The Sciences

Astronomers Discover an Earth-Size World Orbiting a Sun-Like Star in the 'Habitable Zone.' But Is It Earth 2.0?

ImaGeo iconImaGeoBy Tom YulsmanJul 24, 2015 3:52 AM


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An artist's concept of how planet Kepler-452b might look. It's the first near-Earth-size world to be found in the habitable zone of a star that is similar to our sun. (Source: NASA/JPL-Caltech/T. Pyle) Until today, for all we knew as humans, Earth was the only rocky planet in the universe orbiting a reasonably friendly star within a zone that was neither too close nor too far for life to thrive. Now we know there's a good chance that the home planet is not unique. Note the caveat. Please keep reading to find out why it's necessary.... Astronomers with NASA's Kepler Mission announced today that they've found the first near-Earth-size planet in the "habitable zone" orbiting a Sun-like star. They've dubbed it Kepler-452b. It's 6 billion years old, 1.5 billion more than Earth. It's also about 60 percent larger in diameter, and its mass is may be five times that of Earth, give or take. So, about that caveat: Astronomers can't yet say what Kepler-452b is made of. For it truly to be just like Earth, it would have to be made of rock. And that's why we still do not know for sure, despite today's announcement, whether there really are other Earth-like planets circling stars like our Sun within a region where it's not too hot or too cold for liquid water to exist on the surface. Liquid water is thought to be a requirement for life. But Jon Jenkins of NASA’s Ames Research Center, home of the Kepler project, told the New York Times that there's a 50 percent to 62 percent chance of Kepler-452b being rocky. Or as NASA puts it, "previous research suggests that planets the size of Kepler-452b have a good chance of being rocky."

Earth 1.0 and Earth 2.0?... An artist's concept compares Earth (left) with Kepler-452b, which is about 60 percent larger in diameter. (Source: NASA/JPL-Caltech/T. Pyle) So I guess we can go as far as saying that we humans now know — for the very first time — that there's another planet and star out there that closely resemble the Earth and Sun. Or, to put it another way (with an appropriate caveat): Chances are Earth has a close cousin where life might be possible. And while that may not be historic, it is pretty darn cool. Before 1994, we didn't even know for sure whether there were any planets outside our own solar system. It stood to reason — and theory — that there were. But none had been discovered. That was a bit disconcerting. Despite all those teeming aliens inhabiting science fiction stories and movies, it was possible back then to imagine that solar systems featuring Earth-like planets in the habitable zone were exceedingly rare in the cosmos — and thus intelligent life was too. Then, in 1994, Alex Wolszczan of Penn State University made the first widely accepted discovery of planets orbiting another star. But this "solar system" is profoundly different from our own. The planets orbit a pulsar — the dead star that remains after a supernova explosion. Suffice it to say that such a cataclysmic event would not be friendly to children and other living things... Even so, Wolszczan's discovery was an important milestone. But the question of whether Sun-like stars harbored planets was left unanswered. Until October of 1995, when Michael Mayor, Didier Queloz and colleagues at the Geneva Observatory in Switzerland announced that they had indirectly detected the presence of a Jupiter-like planet circling a Sun-like star, designated 51 Pegasi. In discovering this solar system, the researchers didn't actually look through a big telescope and spy a giant, Jupiter-like orb of gas circling 51-Peg. Instead they detected how the planet's gravity pulls subtly on its parent star, causing an ever-so-slight wobble. (The discovery of Kepler-452b was achieved by another technique: detection of a slight fall-off in light reaching the Kepler spacecraft's telescope as the planet moves in front of its parent star.) As it turned out, the 51 Peg solar system was like nothing any astronomer had imagined. The planet was 150 times more massive than Earth, yet it orbited 20 times more closely to its parent star than Earth circles the Sun. It was, in a word, bizarre. "I expected there to be a diversity of solar systems, but I never expected anything like 51 Peg," Jack Lissauer, a planet formation expert, told me in 2002, for a book I was writing. ("Origins: The Quest for our Cosmic Roots.") Since then, 1,030 planets have been discovered orbiting around other suns. These include lots of big, gaseous planets, many orbiting in hot orbits, and even smaller, Earth-sized worlds too. Recently, in fact, an Earth-sized planet with a rocky composition was discovered. But it orbits so close to its parent star that temperatures on the surface would be blazingly, unimaginably hot. Today's announcement is another milestone. But it's not quite the big prize — not quite "Earth 2.0." Not yet...

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