Looking for Earth-like planets isn’t anything new for astronomers. But it’s only been within the last few years that they’ve confidently identified candidates. The change from famine to feast is in large part due to NASA’s Kepler telescope, a mission that survived a number of challenges to become a reality. My US News article from earlier this year dives into how the hunt for exoplanets has changed and why Kepler has made such a significant contribution. The full article is below the image, which illustrates the sizes of a few identified exoplanets.
A Place Like Home
by RON COWEN
The hunt for planets like earth is rapidly locating other worlds.
When astronomer Alan Boss got an urgent call in September 1995 to review a provocative manuscript submitted to the scientific journal Nature, he had no idea that a revolution in the study of planets was about to start. The manuscript described observations by Swiss researchers at the Geneva Observatory who claimed to have found the first planet orbiting an ordinary star beyond the solar system. Boss, a planet formation expert at the Carnegie Institution for Science in Washington, D.C., was excited but skeptical. He was all too aware that several times since the 1960s, reports citing evidence of exoplanets, or planets orbiting a star other than the sun, had come to naught. By the fall of 1995, one research group was ready to throw in the towel after a 13-year search had failed to find even a single body.
That was deeply unsettling for astronomers like Boss. According to the leading theory of the solar system’s formation, which he had recently helped update, planets arise from a swirling disk of gas, dust, and ice that surrounds a young sun and should be common throughout the galaxy. So if exoplanets turned out to be truly scarce, researchers would have to face the fact that they had misunderstood how the solar system came to be. Not to mention that their odds of finding life outside the solar system would be drastically reduced. Science’s search for other civilizations would be in vain, and there would be no place for humans to go should living conditions on this crowded planet take a turn for the worse.
The intriguing manuscript sent to Boss summarized evidence that the tug of an unseen planet was causing a minuscule wobble in the motion of the star 51 Pegasi. Though he had a few questions for the authors, Boss could find no fatal flaw in their article. In early October, the Swiss team announced the discovery at a meeting in Florence; just 13 days later, a U.S. team led by astronomers Geoff Marcy, now at the University of California–Berkeley, and Paul Butler, now at Carnegie, said they had confirmed the wobble. News of the first bona fide planet discovered orbiting around a sunlike star thrilled astronomers and made the front page of newspapers around the world.
Since that landmark finding, astronomers have discovered more than 750 exoplanets, with another 2,300 candidates awaiting confirmation. Researchers use an assortment of telescopes on the ground and in space in their search, but the recent flood of discoveries is largely credited to NASA’s Kepler telescope, which was launched early in 2009 and follows Earth in its path around the sun. Milestone discoveries have come furiously in the last couple of years: the first multiple-planet system around a sunlike star; the first rocky planet; the first exoplanet on which liquid water, a key ingredient for life, might exist.
“People call this the golden age of astronomy for exoplanets, but it’s beyond gold,” says Boss. Indeed, exoplanets now appear to be abundant beyond the wildest dreams of the most optimistic of planet hunters. A recent study estimates that on average, each of the more than 100 billion stars that glitter in the Milky Way harbors at least one planet. And the sheer diversity of the discoveries to date—how they’re distributed around a star, their density, and their spacing if they reside within a multiple-planet system—suggests that an entire solar system like Earth’s is not at all a rare entity, says planetary theorist Doug Lin of the University of California–Santa Cruz.
The orbs detected so far range from small rocky planets the size of Mercury to giant blobs of gas several times heavier than Jupiter. Many have great swooping orbits rather than a circular, sedate path like Earth’s that keeps temperature swings relatively mild, making life possible. This solar system is one of a “lucky subset with circular orbits,” says Marcy. Some members of the new celestial pantheon orbit two stars. Like Luke Skywalker’s home planet Tatooine in the Star Wars series, these planets are treated to a daily double feature: two sunsets and two sunrises. They and their stars emerged from the same primordial cloud of gas and dust, scientists believe; part of the material formed a planet while the rest coalesced into stars that remain gravitationally bound to each other though they now lie far apart.
Galvanized by the rapid progress, astronomers now are racing to find a potential twin to Earth. They’re pinning their hopes on the Kepler telescope, an observatory that almost didn’t get launched because few believed it would really work. Kepler is the brainchild of veteran planet hunter William Borucki, an astronomer at NASA’s Ames Research Center in Mountain View, Calif. Borucki’s tenacity kept the Kepler mission alive for 17 years after NASA repeatedly rejected the concept as too expensive and unlikely to find small planets.
Borucki first wrote about his vision for the telescope in 1984, a time when looking for planets seemed like the stuff of science fiction. He proposed that a properly positioned telescope, flying beyond Earth’s light-distorting atmosphere, could discover planets by staring continuously at one group of sunlike stars. Periodic dips in the brightness of individual stars would indicate a planet has passed in front of its sun or “transited,” blocking a tiny fraction of light. This reduction in brightness would reveal the planet’s size, while the frequency of the transits would indicate the planet’s orbital period and its distance from the star. The Hubble Space Telescope (story, Page 26) has sent back spectacular glimpses of galactic wonders for nearly two decades, but its small field of view and close orbit around Earth prevent it from continuously viewing any particular group of stars.
