Earth Like Planet

The star known as Gliese 581 is utterly unremarkable in just about every way you can imagine. It's a red dwarf, the most common type of star in the Milky Way, weighing in at about a third of the mass of the sun. At 20 light years or so away, it's relatively nearby, but not close enough to set any records (it's the 117th closest star to Earth, for what that's worth). You can't even see it without a telescope, so while it lies in the direction of Libra, it isn't one of the shining dots you'd connect to form the constellation. It's no wonder that the star's name lacks even a whiff of mystery or romance. But Gliese 581 does have one distinction — and that's enough to make it the focus of intense scientific attention. At last count, astronomers had identified more than 400 planets orbiting stars beyond the sun, and Gliese 581 was host to no fewer than four of them — the most populous solar system we know of, aside from our own. That alone would make the star intriguing. But on Wednesday, a team of astronomers announced that it had found two more planets circling the star, bringing the total to six. And one of them, assigned the name Gliese 581g, may be of truly historic significance. For one thing, the planet is only about three or four times as massive as our home world, meaning it probably has a solid surface just like Earth. Much more important, it sits smack in the middle of the so-called habitable zone, orbiting at just the right distance from the star to let water remain liquid rather than freezing solid or boiling away. As far as we know, that's a minimum requirement for the presence of life. For thousands of years, philosophers and scientists have wondered whether other Earths existed out in the cosmos. And since the first, very un-Earthlike extrasolar planet was discovered in 1995, astronomers have been inching closer to answering that question. Now they've evidently succeeded (although to be clear, there's no way at this point to determine whether there is life on the new planet). "We're pretty excited about it," admits Steve Vogt of the University of California at Santa Cruz, a member of the team, in a masterpiece of understatement. "I think this is what everyone's been after for the past 15 years."

Planetary scientist James Kasting of Penn State University, who wasn't involved with the discovery, agrees. "I think they've scooped the Kepler people," he says, referring to the telescope that launched into space early last year on a mission to determine how common Earthlike planets might be. The "Kepler people" have a number of candidate Earths in the can but are still working to confirm them. Being first isn't the main reason Vogt is excited, however. "Someone had to be first," he says. "But this is right next door to us. That's the big result." What's particularly big about it is a matter of simple arithmetic. With only 116 stars closer to Earth than this one, it was hardly a sure thing that so small a sample group would produce two habitable planets, including Earth. And two such planets may be an undercount, Vogt says, since just nine out of those 100-plus stars have been studied in any detail. Indeed, one of Gliese 581g's sister planets, known as Gliese 581d (O.K., they don't put a lot of creative energy into naming these things), could conceivably be a habitable world itself. One of the four planets known to orbit Gliese 581 before the latest discovery, 581d was found by a team of Swiss astronomers in 2007 and was thought to be outside the habitable zone and thus too cold for liquid water. But a reanalysis last year brought it into the zone, albeit just barely. The problem is, 581d is too big to be Earthlike; it's probably made mostly of nonwater ice, like Neptune and Uranus, which makes a poorer candidate for life than 581g. (Comment on this story.) Lost in the excitement over possible life on the new world is what a remarkable achievement its mere discovery was. Detecting a planet this small is monstrously hard — and would have been impossible when Vogt and co-discoverer Paul Butler of the Carnegie Institution of Washington first got into the planet-hunting game in the early 1990s. The instruments you use to detect tiny back-and-forth motions in the star — motions caused by the orbiting planet's gravitational tugs, which are often the only way to infer that the worlds exist at all — simply weren't sensitive enough. Since then, says Vogt, "I've been busting my gut to improve the instruments, and Paul has been busting his gut to do the observations." In all, those observations span more than 200 nights on the giant Keck I telescope in Hawaii over 11 years, supplemented by observations from the Geneva group — and that painstaking work finally confirmed 581g's existence.

