From: baalke@zagami.jpl.nasa.gov   
      
   McDonald Observatory   
   University of Texas   
      
   Contact:   
   Rebecca A. Johnson   
   ph: 512-475-6763   
   fax: 512-471-5060   
   rjohnson@astro.as.utexas.edu   
      
   19 November 2003   
      
   Scientists Develop Cheap Method for Solar System Hunt   
      
   Using McDonald Observatory Telescope   
      
   AUSTIN, Texas -- University of Texas at Austin astronomers have invented an   
   inexpensive method to determine if other solar systems like our own exist.   
      
   Among the more than 100 stars now known to have planets, astronomers have found   
   few systems similar to ours. It's unknown if this is because of technological   
   limitations or if our system is truly a rare configuration. The McDonald   
   Observatory astronomers' novel search method uses a Depression-era telescope   
   mated with today's technology.   
      
   Astronomers Don Winget and Edward Nather, graduate students Fergal Mullally and   
   Anjum Mukadem, and colleagues are looking for the "leftovers" of solar systems   
   like ours. Their method searches for the pieces of such a solar system after   
   its   
   star has died, by exploiting a trait of ancient, burnt-out Suns called "white   
   dwarfs."   
      
   University of Texas astronomers Bill Cochran and Ted von Hippel are also   
   involved, along with S.O. Kepler of Brazil's Universidade Federal de Rio Grande   
   dol Sul and Antonio Kanaan of Brazil's Universidade Federal de Santa Catarina.   
      
   Astronomers know that as Sun-like stars use up their nuclear fuel, their outer   
   layers will expand, and the star will become a "red giant" star. When this   
   happens to the Sun, in about five billion years, they expect it will swallow   
   Mercury and Venus, perhaps not quite reaching Earth. Then the Sun will blow off   
   its outer layers and will exist for a few thousand years as a beautiful, wispy   
   planetary nebula. The Sun's leftover core will then be a white dwarf, a dense,   
   dimming cinder about the size of Earth. And, most important, it likely will   
   still be orbited by the outer planets of our solar system.   
      
   Once a Sun-like system reaches this state, Winget's team may be able to find   
   it.   
   Their method is based on more than three decades of research on the variability   
   (that is, changes in brightness) of white dwarfs. In the early 1980s,   
   University   
   of Texas astronomers discovered that some white dwarfs vary, or "pulsate," in   
   regular bursts. More recently, Winget and colleagues discovered that about   
   one-third of these pulsating white dwarfs (PWDs) are more reliable timekeepers   
   than atomic clocks and most millisecond pulsars.   
      
   These pulsations are the key to detecting planets. Planets orbiting a stable   
   PWD   
   star will affect observations of its timekeeping, appearing to cause periodic   
   variations in the patterns of pulses coming from the star. That's because the   
   planet orbiting the PWD drags the star around as it moves. The change in   
   distance between the star and Earth will change the amount of time taken for   
   the   
   light from the pulsations to reach Earth. Because the pulses are very stable,   
   astronomers can calculate the difference between the observed and expected   
   arrival time of the pulses and deduce the presence and properties of the   
   planet.   
   (This method is similar to that used in the discoveries of the so-called   
   "pulsar   
   planets." The difference is, the pulsar companions are not thought to have   
   formed with their stars, but only after those stars had exploded in   
   supernovae.)   
      
   "This search will be sensitive to white dwarfs which were initially between one   
   and four times as massive as the Sun, and should be able to detect planets   
   within two to 20 AU from their parent star. This means we'll be probing inside   
   the habitable zone for some stars," Winget said. (An AU, or astronomical unit,   
   is the distance between Earth and the Sun.) "Basically, detecting Jupiter at   
   Jupiter's distance with this technique is easy. It's duck soup," he said.   
      
   Easy, but not quick. Outer planets, orbiting their stars at large distances,   
   can   
   take more than a decade to complete one orbit. Therefore, it can take many   
   years   
   of observations to definitively detect a planet orbiting a white dwarf.   
      
   "You need to look for a long time for a full orbit," Winget said. "A half-orbit   
   or a third of an orbit will tell us something's going on there. But for a   
   planet   
   at Jupiter's distance, a half-orbit is still six years." Winget added that for   
   this method, "detecting Jupiter at Uranus' distance is easier, but takes even   
   longer."   
      
   For the PWD planet search, Nather conceived a specialized new instrument for   
   McDonald Observatory's 2.1-meter Otto Struve Telescope. He and Mukadam designed   
   and built the instrument, called Argos, to measure the amount of light coming   
   from target stars. Specifically, Argos is a "CCD photometer" -- a photon   
   counter   
   that uses a charge-coupled device to record images. Located at the prime focus   
   of the Struve Telescope, Argos has no optics other than the telescope's   
   2.1-meter primary mirror. Copies of Argos are now being built at other   
   observatories around the world.   
      
   Mullally continues the search for planets around white dwarfs with Argos on the   
   Struve Telescope. He has 22 target stars, most of which were identified through   
   the Sloan Digital Sky Survey. When the team finds promising planet candidates   
   with Argos, they will follow up using the 9.2-meter Hobby-Eberly Telescope   
   (HET)   
   at McDonald Observatory.   
      
   "If we find large planets orbiting at large distances, that's a good clue that   
   there might be smaller planets closer in. In that case, what you do is pound   
   away on that target with the largest telescope you have access to," Winget   
   said.   
   The HET will enable more precise timing of the PWD's pulses, and thus be able   
   to   
   pinpoint smaller planets.   
      
   This search will be able to study types of stars unable to be studied with the   
   doppler spectroscopy method -- the most successful planet search method to date   
   -- Winget said. Because of idiosyncrasies in the make-up of Sun-like stars, the   
   doppler spectroscopy method is not very sensitive in looking for planets around   
   stars twice as massive as the Sun. Roughly half of the stars in Winget's study   
   will be white dwarfs that were originally these types of stars. For this   
   reason,   
   the PWD study at McDonald can be instrumental in scouting and assessing targets   
   and observing strategies for NASA space missions planned in the next two   
   decades, specifically the Space Interferometry Mission, Terrestrial Planet   
   Finder and Kepler spacecraft.   
      
   This research is funded by a NASA Origins grant, as well as an Advanced   
   Research   
   Project grant from the State of Texas. Through funding from the Texas Higher   
      
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