Three Penn State astronomers have been involved in the planning of the new project: Evan Pugh Professor Alex Wolszczan, Professor Niel Brandt, and Distinguished Professor Donald Schneider. "This new survey promises to bring important advances in fields as diverse as the study of planets outside our solar system and the nature of the mysterious "dark energy" that appears to pervade the universe," Schneider said. Wolszczan, who in 1992 discovered the first planets ever found outside our solar system, noted that, "The SDSS-III planet survey has the potential to identify a large number of new solar systems, which would be prime targets for detailed investigation using large instruments such as the Hobby-Eberly Telescope." The leader of the planet search team, Jian Ge of the University of Florida, developed the basic design for the planet survey while on the faculty at Penn State from 2000 to 2004.

"The cosmological measurements in SDSS-III could rewrite fundamental physics, either pinning down the properties of an exotic form of energy that fills the universe or showing that Einstein's theory of gravity fails at cosmological distances," explains Daniel Eisenstein of the University of Arizona and director of the newly formed collaboration. The Alfred P. Sloan Foundation of New York has approved a $7 million grant in support of SDSS-III, conditional on raising the additional funds from collaboration members and federal agencies needed to complete the project. The SDSS-III program will be announced at the American Astronomical Society meeting in Austin, Texas.

SDSS-III is slated to run from mid-2008 to mid-2014. Its four component surveys will operate from the 2.5-meter telescope at Apache Point Observatory in New Mexico, using optical fibers to capture the light of hundreds of objects simultaneously. This technique has allowed SDSS and SDSS-II to create the largest three-dimensional map of the present-day universe.

The largest of the four surveys, the Baryon Oscillation Spectroscopic Survey (BOSS), will measure the expansion of the universe with unprecedented precision. A decade ago, Eisenstein explains, astronomers made the startling discovery that the expansion of the universe is speeding up. "It's like tossing a ball in the air, waiting for it to fall, and instead seeing it accelerate upwards and disappear from sight."

Cosmologists attribute this acceleration to so-called "dark energy," which pervades otherwise empty space and exerts repulsive gravitational force. Dark energy could be the cosmological constant proposed by Albert Einstein in 1917, or it could be a new form of energy whose properties evolve with time. Distinguishing these possibilities, or determining whether the theory of gravity itself is at fault, requires measuring the history of cosmic expansion with very high precision, explains David Schlegel of Lawrence Berkeley National Laboratory, who is the principal investigator of the BOSS component of SDSS-III.

In 2005, the SDSS achieved one of the first clear detections of "baryon acoustic oscillations," a feature imprinted on the clustering of galaxies by sound waves that traveled in the early universe. BOSS will use this feature as a "yardstick in the sky" to measure cosmic distances, says Schlegel. "Our measurements should reach one-percent accuracy and extend to distances of ten billion light years, giving us strong tests of dark-energy theories."

While new, more sensitive instruments are being constructed for BOSS, SDSS-III will carry out a one-year extension of the SDSS-II SEGUE project, a survey mapping the outer Milky Way. "The Galaxy's stellar halo is much more complex than anyone realized a decade ago, and we want to understand what that is telling us about the formation of the Milky Way," explains Constance Rockosi of the University of California at Santa Cruz, the principal investigator of SEGUE-2.

Interstellar dust blocks visible light coming from stars in the inner Milky Way. Infrared light penetrates this dust, revealing stars even from heavily obscured regions near the galactic center. The APOGEE survey, another of the SDSS projects, will employ a unique new instrument that observes infrared light from 300 stars simultaneously, enabling a survey of 100,000 stars across the entire galaxy. "When stars die, the chemical elements forged by nuclear reactions in their cores are released into space," explains Steven Majewski of the University of Virginia, the principal investigator of APOGEE. "The APOGEE measurements will provide detailed chemical 'fingerprints' for each target star, which in turn will reveal the properties of the stars that preceded them. It's the ultimate exercise in forensic archeology."

And what about planets orbiting those stars? Of the 200 or so planetary systems currently known, most are very different from our own solar system, notes Jian Ge of the University of Florida. The majority of known planets are gaseous giants, like Jupiter, but they follow elongated (instead of circular) trajectories and orbit much closer to their parent stars. Ge is the principal investigator of the SDSS MARVELS project, which will search more than 10,000 stars for orbiting giant planets, a three-fold increase on the number searched by all other telescopes to date. "By systematically monitoring such a large number of stars," says Ge, "MARVELS will address two of the biggest questions in planetary science: how do giant planets form, and why are so many in such unusual orbits?"

Jim Gunn of Princeton University, who has led nearly two decades of construction and operation of the Sloan Digital Sky Survey, is excited about its new ventures. "It's amazing to see that the SDSS can transform scientific fields we hadn't even conceived of 20 years ago," he said.