PRECISION PHOTOMETRY TECHNIQUE TO AID SEARCH FOR EXTRASOLAR PLANETS
CAMBRIDGE, MA--As astronomers continue to findevidence for planets outside our solar system,scientists from the Harvard-Smithsonian Center forAstrophysics (CfA), Mount Wilson Observatory, andTennessee State University have discovered that thosestars suspected of having planets may also sharecertain traits. With this knowledge, observers shouldbe able to hone their searches and increase theirchances of finding other planets.
Using a robotic instrument, known as an automaticphotoelectric telescope (APT), at the Fred WhippleObservatory near Amado, AZ, and the 60-inch (1.52-m)telescope at the Mt. Wilson Observatory near LosAngeles, the team of scientists was able to measureto new levels of precision both the surface magnetismand brightness variations of the seven starsidentified to have orbiting companions. This is acrucial step toward detecting other possible planetarysystems.
Planets around distant stars are too small and toofaint to see directly, as the light from the starsimply overwhelms any planetary glow, so scientistsmust look for other evidence of their presence. Oneapproach depends on the gravitational tug-of-warbetween a star and its companion which creates ameasurable wobble in the star's velocity. Suchvelocity measurements, however, can only find planetsthe size of Jupiter or larger.
According to Sallie Baliunas of the CfA who led theteam of researchers, the APT's measurements of tinyvariations of a star's brightness can revealEarth-sized objects orbiting that star. This ispossible by detecting a minute dimming in the lightfrom a star as an Earth-sized planet crosses in frontof it. While the seven suspected planetary systemsBaliunas and her colleagues examined did not revealany Earth-sized planets, the scientists remain hopefultheir technique will reveal such objects. The taskis not easy, since to detect a planet using thismethod, a planet would have to cross the face of astar almost exactly along our line of sight, anorientation that occurs only a fraction of the time.
Fortunately, Baliunas and her colleagues found thatthose stars harboring planets share the trait ofbeing "magnetically quiet," that is, they have littlesunspot or other magnetic activity. In addition, theresearchers determined that six of the seven starswith low-mass orbiting companions were about 5 billionyears old, or about the same age as our Sun. Thissuggests astronomers can focus their investigations onthe middle-aged, magnetically quiet stars, thus makingsearches for planetary systems more efficient.
"This work not only helps support the suggestion ofplanets around Sun-like stars, but it tells ussomething about the properties of these stars," saidBaliunas. "That gives better targets for the projectsthat are attempting to pick up radio signals fromextra-terrestrial intelligence, since the older age ofthese stars suggests any life on surrounding planetsmight have had enough time to develop communicationstechnology."
The technique's unprecedented levels of photometricprecision may have already helped defuse a possiblecontroversy about the best known of the extra-solarplanets objects--the Jupiter-sized body thought to beorbiting the star 51 Pegasi. Discovered in late 1994,the object has served as a model for finding low-massstellar companions using radial velocity observations.However, at least one critic has claimed thatvariations in the optical signal from 51 Pegasi mightreally be due to natural oscillations in the staritself. To test this theory, the research teamobserved the star with its high-precision photometrytechnique and found no evidence for cyclic brightnessvariations that should result from such oscillations.
The research team includes Baliunas, Robert Donahue,Steven Saar, and Willie Soon of the CfA, and GregHenry and Frank Fekel of the Center for Excellence inInformation Systems at Tennessee State University, anHBCU (Historically Black College or University).
This work was supported by NASA, the National ScienceFoundation, and the Richard C. Lounsberg Foundation,and was reported in the January 1, 1997 issue of TheAstrophysical Journal and the January 15, 1997 issueof The Astrophysical Journal Letters.
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