Unveiling an extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. Until 2010, astronomers have made detections of 462 exoplanets. The vast majority have been observed through radial velocity observations and other indirect methods rather than actual imaging. Most are giant planets thought to resemble Jupiter. In this acticity, we propose to unveil a planet using another method: a transit of the planet in front of its star.
 If you wonder how to discover a new, extrasolar planet, try this !
Roger Ferlet (IAP); Olivier Marco, Ester Aranzana Martinez, Sandra Greiss, & Jeehae Chun (University Pierre Marie Curie)
What is an exoplanet (extrasolar planet)?
Roughly, it's a planet orbiting a star, outside the solar system.Until December 2008, 335 exoplanets have been detected. Their wide variety leads to their classification by mass, nature, size, etc.
The detection of exoplanets is hard because of the huge distance between the observer and the planet. Nevertheless, there are different methods of detection and the most effective ones are:
- radial velocity: the first method used to detect an exoplanet (by M. Mayor and D. Queloz in 1995) and it's still the most efficient one. It enables to find information about mass of the planet. On the site of EU-HOU, you can find an high-school level exercise about the detection of exoplanets by this method.
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the transit: complementary to the radial velocity method. It reveals 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 its classification.
Outline of different phases of an exoplanet transit
Light curve of an exoplanet transit observed by the Corot telescope (may 2007)
Exoplanet HD 189733b
The data are a sample of images of a star HD 189733 obtained by the space telescope Spitzer. We will extract data from these images and use SalsaJ software to handle and analyse it with Excel graph.
The exoplanet orbiting the star HD 189733 was discovered on October 5, 2005 by transit method in France. The planet is classifed as a hot Jupiter class Jovian planet, orbiting very close with a short period of 2.2 days. It’s approximately 63 light-years away in the constellation of Vulpecula (the Fox).
Its location is indicated in this deep (wide-angle) image of the sky centered 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 exercises:
The data are a series of 20 exposures of the same 3 stars, taken by Spitzer telescope. One has to perform photometry measurements of each of the 3 stars, for all the 20 images. Along time, the flux of 2 stars will keep (roughly) constant 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 !
To improve the plots, it is useful to apply a kind of normalisation, that is to subtract the mean value of the flux so that they scale is similar for the 3 stars. See the resulting plot here:
Data (20 FITS images) can be retrieved from this zip file: 20image_spitzer 4.77 Mb
A full description of the work can be read in this PDF file: exoplanet_transit_english 3.07 Mb
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