Rising Mercury in Transit
|The Mercury transit in perspective with sunspot
Around sunrise, Wednesday, May 7, 2003, the planet Mercury will pass in front of the Sun as a small dark point. This is a kind of eclipse of the Sun - however, it is not caused by the Moon but by a planet. This kind of astronomical event is called a transit and it occurs approximately once every 7 years.
Live web coverage of the May 7 transit is available.
For those who try to discover distant planets, such transits are fortuitous. For instance, if the exoplanet is known to move across ("transit") the star's disk, as seen from the Earth; the orbital plane must then necessarily be very near the line-of-sight. During such an exoplanet transit, the observed brightness of the star will decrease slightly because the planet blocks a part of the stellar light. The larger the planet, the more of the light is blocked and the more the brightness of the star will decrease.
This phenomenon is exactly the same that happens in our own solar system this year with Venus and Mercury, pass in front of the solar disk, as seen from the Earth. A solar eclipse (caused by the Moon moving in front of the Sun) is a more extreme case of the same type of event.
|Dip in brightness as prospective planet transits in front of parent star Credit: ESO
A study of the way this brightness changes with time (astronomers refer to the "light curve"), when combined with radial velocity measurements, allows a complete determination of the planetary orbit, including the exact inclination. It also provides accurate information about the planet's size, true mass and hence, density.
Three exoplanets have been discovered so far using the transit method.
The chances that a particular exoplanet passes in front of the disk of its central star as seen from the Earth are small. However, because of the crucial importance of such events in order to characterize exoplanets fully, astronomers have for some time been actively searching for stars that experience small regularly occurring "brightness dips" that might possibly be caused by exoplanetary transits.
This event will be visible from Europe, Africa and Asia. In fact, this transit of Mercury is a fine "prelude" to the even more fascinating and important celestial event next year on June 8, 2004, when the larger planet Venus will pass in front of the Sun - very easily observable from the same continents
Related Web Pages
European Southern Observatory
Astrobiology Magazine New Planets
Extrasolar Planets Encyclopedia