Space Telescope Used To Trace Formation, Evolution Of Planetary Systems

"We will look for signatures of planets that sculpt circumstellar dust in a sample of 300 solar-like stars ranging in age from 3 million to 3 billion years," said Michael R. Meyer of the University of Arizona Steward Observatory.

Meyer is principal investigator for a team that will use the Space Infrared Telescope Facility (SIRTF) in the Legacy Science Program project, "The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context."

Five other teams, including one headed by UA astronomer Robert C. Kennicutt Jr., also were chosen for the SIRTF Legacy Science Program. The six projects comprise more than 3,000 hours of observations, or about half the time available during SIRTF's first year of operation.

The fourth and last of NASA's Great Observatories, SIRTF is scheduled for launch April 26 or soon after. It will view the universe at very long wavelengths, the far infrared, and see objects that are too cool, too dust-enshrouded or too far away to otherwise be seen.

The three previous Great Observatories are the Hubble Space Telescope, Compton Gamma Ray, and Chandra X-ray Observatories. SIRTF is managed for NASA by the Jet Propulsion Laboratory, Pasadena, Calif.

The SIRTF telescope is designed to operate at a temperature of only a few degrees above absolute zero. It carries three science instruments. One of these is a highly sensitive camera called MIPS that uses imaging arrays at far-infared wavelengths and will see the coolest objects in space. It was built by a team headed by the UA astronomy Professor George H. Rieke.

"SIRTF has unprecedented sensitivity at long infrared wavelengths," Meyer said. "It will be a powerful instrument for viewing circumstellar dust as it evolves from the earliest stages of planet formation toward mature planetary systems like our own."

Because SIRTF will detect very cold dust, Meyer's project may reveal planets far from their stars -- as far as Uranus and Neptune are from our sun. These planets have such long orbital periods that it would take many decades to detect them using radial velocity techniques that since 1995 have successfully been used to find Jupiter-like planets orbiting very close to their stars.

"We would like to locate things that are directly analogous to our own solar system. SIRTF will provide really the first evidence of the diversity of solar systems going out to these large radii," Meyer said.

"We won't actually see beautiful resolved images of a circumstellar disk around a young star of interest," he explained. "We'll simply see radiation from the dust at different wavelengths and then construct spectral energy distributions, curves of how much energy there is per wavelength, which allows us to model distribution of dust.

"Dust close to the star will be very hot and will emit at shorter wavelengths. Dust that is very cold and far from the star will emit at very long wavelengths. So by looking at ratios of hot dust to warm dust to cold dust, we'll be able to build a model for the dust distributions in the system."

Our own solar system provides an example, he added. It has an inner dust disk called the zodiacal dust disk, generated by asteroids colliding in the asteroid belt between Mars and Jupiter. These dust grains are heated as they stream toward the sun. As they increase in temperature, they emit more light at shorter infrared wavelengths.

Our solar system also has an outer dust disk beyond the orbit of Neptune, where Kuiper Belt objects smash against each other and generate dust. This dust is cold, and emits most of its light at much longer wavelengths.

In between, planet Jupiter sweeps our solar system clean of dust, creating a huge gap in the dust disk around our sun.

"If you were observing our system from afar, you would see that in the spectral energy distribution. You'd see a lot of hot and warm dust inside the orbit of the asteroid belt. You'd see a lot of very cold dust associated with the Kuiper Belt. And you'd see a gap created by Jupiter in between.

That's exactly the kind of signature we're looking for," Meyer said. "If we see large gaps in the dust distribution, that would be a smoking gun, evidence that planets there could be there, sculpting the dust."

Meyer and his colleagues have modeled how our solar system might have looked when it was one million years old, and how it changed through time, at 100 million years, at one billion years, at 4.5 billion years. It's like a blueprint in the search for other planetary systems.

Their Legacy project may also help settle a current raging debate about whether or not Jupiter-mass planets form very quickly in gas disks around young stars.

Recent ground-based observations support the idea that such planets form as quickly as one million years from the molecular hydrogen in circumstellar disks.

That conflicts with findings from the European Space Agency's infrared "ISO" mission, which suggested that molecular hydrogen disks around stars have long lifetimes, and so could form planets like our Jupiter over timescales longer than 10 million years.

Using the Infrared Spectrograph on SIRTF, Meyer and his colleagues will measure the mass of molecular hydrogen in circumstellar disks of stars in their study.

"If we don't see any gas around stars older than 3 million years, then perhaps planets like our own Jupiter may have trouble forming," Meyer said. "On the other hand, if we see gas-rich disks within 10 million to 20 million years, chances are greater that there may be more solar systems like our own whether planets form quickly or not."

Members of the 'FEPS' Legacy team include:

* UA Steward Observatory -- Michael R. Meyer (principal investigator), Dean Hines, J. Serena Kim, Murray Silverstone, Erick Young, E. Mamajek, A. Moro-Martin

* UA Lunar and Planetary Laboratory -- Jonathan I. Lunine, Renu Malhotra

* California Institute of Technology -- Lynn Hillenbrand (deputy principal investigator), John Carpenter, Sebastian Wolf

* Max Planck Institute, Heidelberg -- Jens Rodmann, Thomas Henning

* NASA Ames Research Center, Franklin & Marshall -- Dana Backman (deputy principal investigator)

* NASA Ames Research Center -- David Hollenbach, Uma Gorti

* National Optical Astronomy Observatory -- Joan Najita, Steve Strom

* Planetary Sciences Institute -- Stuart Weidenschilling

* SIRTF Science Center -- Pat Morris, Deborah Padgett, John Stauffer

* Space Telescope Science Institute --Steve Beckwith, David Soderblom

* University of California - Berkeley -- Martin Cohen

* University of Rochester -- Dan Watson

University of Arizona news is online @ http://uanews.org

FEBS website: http://feps.as.arizona.edu/outreach.html