High-School Students Capture Exoplanet Transit with Ken Hose

I love working on astronomy projects with students. On October 10 we had a clear night and I had 2 high school students over to capture an exoplanet transit. There were a few possible known transits that night and we chose to image Kepler-17 b which was discovered in 2011 by the Kepler Mission. The students were doing this as a school project. I have an observatory with a 12.5” telescope and a CCD camera that the students used.

The aim was to measure the small change in brightness as the exoplanet transited in front of its host star. The idea was to take many back-to-back exposures with a CCD camera before, during, and after the predicted transit. We used a special photometry software program to extract the change in brightness from one exposure to the next. The students were able to plot the data in Excel and generate a U-shaped light curve representing the change in brightness over time.

The transit light curve can be used to estimate some of the orbital parameters of the exoplanet. One can estimate planet radius, semi-major axis (orbital radius for a circular orbit) and orbital period, among other things. There are 4 light curve measurements required for the estimates: the transit duration (minutes), the transit depth (in magnitudes), the bottom part of the U-shaped transit (minutes), and the period (days). Of course the period requires more than one transit observation, which I happened to have.

The calculations of the orbital parameters compared reasonably well to published values. The calculations gave a value of planet radius of 1.53 times the radius of Jupiter (vs. 1.31RJ published), and an orbital radius of 0.025 AU (vs. 0.0259 AU published). So this is a very large planet orbiting very close to its host star.

The host star for Kepler-17b has a spectral type of G2V which is the same as our Sun. This means that the host star will have about the same size and luminosity as our Sun. This makes it easy to calculate the relative amount of solar flux reaching the exoplanet. Since the exoplanet is orbiting at 0.025 AU we can use the inverse square law to calculate that the planet will receive about 1,600 times more solar radiation that we receive on Earth. It must be a very hot place!

Most people are surprised that it is possible to detect exoplanets with amateur equipment. But it is possible for amateurs (and students) to do real science. I think it is amazing that we were able to estimate physical parameters for a planet like Kepler-17b which is about 2600 light years away.

Ken Hose