Photometric Detection of extrasolar Planets by the Transit Method

 The aim of the transit method is to measure the brightness drop of a star, which results from the transit of a planet across the star´s disk.

 Transit LC

The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial velocity method one can determine the density of the planet, and hence learn something about the planet's physical structure.

The exact shape of the transit depends on the size ratio of planet and central star, the inclination and the stars limb darkening.

Transits are observable if the inclination is very close to 90°. The minimal inclination is lower for planets close to the central star than for planets farther away. Therefore the transit method is especially well suited to find close-in companions.

The transit event is - in a first approximation - a periodic phenomenon. In a system where a known planet transits its host star, a second planet in that system can cause the time between transits to vary. This technique is itself a planet detection method that is very powerful for searching low-mass planets and is most sensitive for planets in mean-motion resonances. For example, for a hot jupiter (3-day orbit), an Earth mass planet in the 2:1 resonance will cause periodic transit timing variations (TTV) that have an amplitude greater than one minute [1]. It can also be used to detect possible trojans of transiting extrasolar planets [2] or „exomoons“ [3].

The transit method is becoming increasingly popular, because even with small telescopes one can achieve great successes. Small ground-based observatories have already exceeded the photometric precision necessary to detect sub-Earth mass planets by the transit timing variation method [4].

We have started high precision photometric observations at the University Observatory Jena in fall 2006. One goal of our observations at the University Observatory Jena is the systematic follow-up of selected known transiting planets. This can lead to the discovery of additional low-mass bodies in these systems. This is possible by detecting and analyzing TTV. Therefore we paid special attention to the accurate determination of transit times in order to identify precise TTVs.


In 2009 we launched an international observing campaign to detect and characterise transit timing variation signal in selected transiting exoplanet. The programme is realised by collecting data from 0.6 - 2.2-m telescopes spread worldwide at different longitudes. Our first efforts to detect transit timing variation signal for the transiting planets XO-1b, TrES-1b and TrES-2b resulted in redetermining transit ephemerides [5, 6].

A first success for the detection of TTVS's is the exoplanetary sytem Wasp-3.

In [7] we present photometric observations gathered in 2009 during the dedicated transit-time-variation campaign. The observed transit timing cannot be explained by a constant period, but by a periodic variation. Numerical Three-body simulations have shown that a configuration with a hypothetical second planet with a mass of about 15 Earth masses, orbiting its star in about 3.75 days can reproduce the observed variations.


Another project of the University Observatory Jena is the observation of young, open clusters. The main goal is the search for close-in, young planets and Brown Dwarfs with the transit method. Young, open star clusters provide an ideal environment because they have a relative high number of stars of same age, metallicity and distance. These observations can give information on the limits for the time scales of planet formation and migration, on the effect of age, environment and metallicity on the frequency of planets and can help to constrain evolutionary models of planets and Brown Dwarfs. For this purpose various clusters for each season with different ages were selected. In addition to Jena, the project is suppoorted by observatories in Japan, China, Taiwan, India, Armenia, Bulgaria, Slovakia, Poland, Spain, USA, Chile and Venezuela. This telescope network (YETIYoung Exoplanet Transit Initiative) enables us to observe the clusters continuously for several days in order not to miss any transit. A summary of the project is given in [8].

From our observations in summer 2009 at the University Observatory Jena one candidate for a young planet was detected. With the help of the international network some more planet candidates are expected.


[1] J. H. Steffen, B. S. Gaudi, E. B. Ford, E. Agol, and M. J. Holman, ArXiv e-prints, 704, arXiv:0704.0632 (2007)

[2] E. B. Ford, and M. J. Holman, The Astrophysical Journal , 664, L51 (2007)

[3] A. Simon, K. Szatmáry, and Gy. M. Szabó, Astronomy and Astrophysics, 470, 727 (2007)

[4] E. Agol, J. Steffen, R. Sari, and W. Clarkson, Monthly Notices of the Royal Astronomical Society, 359, 567 (2005)

[5] St. Raetz, M. Mugrauer, T. O. B. Schmidt, et al., Astronomische Nachrichten, 330, 475 (2009)

 [6] St. Raetz, M. Mugrauer, T. O. B. Schmidt, et al., Astronomische Nachrichten, 330, 459 (2009)

[7] G. Maciejewski, D. Dimitrov, St. Raetz, et al., Monthly Notices of the Royal Astronomical Society, 407, 2625 (2010)

[8] R. Neuhäuser et al., Astronomische Nachrichten, 332, 547 (2011)

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