so absurdly close to its star that a full orbit—a year, from the planet's point of view—took just four days. (In our solar system, Mercury, the planet closest to the Sun, takes eighty-eight days to go once around).

That first planet might have been some sort of bizarre fluke, but over the next few years, more and more of what astronomers were now calling “hot Jupiters” were found, all over the sky. The conventional wisdom about how solar systems form was clearly wrong, or at least incomplete. Theorists had made the classic error of thinking that the single example they had to work with was a typical example.

Observers, however, were thrilled. Until that first planet was found, many doubted that existing telescopes were powerful enough to find alien worlds. Now that observers knew otherwise, they switched by the score from whatever area of astronomy they'd been working in. They began searching for planets, bringing new ideas and energy to what had until then been a backwater of astronomy. And while these first solar systems were weird and crazy, they brought a renewed hope that Earth-like worlds, too, existed out among the stars, places where life, and maybe even intelligent life, had taken hold.