1. Detecting Exoplanets and Exomoons
With the Lutz 0.5-Meter Telescope
By Kimberly Ward-Duong
Mentor: Dr. Stephen C. Tegler
NAU Dept. of Physics and Astronomy
Arizona/NASA Space Grant Research Symposium
Tucson, Arizona
April 17, 2010
Hello, my name is Kimberly Ward-Duong, and my presentation is entitled Detecting Exoplanets and Exomoons with the Lutz 0.5 meter telescope.
First off I’m going to talk about what an exoplanet is, what our projects goals are, how we gathered and reduced our data from the telescope, and the implications of our results.

2. Exoplanets and Their Transits
Extrasolar Planets
443 systems
71 known to transit
Transit/lightcurve relation Tc
Our project:
1. Observe stars with known exoplanet transits
2. Analyze transit lightcurves
“First off, an exoplanet is simply defined as a planet that orbits a star other than our sun, outside of the solar system. From radial velocity and spectroscopic work, we know of at least 443 other stars that have planets orbiting them. 71 of these are known to transit, which simply means they pass in front of their star in our line of sight. Measuring these transits generates what’s known as a lightcurve
First as seen here on the left, we would never see the brightness of the star decrease.
By measuring the decrease in light that occurs when a planet transits in front of its star, we can generate what is known as a lightcurve, seen here. On the y axis we have brightness of the star, plotted over time on the x-axis. For each transit, we see a characteristic dip in light as the planet covers more and more of the star as it moves across. Then as it finishes transiting, the star returns to normal brightness. Detecting such a decrease in light from such a far way away (usually 600 some lightyears) means that the planet is easiest to detect when it’s very large – so most transiters are roughly the size of Jupiter, and VERY close to their stars, so very hot.
From analysis of these lightcurves, we can determine various things about the planet, such as the radio of the planet radius to the star radius, the inclination of the orbit, the radio of size between the orbit and the star’s radius, the orbital period, that is, time it takes to go all the way around the star, and most importantly, the central transit time, where the planet is right in the middle of the front of the star, as seen here. For each full path the planet makes around its star, it should take the same amount of time to come back around, and so the central transit time should be very predictable. But sometimes it isn’t… and that’s where our project comes in.
Image credit:

3. Transit Timing Variation (TTV)
Periodic changes in Tc
New technique to detect multiple planet systems
Ideal for the detection of other Earths, super-Earths
None yet discovered by these means!
The goal of our project is to detect signs of changes in the central transit time, known as transit timing variation, or TTV. This requires the observation of stars with known planet transits, and the analysis of the lightcurves from those transits. This analysis will help us refine the parameters discussed on the previous slide, orbital period, etc, as well as allow us to look for changes in the central transit time.
How this works is if the planet is being tugged on, or perturbed, by another planet or body in the system, then the orbital period will change due to those gravitational effects. Therefore, TTV is a technique to detect multiple planet systems, and actually proves ideal for the detection of other Earth-like planets or super-Earths, which are smaller than Jupiter, but larger than our own planet. However, there are very few known planetary systems with multiple planets, and no one has yet discovered one by these means – so we’re in a prime location to look for these signatures over many observations.

4. Data Collection Atmospheric Research Observatory at NAU
Barry Lutz Telescope
0.5-m (20-in) Ritchey-Chrétien telescope
Apogee CCD Camera
images
5-7 hours of data per night
Now we move to how we actually collect our data.
We’re lucky to have an on-campus facility, built in 1952 and known as the Atmospheric Research Observatory. It houses a new half-meter RC telescope, known as the Barry Lutz Telescope, equipped with an apogee CCD digital imaging camera for science data, seen here.
For each predicted transit, we head down to the observatory and take some images of the star over the course of the predicted transit, generally with exposure times of seconds, depending on the target and atmospheric seeing. Since most transit last about 3 hours, and we want to get an hour before and after to see the change in light from the norm, we can spend anywhere from 5-7 hours on a given night taking data.
Images courtesy of Dr. Stephen Tegler

5. Data Reduction Process
Creation of detailed analysis procedure
IRAF
Differential photometry
Plot and model lightcurves using MATLAB
Once I started getting data from the telescope, then it was time to reduce it. I would say aside from observing, 95% of my work involved the creation of a detailed computational analysis procedure – I won’t bore you with the nitty gritty details.
In short, to reduce the data, I used IRAF software (Image Reduction and Analysis Facility) to measure the brightness of the target star with the known planet transit. This image is a sample from an observation of the starWASP-11/HAT-P-10, seen here, which has a known transiting planet. I could then compare it to other stars in field, namely these ones, whose brightnesses would be stable over time, and calculate the difference to obtain a lightcurve. This process is known as differential photometry. From there, I could plot the lightcurves in MATLAB, and with the help of my mentor, fit the lightcurve to the predominant theoretical model.
WASP-11/HAT-P-10

6. Results
WASP-11/HAT-P-10
Quadratic limb darkening model fit to data (Mandel & Agol 2002)
Parameters from discovery paper (West et al. 2009)
Free parameter: Tc
Standard deviation: 3 mmag
So here are our results for the transit on the object you just saw – there were about 160 images total, corresponding to each of these data points. You can see the characteristic dip in light, plotted on the y axis, over time on the x-axis. We found that the depth of this object corresponded almost exactly to the published literature on this object: 25 hundreths of a magnitude. The lower plot gives the std dev in the measurements, which is roughly 3 mmags uncertainty. To determine the central transit time, we needed to fit our curve to the industry standard theoretical model proposed by Mandel & Agol in This model takes into account limb darkening, which is the darkening of a star as you go out from the center – usually due to change in density and temperature at the outside of the star.
The model involves 5 parameters, which we spoke of before – the constrained parameters are Rp, I, a/R, and orbital period, the values of which we took from the discovery paper of this object, West et al The model leaves central transit time as a free parameter, so we can determine its value from the best fit of the model.

7. Discussion/Conclusion
Observed transit timing variation?
Yes! Best fit model Tc early by 5 minutes
1.8 sec period difference
Possible explanations
More observations needed to verify/explain findings
Long term project plans
Goal: Observe transits/object
Further refine modeling specifications
Technique may also prove useful for finding an exomoon
So the big question is, did we observe ttv?
And the answer is yes! Our best-fit model gave a central transit time that was five minutes earlier than the predicted time. This correlates to the orbital period of the planet being about 1.8 seconds different for our data set than from the published literature. Since there have been 170 wasp-11 “years” since its discovery, this ends up being about 5 minutes difference.
So why are we seeing this?
The most exciting answer would be that it’s evidence of another planet tugging and affecting the orbit of our planet. That may be a bit unlikely, especially for one particular transit, so we think it may actually be that the published period is not quite correct, and needs to be refined with further measurements.
One other exciting possibility is that of orbital decay – WASP-11 is a gas giant, and very close to its parent star – almost 10x closer to its star than mercury is to our sun. Thus, there may be extreme tidal forces acting upon the planet, causing its orbital radius to decrease, and therefore its period to shorten. But to see if that’s the case will also take many more observations.
Image credit:

8. Thank you. And many thanks to: Dr. Stephen Tegler Dr. Nadine Barlow Ms
Thank you! And many thanks to: Dr. Stephen Tegler Dr. Nadine Barlow Ms. Kathleen Stigmon Arron Shiffer David Tollefsen Alex McCanna NASA and the Arizona Space Grant Consortium Questions?
Artist’s impression of exoplanet HD b
(Giovanni Tinetti)
(Title slide image credit: STScI artist rendition)