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Detection of extrasolar
planets: transit method
This exercise has been originally prepared by Roger Ferlet (IAP) and Olivier Marco with three
undergraduate students: Ester Aranzana Martinez, Sandra Greiss and Jeehae Chun (University Pierre &
Marie Curie). It has been tested in high school by Michel and Suzanne Faye, and subsequently
improved by Stefano Bertone, Gilles Chagnon and Anne-Laure Melchior.
An extrasolar planet, or exoplanet, is a planet outside the Solar System orbiting a star. Until 22nd March
2012, astronomers have detected1 763 exoplanets. The vast majority of them have been observed
through radial velocity observations and other indirect methods, as actual instruments do not have the
sensibility and resolution to image them, and they are giant planets orbiting very close to their star.
Here, we propose to unveil a planet using another method: a transit of the planet in front of its star.
How to detect an exoplanet (extrasolar planet) with the transit method?
Complementary to the radial velocity method, the
principle of the detection of extrasolar planets with the
transit method is based on the luminosity variation of
the star when the planet passes in front of it. Moreover,
the radius of the planet can be determined with this
method; hence it helps its classification.
Light curve of an exoplanet transit observed by
the Corot telescope (May 2007)
Hunt for exoplanet HD 189733b
The data are a time sequence of 20 images of a star HD 189733 obtained by the space telescope
Spitzer. We will perform measurements from these images with the SalsaJ software and analyse them
with Excel.
The exoplanet orbiting the star HD 189733 was discovered on October 5, 2005 by transit method in
France. The planet is classified as a hot Jupiter class Jovian planet, orbiting very close to its star with a
short period of 2.2 days. It is approximately 63 light-years away in the constellation of Vulpecula (the
Fox). It is centred on the Northern constellation of Cygnus. The physical characteristics of this planet
are defined by:
mass: 1.13 MJ
radius: 1.138 RJ
Surface gravity: 21.2 m/s²
Temperature: 1117K
Goals and objectives of this exercise:
The data are a temporal series of 20 images exhibiting 3 stars, taken by Spitzer telescope. One has to
perform photometry measurements of each of these 3 stars, for all the 20 images. The flux of 2 stars
1
Check the Web site http://exoplanet.eu/ for the last number of detected exoplanets.
This project has been funded with support from the European Commission. This publication [communication] reflects the views only of the
author, and the Commission cannot be held responsible for any use which may be made of the information contained therein.
will stay constant (within the noise level) as a function of time, while the flux of the 3rd star will drop,
because of the planet transit. Plotting the flux versus time, one should highlight the variation and thus
discover the exoplanet!
Do it yourself!
1. Open SalsaJ.
2. There are 20 images taken with the NASA satellite
Spitzer enabling to study the transit of an exoplanet
named HD 189733b*. To open them and tile them up,
use the macro in Pluging/macro/Extrasolar Planet
transit.
3. Identify the 3 main stars present on these images and
named 1, 2, 3 as indicated on this image.
Star 1
Star 3
Star 2
4. Click on the Photometry icon
: a photometry window will appear and your pointer
will become a brightness sensor. You can click on the stars to measure their intensity.
Measure the intensity (in arbitrary unit) of Star 1 on the sequence of 20 images.
5. Open the Answerfile_transit_empty.xls file and rename it. Copy and paste the results
from the Photometry window in the answerfile in the RawMeasurementSalsaJ sheet.
Then, select carefully the Star 1 measurements and copy them in the Answerfile.
6.
The Answerfile subtracts the average flux of each star and plot the variation of the
normalised flux as a function of time. Verify if what is plotted correspond to your data.
7. Proceed similarly for Star 2 and Star
3. You can use Analyse/Clear
Photometry Results to reset the
photometry window. You will then
get three light curves as displayed
here.
8. It is possible to modify the
photometry parameters through
“Photometry Settings”. Click on
Forced Star Radius, and choose a
radius of 6 or 18 pixels to measure
the whole sequence again. What is
changed?
Interpretation
The intensities of Star 2 and Star 3 exhibit fluctuations compatible with noise, while Star 1 reveals a
significant drop. Since all stars are observed at the same time, a systematic effect would have been seen
on the 3 curves. Furthermore, these data have been acquired in space and are not affected by the
changes of atmospheric conditions. The Star 1 variations are significant (larger than the noise level,
and lasting several hours) and correspond to a transit phenomenon.
During a transit, there is four points of contact, when the
circumference of the planet touches the circumference of
the star at a single point:
(1) the planet is entirely outside the star, moving inward;
(2) the planet is entirely inside the star, moving further
inward;
(3) the planet is entirely inside the star, moving outward;
(4) the planet is entirely outside the star, moving outward.
9. Can you identify the four points of contact of the transit on your light curve?
10. Knowing that the diameter of the star is 0.788±0.051 Rsun. Estimate the diameter of the
transit body with respect to the Sun diameter. Speculate on the nature of the transiting
body.
11. What can you say about the relative position of the Earth, the star HD 189733 and the
transiting body?
12. Check your results with the Extrasolar Planets Encyclopedia (http://exoplanet.eu/).
13. The following figure displays a ground-based follow-up of this star exhibiting a transit.
How does it compare to your data?