In the last several years, more than 30 planets have been discovered orbiting other stars. All discoveries to date have been by the radial velocity method whereby extremely small variations in the speed of the star relative to Earth are used to infer the presence of an unseen orbiting companion. More than 20% of the planets discovered orbit their parent stars for periods of less than a week. For these short-period orbits, 10% will be oriented such that the planet will periodically pass in front of the star as seen from Earth. An alternate method of detecting extrasolar planets employing high-precision measurements of the brightness of the stars can confirm the existence of the planet and obtain its mass and radius. This technique was convincingly demonstrated when the first-ever measurement of the dimming of a star (HD 209458) because of the passage of an orbiting planet occurred in late 1999. This "extrasolar planetary transit" was discovered independently by two groups and widely reported in the news media.
Soon after the announcement, Ames conducted an archival search of the brightness data of the star HD 209458. The data, collected by the European Hipparcos satellite between 1989 and 1992, revealed a photometric dimming consistent with the observed radial-velocity measurements and ground-based transit observations. The long baseline in time between the Hipparcos measurements and the present allowed a precise determination of the orbital period of the planet. These results will be published in the Astrophysical Journal Letters. The successful confirmation of an extrasolar planetary transit in the Hipparcos data suggests that it may be possible to discover more extrasolar planets around sun-like stars using data from the Hipparcos satellite or NASA's planned Full Sky Astrometric Explorer (FAME) satellite.
A novel method for obtaining high-precision photometric measurements of bright stars using a spot filter and charge coupled device detectors on ground-based telescopes has been developed. A demonstration of the technique was performed on the sun-like star HD 187123 in the fall of 1999. Athough no transit of an extrasolar planet was seen, the required precision was achieved, as shown in figure 1. Additional observations were made of the stars bearing extrasolar planets HD 217107, 51 Pegasi, Upsilon Andromedae, and Tau Bootes, without result.
Point of Contact: T. Castellano
(650) 604-4716
tcastellano@mail.arc.nasa.gov
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