Borucki built a ground-based observatory and conducted experiments that proved the transiting method could work, though land-based telescopes could not be a long-term solution as the Earth’s atmosphere distorts the flickers of starlight. Finally, the project was approved, and in May 2009, the Kepler began regular operations. From its current vantage point, more than 30 million miles away, Earth is a tiny blue marble. Kepler’s primary mission is to focus on a narrow patch of roughly 156,000 stars in the Milky Way in hopes of finding other blue marbles that might mimic Earth in size, temperature, and orbit.
The telescope, which can record a reduction in starlight as small as 0.002 percent—akin to detecting an insect crawling across a car headlight from 3,000 miles away—had found 60 confirmed planets as of early 2012, as well as the more than 2,300 additional candidates. Initially, most were gaseous giants like Jupiter and Saturn. Then came smaller, rocky bodies whose molten surfaces cannot support life as humans know it. A precious few of these candidates of unknown size and composition reside in the so-called habitable zone, where water can exist as a liquid on a solid surface.
Lisa Kaltenegger, an astronomer at the Max Planck Institute for Astronomy in Germany and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and an expert on the conditions necessary to detect life beyond Earth, remembers exactly where she was in 2009 when she heard about the spotting of the first potentially habitable exoplanet. About to hike across a glacier field and ice crevasse on South Island in New Zealand, she first checked her E-mail. A friend had written to say that astronomers had just determined that of one of the six planets circling a star called Gliese 581 was positioned within the habitable zone of the cool, red star. The first “Goldilocks” discovery—an exoplanet neither too hot nor too cold—opened “a new chapter in scientific discovery,” she says, that shifted focus from giant gaseous planets to rocky planets that could maintain water. “I allowed myself to completely unscientifically paint a picture in my head of what such a world could be like,” she recalls—a red sun and “loads of carbon dioxide obscuring the clear view.”
Of course, residing in the habitable zone doesn’t mean a planet actually harbors life, cautions Kaltenegger. That’s where monitoring planet transits across their stars offers a further advantage: The starlight filtering through, and reflecting off, a passing planet’s atmosphere can reveal the atmosphere’s composition, and indicate whether biological activity could have contributed to the chemical constituents. Astronomers have already recorded water vapor and sodium in the puffy atmospheres of gaseous Jupiter-like planets using transits. Now, they face the more difficult task of searching for tiny amounts of ozone and carbon dioxide, which can be produced by living organisms, in the much thinner atmosphere of rocky planets. Existing telescopes aren’t big enough; the effort must wait for the launch of the James Webb Space Telescope in 2018. Featuring a large, segmented mirror that will unfold in space, the Webb telescope will be able to make images (which Kepler doesn’t) and gather six times as much light as the Hubble while orbiting the sun some 940,000 miles from Earth.
Mass appeal. One reason early discoveries were dominated by big, gassy planets is that they exert the strongest tug on their parent stars, creating the most detectable wobble. The most common type of planet found by the Kepler is much smaller. To take the next step and determine whether a planet is gaseous or one of the rocky gems that could be an Earth analog, scientists must know both its mass and size. But the standard technique for determining mass—measuring how much the planet tugs on its star—doesn’t work for many of the Kepler planets because they are so puny.
A newer method relies instead on the extent to which planets in a solar system tug on each other. The heavier a planet, the more it delays or advances the transit times of the others in the system. That suggested a new way to figure out a planet’s mass. In April 2010, Jason Steffen, one of the researchers who devised the technique and is now an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill., found that the three planets in a system called Kepler-9 sometimes passed in front of their star ahead of or behind schedule. By analyzing the data further, Steffen was able to determine the mass of each planet.
The method worked especially well with the exoplanets in a solar system unveiled in February 2011 called Kepler-11, whose five innermost planets are packed so tightly that they all orbit their star at a distance shorter than that of the sun to its nearest planet, Mercury. Their mutual tug plays havoc with their transit times, enabling the team to determine their masses with unprecedented accuracy. In January 2012, Steffen’s team and two others announced that they’d identified 21 previously unknown planets and calculated the maximum mass of each orb.
Since many of the sunlike stars of interest are younger and rowdier than the one in this solar system—and thus more prone to sudden flares—scientists estimate that the Kepler telescope will have to watch twice as many transits as previously anticipated to be sure it’s really seeing planets rather than just stellar temper tantrums. But the Kepler mission is slated to end in November 2012. It would need to operate until mid-2016 to accurately estimate how many Earth-like planets are in the Milky Way, says Ron Gilliland, an adjunct professor of astronomy at Pennsylvania State University–University Park. With NASA strapped for cash (box, Page 10), it’s unclear whether that will happen. But space enthusiasts can take comfort from one fact that has been established, says planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo. Even if a solar system with a habitable Earth twin turns out to be a 1 in 1 million find, he notes, with 100 billion-plus stars in the galaxy, that equals a lot of possibilities.