Mirror Earth: Science Closes in On Our Planetary Twin

The hunt for a habitable world like ours heats up—and bears results

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The number of confirmed exoplanets, or worlds orbiting distant suns, shot past 1,000 last week, according to the semi-official Extrasolar Planets Encyclopedia. That’s pretty heady stuff considering that just over 20 years ago we didn’t know of any at all.  But the ultimate goal of finding a true Earthlike world has continued to elude astronomers.

That landmark discovery, however, appears to be closer than ever, particularly after the release of a pair of papers in the journal Nature this week, concerning a planet known as Kepler 78b. The new world is not only close to Earth in size (it’s just 20% bigger), but it has almost exactly the same density. The implication, says University of Hawaii astronomer Andrew Howard, lead author of one of the papers: “We think it’s made of the same stuff as Earth—primarily rock and iron.”

What makes this find truly compelling, however, is the very fact that there is not just one paper, but two. And that second one, based on independent observations from a European team, reaches exactly the same conclusion. “When results agree like this,” says Howard, “that’s really the gold standard.”

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The planet was found earlier this year in data beamed down by the now-defunct Kepler satellite, but Kepler can tell scientist only how physically large an exoplanet is, based on what percentage of light it blocks as is it moves in front of its parent star. Before they began finding exoplanets in droves, theorists mostly assumed size and composition went hand in hand—that a planet in the size range of Jupiter, for example, must be like Jupiter in all respects.


Turns out they were wrong. Among the very first worlds discovered by Kepler, for example, one, Kepler 7b, is 50% bigger than Jupiter, but with the density of Styrofoam. Another, Kepler 10b is about the size of Earth but is nearly as dense as pure iron. Yet another, GJ 1214b (this one wasn’t found by Kepler), is less than three times the size of Earth, and has a density that suggests it’s probably half-rock and half-water. “There have been a lot of surprises,” Howard admits.

But it would have been downright astonishing if there were no true twins of Earth out there in the Milky Way, and both Howard’s team and the competing team, led by the University of Geneva’s Francesco Pepe, nailed down the composition of Kepler 78b with the same technique: the planet’s tug on its star as it orbits, pulling it almost imperceptibly in one direction and then the other. By measuring subtle changes in the star’s color induced by those wobbles, the astronomers can judge how fast the star moves as it approaches and retreats—and that speed depends on the planet’s mass.

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It also depends on how tight the planet’s orbit is: the closer it is to the star, the more leverage it exerts. And Kepler 78b is crazy close—so close that its “year” last just 8.5 hours. “I mentioned this to a kid in my neighborhood,” says Howard, “and he said ‘wow, that would be great! I would have 10,000 birthdays already. That’s a lot of presents!’”

The simultaneous publication in Nature is not a coincidence. When the planet’s discovery was announced last spring, says Howard, his team asked the astronomers in Geneva to join them in a supporting role in measuring its mass. The Swiss liked the idea of a partnership—provided they were the ones taking the lead spot. “Neither of us wanted to play second fiddle to the other,” Howard says, “so we agreed to observe independently and submit our results to the same journal on the same day.”

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Finding an Earth-size planet with an Earthlike composition is a major milestone in exoplanetology, but as a University of Maryland astronomer observes in a commentary, also published in Nature, the extra leverage—and close proximity to the parent star—that made the planet relatively easy to weigh “comes at the price of a hellish environment.” With a surface temperature of up to 8,500°F (4,700°C), says Howard, it’s “one of the hottest that’s ever been discovered.”

Kepler 78b is clearly not a place you’d expect to find life, in other words, and it may be that no current technology can discover such a planet. But a new space telescope called the Transiting Exoplanet Survey Satellite (TESS), coupled with powerful new wobble-measuring spectrographs on huge ground-based telescopes, could find a truly habitable Earthlike world. TESS won’t fly until 2017 at the earliest, but that’s nothing in cosmic time. If there’s a second Earth out there to be found, it’ll still be waiting when the new satellite opens its eyes.

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