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
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
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.