Mirror Earth Read Free Page A

reporters the number of new planets—or, rather, “planet candidates,” in the very careful language Kepler scientists prefer to use—Borucki explained, like a pollster projecting the outcome of an election based on just a small sample of voters, that Kepler is looking at only about 1/400th of the sky. If the spacecraft had been able to monitor the whole, he said (or if NASA had sent up four hundred identical Keplers, pointing in all different directions), they’d be talking not about twelve hundred planets, but about the more than four hundred thousand the probe would undoubtedly have seen.
    It was a terrific story—two weeks earlier. The talk at the American Association for the Advancement of Science meeting was pretty much just a replay. For a science reporter—especially a reporter for the Associated Press, where late-breaking news is a specialty—that just wasn’t good enough. Borenstein couldn’t write a story that said, in essence, “The Kepler results announced two weeks ago are still true.” So, while the editor of
Discover
magazine and I stood by, looking on in comradely amusement, Borenstein kept pushing the Kepler team leader to say something new. Borucki was clearly reluctant to be pushed.
    â€œOkay, so if I understand you correctly,” the reporter asked, “you said you’ve found 1,235 planet candidates, right?”
    â€œThat’s right,” said Borucki. He’d said this two weeks earlier. Borenstein knew it. But like a prosecutor in a courtroom, Borenstein was building his case.
    â€œAnd of those, fifty-four are in the habitable zone of their stars?”
    â€œYes, that’s correct.”
    Again, this news was two weeks old, but it was really important. The habitable zone is the orbital band surrounding a star where the temperature allows water to exist as a liquid rather than as ice or vapor. It’s sometimes called the Goldilocks Zone, even by astronomers—even on NASA’s website—since like the porridge in the fairy tale, it’s not too hot and not too cold, but just right. Biologists have long insisted that water is essential for life, because nutrients can dissolve in it easily, to be distributed to every part of an organism. That’s what blood does for most animals, and blood is mostly water. Life on Earth wouldn’t be possible, says the conventional wisdom, if most of our water boiled off into the atmosphere or froze solid. Earth is inhabited because we live within the Sun’s habitable zone.
    If you’re interested in finding life on other worlds—and that’s what just about every scientist who hunts for planets is ultimately looking for—planets in the habitable zone are what you want. Planets about the size of Earth in the habitable zone are even better. The question of whether life exists beyond Earth is one of the oldest mysteries of nature, dating back at least to the ancient Greeks, and probably even further. At sometimes in history, the notion of alien life has been considered heretical; at others, learned men took it as a given that planets, both within and outside the solar system, were home to intelligent beings. The eighteenth-century astronomer William Herschel, who discovered the planet Uranus, was convinced that even the Sun was inhabited (he had a handy explanation for why the Sun creatures weren’t incinerated).
    Kepler isn’t capable of answering the question of whether life exists on other worlds, but it can take the first step by finding an Earth-like planet, a Mirror Earth, where life could be thriving, at least in principle. Kepler was designed with several scientific objectives in mind, but number one on the list that appears on the mission website is this: “Determine the abundance of terrestrial [that is, Earth-size] and larger planets in or near the habitable zone of a wide variety of stars.”
    â€œSo, as I understand it,” continued