Download preprint accepted by Astrophysical Journal Letters, 23 November, 1999 (310Kb PS file or 162Kb PDF file).

Download ASCII file of HD 209458 photometric data.

Planetary Transit Across Star HD 209458 Detected by STARE Project Astronomers

At the end of August, 1999, STARE Project Astronomers Dr. Tim Brown and David Charbonneau started observing the star HD 209458 on the advisement of Dr. David Latham of the Harvard-Smithsonian Center for Astrophysics. Latham inferred from his radial-velocity analysis of the motion of the star (refer to Exoplanet Search Methods for more information) that a planetary companion was present and possibly orbiting in an edge-on orientation. When an orbit is aligned this way, the planet will pass between its star and the Earth (transit) once each orbit, causing a slight dimming of the star's light as in the figure below (see STARE Search Method for a more detailed explanation). After analyzing data collected by the STARE telescope, such a dimming was discovered occurring on the nights of September 9th and 16th. The times of these transits coincided exactly with the expected times predicted by the radial-velocity analysis. A partial transit in November was observed by G. Henry (Tennessee State University). Further details on this observation are available at the TSU Automated Astronomy Group website.



Figure based on one by Hans Deeg, from 'Transits of Extrasolar Planets'

When the properties of the parent star are known (such as radius, color, and limb darkening), subsequent analysis of the transit light curve (below) can be used to determine many properties of the planet not discernible by radial-velocity measurements alone. In the case of HD 203458, the radius of the planet is estimated at 1.27 times that of Jupiter (or around 91,000 km). This measurement, coupled with the mass determined by both the radial velocity and transit parameters (approximately 63% that of Jupiter, or 1.2 x 10^27 kg), allows for the calculation of surface gravity (at 9.7 m/s^2, just slightly lower than Earth's) and density (approximately 0.38 g/cm^3, so low that if there was an ocean large enough, the planet would float!)


Superposed lightcurves of star HD 209458 showing transits occurring on 9 and 16 September, 1999

This first planetary transit discovery is very exciting in that it gives astronomers a wealth of new information about the properties and formation of other solar systems (such as the make-up of extra-solar planetary atmospheres). Please revisit the STARE web page in the future for updates on this and any other transit discoveries.