AROUND TWO SUNS THRIVE NASA TELESCOPE TO FIND PLANETS

Using the Spitzer Space Telescope NASA astronomers have observed that planetary systems - dusty disks of asteroids, comets and possibly planets - are so abundant in double star systems as single stars in systems such as ours. Since more than half of all stars are twins, or binaries, the finding suggests that the universe is full of planets with two suns. Sunsets on some of those worlds would resemble that of Luke Skywalker's planet, Tatooine, where two fiery balls dip below the horizon one by one.

"It seems that there is no bias against the formation of planets in binary systems," said Dr. David Trilling of the University of Arizona in Tucson, the first author on the research appearing in the April 1 edition of the Astrophysical Journal. "There may be countless planets out there with two or more suns."

Previously, astronomers knew that planets can form in exceptionally wide binary systems in which stars are 1,000 times farther than the distance from Earth to the Sun, or 1,000 astronomical units. Of the approximately 200 planets discovered outside our solar system, something like 50 are in orbit around one of the members of a wide stellar duo.

The new Spitzer study focuses on binary stars closer together, with separations between 0 and 500 astronomical units. So far, almost nothing was known about the effect that the proximity of the stars could have on the growth of planets. The usual techniques of planet hunting generally do not work well in these stars. In 2005 a NASA-funded astronomer found the only planet candidate who resides in a multiple-star system.

Trilling and his colleagues used Spitzer's infrared eyes that can detect heat, not to search for planets, but dusty disks in double star systems. The so-called debris disks are composed of rock fragments of asteroids that were not used to form planets. Their presence indicates that the process of building planets has occurred around a star or stars, possibly resulting in intact, mature planets.

In the most comprehensive survey of this kind, the team looked for disks in 69 binary systems between 50 and 200 light years from Earth. All the stars in the sample are somewhat younger and more massive than our sun, which is in the midlife. The data show that something like 40 percent of the systems had disks. This fraction is somewhat greater than that of a comparable sample of single stars.

Furthermore, astronomers were surprised to discover that the frequency of disks is greater (about 60 percent) in the binary closer together. The closest of these stellar companions are at distances of 0 to 3 astronomical units. Spitzer detected disks orbiting both members of the star pairs, rather than just one. Tight star systems like these are where planets, if present, will experience sunsets like Tatooine.

"We were surprised to find that the group of binaries with stars close together had more disks," said Trilling. "This could mean that planet formation is favored in these systems compared to single star systems. You can also simply mean that close binaries are more dust. Future observations will give us better answers."

The Spitzer data also reveal that not all binary systems are friendly places for planets to form. The telescope detected far fewer disks in binary systems of intermediate separation, where the stars have separations between 3 and 50 astronomical units. Since Spitzer disks found around a star in wide binary systems and disks around both stars in nearby systems, it is possible that the stars have to be either very close or too distant to form planets.

"For a planet in a binary system, the position is most important," said one of the co-authors, Dr. Karl Stapelfeldt of the Jet Propulsion Laboratory of NASA in Pasadena, California.

"For the most part, the binary systems have been ignored," added Trilling. "They are more difficult to study, but may be the most common sites for planet formation in our galaxy."

Other authors on the paper include: John Stansberry, George Rieke and Kate Su of the University of Arizona, Dr. Richard Gray of the Appalachian State University in Boone, NC, Dr Chris Corbally of the Vatican Observatory in Tucson ; Drs Goeff Bryden, Andy Boden and Chas Beichman of JPL and Dr. Christine Chen, National Astronomy Observatory in Tucson UNDERCAR optics.

JPL manages Spitzer for the Science Mission Directorate at NASA Headquarters in Washington. Telescope Science operations are conducted at the Spitzer Science Center, located at the California Institute of Technology (Caltech, for its abbreviation in English) in Pasadena. The multiband imaging photometer for Spitzer was built by Ball Aerospace Corporation (located in Boulder, Colorado), University of Arizona and Boeing North America (located in Canoga Park, California). Co-author Rieke is the principal investigator.