From: baalke@zagami.jpl.nasa.gov   
      
   Media Relations   
   University of California-Berkeley   
      
   Contacts:   
   Debra Fischer   
   Via IAU GA Media Room (details below), 21-22 July only   
   fischer@astro.berkeley.edu   
      
   Jeff Valenti   
   valenti@stsci.edu   
   STScI: +1-410-338-2622   
      
   Helen Sim   
   Media Liaison   
   IAU General Assembly   
   iaumedia@netscape.net   
   Telephone: +61-419-635-905   
      
   Robert Sanders   
   PIO   
   rls@pa.urel.berkeley.edu   
   Telephone: +1-510-643-6998   
      
   21 July 2003   
      
   Stars rich in heavy metals tend to harbor planets, astronomers report   
   By Robert Sanders, Media Relations   
      
   Sydney, Australia -- A comparison of 754 nearby stars like our sun -- some with   
   planets and some without -- shows definitively that the more iron and other   
   metals there are in a star, the greater the chance it has a companion planet.   
      
   "Astronomers have been saying that only 5 percent of stars have planets, but   
   that's not a very precise assessment," said Debra Fischer, a research   
   astronomer   
   at the University of California, Berkeley. "We now know that stars which are   
   abundant in heavy metals are five times more likely to harbor orbiting planets   
   than are stars deficient in metals. If you look at the metal-rich stars, 20   
   percent have planets. That's stunning."   
      
   "The metals are the seeds from which planets form," added colleague Jeff   
   Valenti, an assistant astronomer at the Space Telescope Science Institute   
   (STScI) in Baltimore, Md.   
      
   Fischer will present details of the analysis by her and Valenti at 1:30 p.m.   
   Australian Eastern Standard Time (AEST) on Monday, July 21, at the   
   International   
   Astronomical Union meeting in Sydney, Australia.   
      
   Iron and other elements heavier than helium -- what astronomers lump together   
   as   
   "metals" -- are created by fusion reactions inside stars and sown into the   
   interstellar medium by spectacular supernova explosions. Thus, while metals   
   were   
   extremely rare in the early history of the Milky Way galaxy, over time, each   
   successive generation of stars became richer in these elements, increasing the   
   chances of forming a planet.   
      
   "Stars forming today are much more likely to have planets than early   
   generations   
   of stars," Valenti said. "It's a planetary baby boom."   
      
   As the number of extrasolar planets has grown -- about 100 stars are now known   
   to have planets -- astronomers have noticed that stars rich in metals are more   
   likely to harbor planets. A correlation between a star's "metalicity" -- a   
   measure of iron abundance in a star's outer layer that is indicative of the   
   abundance of many other elements, from nickel to silicon -- had been suggested   
   previously by astronomers Guillermo Gonzalez and Nuno Santos based on surveys   
   of   
   a few dozen planet-bearing stars.   
      
   The new survey of metal abundances by Fischer and Valenti is the first to cover   
   a statistically large sample of 61 stars with planets and 693 stars without   
   planets. Their analysis provides the numbers that prove a correlation between   
   metal abundance and planet formation.   
      
   "People have looked already in fair detail at most of the stars with known   
   planets, but they have basically ignored the hundreds of stars that don't seem   
   to have planets. These under-appreciated stars provide the context for   
   understanding why planets form," said Valenti, who is an expert at determining   
   the chemical composition of stars.   
      
   The data show that stars like the sun, whose metal content is considered   
   typical   
   of stars in our neighborhood, have a 5 to 10 percent chance of having planets.   
   Stars with three times more metal than the sun have a 20 percent chance of   
   harboring planets, while those with 1/3 the metal content of the sun have about   
   a 3 percent chance of having planets. The 29 most metal-poor stars in the   
   sample, all with less than 1/3 the sun's metal abundance, had no planets.   
      
   "These data suggest that there is a threshold metalicity, and thus not all   
   stars   
   in our galaxy have the same chance of forming planetary systems," Fischer said.   
   "Whether a star has planetary companions or not is a condition of its birth.   
   Those with a larger initial allotment of metals have an advantage over those   
   without, a trend we're now able to see clearly with this new data."   
      
   The two astronomers determined metal composition by analyzing 1,600 spectra   
   from   
   more than 1,000 stars before narrowing the analysis to 754 stars that had been   
   observed long enough to rule a gas giant planet in or out. Some of these stars   
   have been observed for 15 years by Fischer, Geoffrey Marcy, professor of   
   astronomy at UC Berkeley, and colleague Paul Butler, now at the Carnegie   
   Institution of Washington, in their systematic search for extrasolar planets   
   around nearby stars. All 754 stars were surveyed for more than two years,   
   enough   
   time to determine whether a close-in, Jupiter-size planet is present or not.   
      
   Though the surfaces of stars contain many metals, the astronomers focused on   
   five -- iron, nickel, titanium, silicon and sodium. After four years of   
   analysis, the astronomers were able to group the stars by metal composition and   
   determine the likelihood that stars of a certain composition have planets. With   
   iron, for example, the stars were ranked relative to the iron content of the   
   sun, which is 0.0032%.   
      
   "This is the most unbiased survey of its kind," Fischer emphasized. "It is   
   unique because all of the metal abundances were determined with the same   
   technique and we analyzed all of the stars on our project with more than two   
   years of data."   
      
   Fischer said the new data suggest why metal-rich stars are likely to develop   
   planetary systems as they form. The data are consistent with the hypothesis   
   that   
   heavier elements stick together easier, allowing dust, rocks and eventually   
   planetary cores to form around newly ignited stars. Since the young star and   
   the   
   surrounding disk of dust and gas would have the same composition, the metal   
   composition observed from the star reflects the abundance of raw materials,   
   including heavy metals, available in the disk to build planets. The data   
   indicate a nearly linear relationship between amount of metals and the chance   
   of   
   harboring planets.   
      
   "These results tell us why some of the stars in our Milky Way galaxy have   
   planets while others do not," said Marcy. "The heavy metals must clump together   
   to form rocks which themselves clump into the solid cores of planets."   
      
   The research by Fischer and Valenti is supported by the National Aeronautics   
   and   
   Space Administration, the National Science Foundation, the Particle Physics and   
   Astronomy Research Council (PPARC) in the United Kingdom, the Anglo-Australian   
   Observatory, Sun Microsystems, the Keck Observatory and the University of   
   California's Lick Observatories.   
      
   Images   
      
      
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