PEERING DEEPLY The primary mirror of the Kepler telescope. The craft’s mission, set to begin Friday, is to discover Earth-like planets in Earth-like places.
Someday it might be said that this was the beginning of the end of cosmic loneliness.
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COSMOS CENSUS TAKER The Kepler spacecraft is coated with solar panels.
Presently perched on a Delta 2 rocket at Cape Canaveral is a one-ton spacecraft called Kepler. If all goes well, the rocket will lift off about 10:50 Friday evening on a journey that will eventually propel Kepler into orbit around the Sun. There the spacecraft’s mission will be to discover Earth-like planets in Earth-like places — that is to say, in the not-too-cold, not-too-hot, Goldilocks zones around stars where liquid water can exist.
The job, in short, is to find places where life as we know it is possible.
“It’s not E.T., but it’s E.T.’s home,” said William Borucki, an astronomer at NASA’s Ames Research Center at Moffett Field in California, who is the lead scientist on the project. Kepler, named after the German astronomer who in 1609 published laws of planetary motion that now bear his name, will look for tiny variations in starlight caused by planets passing in front of their stars. Dr. Borucki and his colleagues say that Kepler could find dozens of such planets — if they exist. The point is not to find any particular planet — hold off on the covered-wagon spaceships — but to find out just how rare planets like Earth are in the cosmos.
Jon Morse, director for astrophysics at NASA headquarters, calls Kepler the first planetary census taker.
Kepler’s strategy is, in effect, to search for the shadows of planets. The core of the spacecraft, which carries a 55-inch-diameter telescope, is a 95-million-pixel digital camera. For three and a half years, the telescope will stare at the same patch of sky about 10 degrees, or 20 full moons, wide, in the constellations Cygnus and Lyra. It will read out the brightnesses of 100,000 stars every half-hour, looking for the telltale blips when a planet crosses in front of its star, a phenomenon known as a transit.
To detect something as small as the Earth, the measurements need to be done with a precision available only in space, away from the atmospheric turbulence that makes stars twinkle, and far from Earth so that our home world does not intrude on the view of shadow worlds in that patch of sky. It will take three or more years — until the end of Barack Obama’s current term in office — before astronomers know whether Kepler has found any distant Earths.
If Kepler finds the planets, Dr. Borucki explained, life could be common in the universe. The results will point the way for future missions aimed at getting pictures of what Carl Sagan, the late Cornell astronomer and science popularizer, called “pale blue dots” out in the universe, and the search for life and perhaps intelligence.
But the results will be profound either way. If Kepler doesn’t come through, that means Earth is really rare and we might be the only extant life in the universe and our loneliness is just beginning. “It would mean there might not be ‘Star Trek,’ ” Dr. Borucki said during a recent news conference.
The need, indeed even the possibility, of a planetary census is a recent development in cosmic history. It was only in 1995 that the first planet was detected orbiting another Sun-like star, by Michel Mayor and his colleagues at Geneva Observatory. In the years since then there has been a torrent of discoveries, 340 and counting, that has bewildered astronomers and captured the popular imagination.
“What exists is an incredibly random, chaotic, wild range of planets,” said Debra Fischer of San Francisco State University, also a veteran planet hunter who is not a member of the Kepler team. So far none of them qualify as prime real estate for life, and few of them reside in systems that resemble our own solar system. Many of the first planets discovered were so-called hot Jupiters, gas giants zipping around their stars in a few days in tight, blisteringly hot orbits.
Most of the planets have been found by what is called the wobble method, in which the presence of a planet is deduced by observing the to-and-fro gravitational tug it gives its star as it orbits. The closer a planet is to its star, the bigger the tug and the easier it is to detect.
The smallest exoplanet discovered is about three times as massive as the Earth. It is known as MOA-2007-BLG-192-L b, but astronomers don’t know yet whether its home star is real star or a failed star called a brown dwarf.
Last summer Dr. Mayor announced that his team had found three so-called warm super-Earths — roughly four, seven and nine times the mass of the Earth — orbiting within frying distance of a star known as HD 40307 in the constellation Pictor. Indeed, Dr. Mayor proclaimed that according to their data, about a third of all Sun-like stars host such super-Earths or super-Neptunes in tight orbits.
But all this is prelude. Astronomers agree that these planets are oddballs according to any reasonable theory of planet formation. But as Alan Boss of the Carnegie Institution of Washington pointed out, they are easy to detect by the wobble method. The fact that they are there suggests that there are many more modest-size planets to be found in larger, more habitable orbits.
The Kepler mission is a tribute to the perseverance of Dr. Borucki, who began proposing it to NASA in the 1980s, before any exoplanets had been discovered, and kept campaigning for it. “He had the true faith,” Dr. Boss said.
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