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Transits of Extrasolar Planets

Well over 200 planets have been found to be orbiting stars other than our Sun, and it is very feasible that there could be many thousands or even millions more in our Galaxy alone. Since direct imaging is at present not possible, due to the tiny amount of light reflected by these planetary companions from their parent stars, indirect methods have been formulated to determine orbital parameters.

The large majority of detections have been through the radial-velocity method, which observes the gravitational pull of a planet on its star. A handful have however been discovered due to their passing in front of the star and slightly reducing the light flux received at Earth. This is known as the transit method, and it is what was used at ULO to gain comprehensive orbital information from four planets, and partial details of several others.

One of the most important requirements in extrasolar astronomy is a large dataset. The more transits that are recorded, the more statistically significant the data is. The statistical processing applied in this project ensured that the significance of the results were equivalent to those obtained at a larger telescope. It is a fine example of how important and relevant observations can be made in a city such as London, and has inspired us to continue searching.

Observations of the star HD189733 were taken in July and August 2006 during two transit events; a sequence of several hundred images were obtained on each night. Observing were project student Dan Smith (MSci 2006/7) and Dr Steve Fossey.



Photometry of the star's brightness variations, measured relative to other stars in the field, reveals how the star's light fades very slightly (a few percent) while its planetary companion passes in front of it as viewed from Earth.




The planet orbits its host star in just over 2 days, and the total transit lasts just under 2 hours. Modelling of the light curve can provide estimates of the planetary radius: the latest results on this system indicate the exoplanet is about 15 percent bigger than Jupiter, while the star is about 20 percent smaller than the Sun. Such observations are very important for determining the nature of such planets - so-called "hot Jupiters" - and addressing detailed questions concerning their origin in exo-planetary systems.

Links: The Extrasolar Planets Encyclopaedia