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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. In this news note, we describe how amateur astronomer
Ronald Bissinger
of Pleasanton, CA, detected the eclipse of HD 149026 during 3 nights
of observations beginning June 29, 2005. He used
Mira AP software to make
the critical photometric measurements that show the predicted 0.003
magnitude dimming during eclipse..
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 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. We would add just a small footnote: Now they can, using
Mira. |
