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Astron. Astrophys. 338, 479-490 (1998)

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5. Summary

The TEP network has embarked on obtaining extended observational coverage of the eclipsing M star binary CM Dra with the goal of detecting transits of planets which are orbiting in the plane of the binary components. Reported here are the data from the first three years of this project, covering the years 1994 through 1996. Time series of about 200 hours coverage were obtained in each year of observations, containing a total of 17176 data points. This gives the most thorough coverage of any binary eclipsing star observed to date. In this coverage we observed six events in which the brightness of CM Dra dropped between 0.004 mag and 0.01 mag over a time scale of an hour. These events are compatible with planetary transits, but their true nature can only be ascertained by further observational coverage. The absence of any light drops larger than 0.01 mag allows us to rule out the presence of close planets with radii of larger than 2.5 RE (corresponding to about 1.1% the volume of Jupiter), with a confidence of 80% for orbital periods of less than 60 days. The confidence is 98% that such planets are absent with periods of less than 20 days. The light-loss events listed in Table 5 are examples of what lightcurves from the transits of extrasolar planets with sizes between 1.5 and 2.5 RE should look like. But again, their true nature can only be confirmed by observations of repeated transits from the same planet.

Furthermore, the light-loss events in Table 5 only contain events that are visible in the lightcurve. The typical noise in the lightcurve is 2-3 mmag, and events from planets smaller than about 2 RE could be hidden in the lightcurve. Such small planets, although their transits are visually undetectable in the lightcurve, may be found by the application of a matched filter algorithm - i.e. cross-correlations with the quasi periodic signals caused by planetary transits (Jenkins et al. 1996). Detection statistical calculations show, that the confidence that planets with 2RE could be detected in the current data - if they are present - is about 50%. In conclusion, it is therefore definitively possible to use the photometric transit method with ground based 1 meter class telescopes to obtain results about the presence of extrasolar terrestrial-sized planets within a star's habitable zone. With larger telescopes and longer observing times, this method may be extended to the detection of terrestrial-like planets around many other eclipsing binary systems, as well.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998