Racoon Run

The Discovery of Exoplanet XO-1b

Under the direction of Dr. Peter McCullough of the Space Telescope Science Institute in Baltimore, MD, the XO Project (http://www-int.stsci.edu/~pmcc/xo/about/) employs two fully automated cameras on the Haleakala summit on Maui to search for transiting exoplanets.  Each camera uses a commercially available Canon telephoto lens (200mm focal length, f/1.8) and a CCD camera.  Aperture photometry was used to measure the light from over 100,000 stars to a precision of better than 1%.  Analyses of the resulting light curves yielded numerous candidates for further observation.

The small telephoto lenses on Maui provided relatively low resolution light curves of stars that exhibited periodic dimming.  Higher precision measurements of the light curves were then needed to rule out other causes of dimming such as eclipsing binary stars.  A small team of amateur astronomers was recruited by Dr. McCullough to do more precise photometry using backyard telescopes.  Tonny Vanmunster in Belgium, Bruce Gary in Hereford, AZ, P.J. Howell (Boston University) and myself began screening candidates in the spring of 2005.  Out of over 100,000 stars monitored by the small telescopes on Maui I observed over a dozen promising candidates.  

Uncovering XO-1b

Most of the candidates, not surprisingly, turned out to be eclipsing binary stars as was evident from the distinctive shape of their light curves.  See the chart below for an example of the flat bottomed, shallow light curve from an exoplanet transit compared to a deeper, pointed bottomed light curve from an eclipsing binary star system.

Over several weeks one by one I worked through the candidate list and reported negative findings back to the team.  But as my computer displayed a light curve on the morning of June 23, 2005 after my telescope and CCD camera took 337 1-minute images of one particular candidate throughout the night before, there was little doubt in my mind that a new exoplanet had been discovered.  The shape of the light curve was characteristic of a transiting exoplanet and did not look at all like one of an eclipsing binary star system.  But further confirmation from other observers was necessary.

I sent an email to the team with the light curve and in a matter of days other team members were indeed able to obtain the same light curve.  Using the team’s light curves, Dr. McCullough was then able to obtain spectroscopy on the star using the large telescopes at the University of Texas’s McDonald Observatory that confirmed the star was wobbling as it was tugged by a large orbiting exoplanet.  Dr. McCullough and his team had found their first exoplanet!

While spectroscopy is the “gold standard” for exoplanet detection, it can only give an indication of an exoplanet’s mass.  The depth and duration from a transit light curve can provide the size of the exoplanet as well, giving a complete picture of the system.  Furthermore, exoplanets like XO-1b, HD209458b and TrES-1b that transit fairly bright stars provide large ground-based observatories and orbiting space telescopes like the Hubble and Spitzer an opportunity to analyze the parent star’s light shining through the exoplanet’s atmosphere.  Spectrographic analysis of that starlight can reveal the chemical composition of the exoplanet’s atmosphere.

The STScI press release can be found at http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/22/full/

The paper has been accepted for publication in the Astrophysical Journal and can be found at http://arxiv.org/PS_cache/astro-ph/pdf/0605/0605414.pdf