In this news note, we describe how Ronald Bissinger of Pleasanton, CA, detected the eclipse of HD 149026 during 3 nights of observations beginning June 29, 2005. He used Mira to make the critical photometric measurements that show the predicted 0.003 magnitude dimming during eclipse.
On June 30, 2005 a team of astronomers led by Dr. Debra Fischer (San Francisco State University) announced the discovery of a planet orbiting the Sun-like star HD 149026. This discovery was made using the radial velocity method, which measures subtle wobbles in the star's velocity along the line of sight caused by the gravitational influence of its orbiting planet. Based on these measurements, the new "exoplanet", known as HD 149026b, was estimated to have a mass similar to that of Saturn and a diameter about 6% that of the star [research paper]. But there is another way to detect exoplanets — one that does not require a large telescope equipped with a high resolution spectrograph: "Transit Search". The largest, most detectable changes in radial velocity occur for those exoplanets whose orbital plane is inclined to match our line of sight, thus producing an eclipse as the planet transits the star once per orbit. Exosolar planets are too far away to be seen directly, but they may be detected by observing the dimming of starlight during the eclipse. The transit search method can be done using a modest size telescope but requires extremely precise measurements of the star's brightness.
Detecting exosolar planets is an extremely difficult pursuit. This method requires luck of timing to catch an eclipse, the acquisition of high quality image data, and the application of sophisticated techniques for image processing and measurement. In the case of HD 149026, the eclipse blocks approximately 1/300th the star's surface, dimming its light by only 0.003 magnitude, or 0.3%. An eclipse of this degree cannot be detected unless the photometric measurements have a consistency approaching the milli-mag level over a span of many images (0.001 magnitude variation means that the brightness variation is about 0.1% of the signal). Making such precise measurements over many images pushes both the technology and experimental technique to the limit. Exosolar planet surveys have been the dominion of a handful of professional astronomers using relatively large telescopes... until now. Using our Mira AP software, Mr. Bissinger has pushed back the observational frontier and has shown that the field of exoplanet discovery is now open to others using comparatively modest equipment along with Mira for processing and measuring the data. Detecting a predicted eclipse differs from discovering one only in terms of knowing what to observe at what time. If such small fluctuations can be measured, then they can be discovered using appropriate software techniques.
Shortly after the discovery of HD 149026b, Mr. Bissinger was asked by astronomer Dr.' Gregory Laughlin (University of California at Santa Cruz and TransitSearch.org), if he could observe the upcoming transit to assess whether it was possible for an amateur astronomer to detect such marginal events. Accepting the challenge, he acquired some 100 images on the evenings of June 29th, July 2nd, and July 5th, 2005 using his 14 inch Celestron telescope and an SBIG CCD camera. He employed Mira AP for calibration of instrumental effects such as bias, dark, flat field, and cosmetic defects. Then he used Mira AP to make photometric measurements of HD 149026 and comparison stars in the same field of view. His results in the graph below show the eclipse very clearly. Were the photometry not at this level of precision and consistency from image to image, the subtle 0.003 magnitude dip would be lost in the noise. The red curve shows the prediction from a theoretical model of a planet eclipsing 1/300th the area of the star.
The Importance of this Work
The remarkable aspect of these observations is not just detection of the eclipse, but that they were made from Mr. Bissinger's backyard observatory under the bright skies of suburban San Jose, California. Although a few amateur astronomers have previously observed larger exoplanets having deeper eclipses of the order of 0.02 magnitude, Mr. Bissinger has moved the transit search field into a new realm. By observing an eclipse depth of only 0.003 magnitude, he has demonstrated the potential for others to detect the more numerous smaller planets using commercially available software and instrumentation. From a scientific perspective, pushing down the threshold size of detectable exoplanets can provide dramatically better statistics about the frequency and nature of planetary systems around stars other than the Sun.
Says Mr. Bissinger,
"While the usual dark frames and flat fields were used to increase the precision of the photometry done using Mira AP, it was the software’s excellent background correction and the ability to use elliptical apertures that pushed the precision to high levels. During the 40 second exposures, small tracking errors produced slightly elongated star images. Matching the elliptical apertures to the star profiles gave enough extra increase in the signal to noise ratio to make it possible to get measurements of this quality."
Astronomer and director of the AAVSO, Dr. Arne Hendon, has described the precision of Ron's results as being something many professional astronomers dream of but never reach.