None of this proves that there is water on Gliese 581g. "Those are things we just have to speculate about," says Vogt. But he goes on to point out that there's water pretty much everywhere else you look. "There's water on Earth," he says, "and on the moon, and Mars, and on Jupiter's moon Europa and Saturn's moon Enceladus, and in interstellar space. There's enough water produced in the Orion Nebula every 24 seconds to fill the Earth's oceans." It's not hard to imagine, in other words, that Gliese 581g might have plenty of water as well. "It could have quite a good ocean," Vogt says. Certainly, it could be a sterile, nonbiological ocean. But unlike any planet found until now, there's nothing to rule out the idea that it could be teeming with life.

The smallest planet yet spied outside our solar system has been found orbiting a sunlike star about 560 light-years away, astronomers announced today. Known as Kepler-10b, the planet is just 1.4 times Earth's size and 4.6 times its mass. The planet, found using NASA's Kepler spacecraft, is the first of the more than 500 known exoplanets that's definitively rocky—much like Earth, Mars, Venus, or Mercury—the study team says. Launched in March 2009, Kepler was designed to hunt for potentially habitable Earthlike planets. Astronomers have been studying Kepler-10b since its discovery in 2009, when the team detected a periodic dimming of the host star as the planet passed in front of the star. Finding such a small planet this way was no easy feat—seen from a similar distance, Earth passing in front of the sun would cause a 0.01 percent reduction in the star's brightness, said Natalie Batalha of San Jose State University, lead author of an upcoming paper describing the find. "Imagine you have 10,000 light bulbs and you take one away. That's the change in brightness we're looking for," Batalha said today during a meeting of the American Astronomical Society in Seattle, Washington. Still, after using Kepler and other instruments to precisely calculate the new planet's size, mass, and density, Batalha said, "we know without question that this is a rocky world."

Smallest Planet Has Density of Iron Before figuring out the nature of Kepler-10b, the scientists looked at the host star's properties, as revealed by starquakes, acoustic disturbances that make the entire star ring like a bell. "In the same way that we use a sonogram to probe an unborn fetus and earthquakes to probe the interior of the Earth, we use starquakes to probe the interior structure and properties of the star itself," Batalha said. "A tiny star would yield different [vibration] frequencies than a large one, just as when you strum a violin you're going to get a different sound than when you strum a cello." Using starquakes, Batalha and colleagues were able to accurately determine the size, mass, and age of the star, which in turn allowed them to make very fine-tuned estimates of the new planet's characteristics. Astronomers carefully studied the tiny variations in starlight to determine Kepler-10b's size. The observations also revealed that the planet is very close to the star, orbiting once every 20 hours. (Related: "Five New Planets Found; Hotter Than Molten Lava.") Using the W. M. Keck Observatory in Hawaii, the Kepler team made precise measurements of minute shifts in the wavelengths of the host star's light. This data showed how the star nodded back and forth in response to the planet's gravity, allowing the team to calculate the masses of both objects. Based on this combined data, the team concludes that Kepler-10b must be a rocky world with an average density of 8.8 grams per cubic centimeter—about the same as a chunk of iron.

Rocky World is "Planetary Missing Link" Being rocky, however, is no guarantee that a planet will be habitable. In the case of Kepler-10b, one side of the planet always faces the star, so that side would have a surface temperature of 2,500 degrees F (1,370 degrees C), Batalha estimates. It's highly unlikely that such a world would retain an atmosphere, since the searing hot gases would rapidly escape into space. Still, Kepler-10b is an enormously important find, said Geoffrey Marcy, a planet hunter at the University of California, Berkeley. Marcy, who is involved in the Kepler mission, was not a core member of Batalha's team. "In astronomy, we've been discovering giant gas planets for 15 years. But the ultimate goal is to discover habitable worlds, like Earth," he said at the AAS meeting. (Related: "New Planet System Found—May Have Hidden 'Super Earth.'") According to Marcy, Kepler-10b is the "planetary missing link." "It's definitely not a gas giant like Jupiter. Nor is it habitable—it's too hot. This is a transitional planet somewhere between a gas giant and what we've been hoping to find." One other possibly rocky planet, COROT-7b, might be even more Earthlike in size and mass, Batalha agreed. But its star is much more active with flares and other disturbances, making it difficult to nail down important parameters with the needed precision. (See "'Super Earth' May Really Be New Planet Type: Super-Io.") "For Kepler-10, we were lucky," she said. "It's a very quiet star."

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