Racoon Run

Exoplanet Transit Detection by Amateur Astronomers 

It was only in 1995 that the first confirmed detection was made of a planet orbiting a star other than our own Sun.  Without knowing if there were planets around other stars scientists could not know if our solar system, and hence Earth, were unique in the universe.  If our solar system were unique it would be difficult to imagine life as we know it existing anywhere else in the universe.

One method of finding exoplanets, which are planets orbiting other stars, is to detect the very small dimming of the light from a star as an exoplanet crosses, or transits, in front of it. 

Amateur astronomers around the world are now routinely detecting exoplanets using this transit method.  Using commonly available telescopes, digital cameras and webcams, many amateurs are assisting professional astronomers in the search for new exoplanets. 

In 2001, I began working with Dr. Tim Castellano, a NASA astronomer, and Dr. Greg Laughlin, Professor of Astronomy at the University of California, Santa Cruz to help demonstrate that amateur astronomers could in fact assist in the search for new worlds.  Using a telescope with a 12” aperture in my back yard, I detected the signature dip in the light curve of the known transiting exoplanet HD209458b:

I was even able to measure the transit using a much smaller telescope with only a 4” aperture:

Exoplanet TrES-1b

In 2004, the discovery of another transiting exoplanet named TrES-1b was made (see Alonso et al, ApJ. Letters , 613 , L153).  Following the announcement, the transit of TrES-1b was observed by amateur astronomers worldwide. 

On September 25 and October 1, 2004, I and another observer located in Elgin, Oregon, Joe Garlitz, observed the same TrES-1b transits at the same time.  We were hundreds of miles apart, and I used a 14” SCT while Joe used an 8” reflector.  The following plots were made of our light curves immediately before and after the time of mid-transit when the exoplanet crossed the center of the parent star:

The two plots suggest that the TrES-1b exoplanet may have been passing over darker (cooler) areas of the parent star during the transit.  Like sunspots on our own Sun, starspots are found on many other stars as well.  These observations by amateur astronomers may indicate that the parent star of TrES-1b also has starspots.  If so, these observations may also be the first detection of starspots by amateur astronomers using exoplanet transits.

In the months after the discovery of TrES-1b the Hubble Space Telescope observed several transits to further characterize the exoplanet and its parent star.  The HST light curve, far more precise than could be made by any Earth-based telescope looking through our atmosphere, showed similar bumps indicative of starspots.  While made during different transits, superimposing the data above from two amateur astronomers onto the HST light curve further suggests the presence of starspots.  

Modeling of Starspots 

Amateur astronomers now have access to sophisticated models of transiting exoplanets that until recently had been in the sole province of professional astronomers.  A software package called Binary Maker 3, written by David Bradstreet of Eastern University (http://www.binarymaker.com) was developed to model light curves of eclipsing binary stars but can also be used to model exoplanet transit light curves. 

Assuming that the irregularities in the TrES-1b light curve are caused by one or more starspots the input variables of Binary Maker 3 (temperatures, relative masses, inclination, limb darkening) can be adjusted to yield a light curve similar to the ones observed by the amateur astronomers and the Hubble Space Telescope.  Using a stellar temperature of 6,000 degrees K, a planetary temperature of 200 degrees K (results not that sensitive to planetary temperature), and a starspot having a 5.2 degree circular radius located 9 degrees away from the stellar equator, we can obtain the model light curve below for the TrES-1b system:

My Smallest Transit 

On June 29, July 2 and July 5, 2005 I detected my smallest transit.  The exoplanet HD 149026b was announced on June 30th .  While the transits of HD 209458b caused its parent star to dim by 0.89% and TrES-1b’s parent star by 1.72%, the dimming caused by the transit of HD 149026b was only 0.51%, equal to a dip of only 0.003 magnitude.

Such a shallow transit depth of only 0.003 magnitude would be exhibited by an exoplanet about 3-1/2 times the size of Earth revolving around a small red star, so it is theoretically possible for amateur astronomers to find exoplanets perhaps only twice the size of Earth in orbit around small red stars.  

Future Exoplanet Research 

Beyond assisting in the discovery of exoplanets, amateur astronomers can provide additional scientific data to professional astronomers through the continued monitoring of known transiting exoplanets.

A great resource for exoplanet observing can be found at www.transitsearch.org which lists the times of all known exoplanet transits.  An observer can also input their location and see a list of candidates that are suspected of having transiting exoplanets.