The Transit Method

The transit method is based on the observation of a star's small drop in brightness, that occurs when the orbit (dashed line) of one of the star's planets passes ('transits') in front of the star  The amount of light lost -typically between 0.01% and 1%- depends on the sizes of the star and the planet; and the duration of the transit depends on the planet's distance from the star and the star's mass. Since the star's mass and size  can be determined from spectroscopic observations, the planet's size and distance can be determined. Of course, a transit has to occur once for every orbital revolution of the planet around the star. This repeated occurrence of transits is the major diagnostic tool to determine if an observed transit is really from a planet - it has to appear once in each of the planet's 'year'.

The repeatability of transits gives also the opportunity to observe a known transiting planet in the future with improved instrumentation - one day it may be possible to  examine a planet's atmosphere (by spectroscopy of absorption lines from its atmosphere while it is transiting) and check it for indicators of life - such as the presence of free oxygen in the atmosphere.

In the TEP project, we are observing an eclipsing binary star for transit signals. A planet in the plane of the binary star would transit both components. This is symbolized in the logo of the TEP network.

A variety of transit shapes can result from transits across binary stars. They are a consequence from the variety of geometric configurations that are possible, with the binary stars moving around each other, and the planet moving around both.

This graph shows examples of brightness changes that could be caused by a planet with 2 Earth Radii, transiting across both components of CM Draconis. The vertical axis is labeled in milli-magnitudes; 1 mmag is about 0.1% brightness change. Left is the 'normal' case with two dips (one from each component) separated by a few hours; at right a is single long transit - such as it could have occurred if CM Dra is in the configuration shown by the TEP logo. With long-periodic planets, even more complicated shapes and multiple transits may occur. The Fig.1 in the paper in Astronomy & Astrophysics (1998, Vol 338, p 479, preprint in postscript , pdf) gives more examples of these transits. The unique shapes that planets would cause in binary transits serve as an additional tool in the identification of true planetary transits: Each planet (with a specific period and phase) would cause a unique sequence of transits across CM Dra. Once some planet candidate has been found, further transits, including their shapes, can be predicted and verified by telescope observations.

The following two articles give a more detailed overview about the transit method: Additional articles can be found in the references.

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Last update: 25/Mar/2002. For comments, send email to Hans-Jörg Deeg at (NOTE: remove the X from address)