APT observations of the first known transiting planet, HD209458. The solid line in the lower panel is a model and the dashed lines illustrate a variation of the planet diameter by 10%. The upper panel illustrates a reference star near HD209458 and shows that, for this particular observation, we achieve an RMS variation of 0.003 magnitudes.

Our team is searching for new planets orbiting nearby stars using our very own Automated Patrol Telescope. On about 30% of the clear nights at Siding Spring Observatory (near Coonabarabran in northern NSW) we observe a patch of sky near the plane of the Galaxy. In each image we take, we can measure the brightness of thousands of stars. Using a new observing technique developed by Michael Ashley, and data processing software from our collaborator, Mike Irwin at Cambridge, we can make these measurements with a precision that is currently unrivaled by any similar projects being developed elsewhere.

If one of our target stars hosts a planet in an orbit that is edge-on to our line of sight, the planet will periodically eclipse (transit) the star. We can detect this as a slight dip in the apparent brightness of the star. In favourable cases, the depth of this dip is considerably greater than our measurement error, and a significant detection can be made. Such a detection yields the planetary orbital period and size of the planet. However, other systems, such as binary stars, can mimic a planetary transit. However, follow-up spectroscopic observations (measuring periodic changes in the star’s radial velocity) can determine the orbiting object’s mass, and thus confirm whether or not it is a planet.

Marton Hidas, John Webb and Michael Ashley