The last time we did a story on extrasolar planets (or exoplanets) in October, 2009, there had been 374 planets outside our solar system discovered. As of June 28, 2010 that number had risen to 464. The numbers look set to get a further boost thanks to a new technique that allows planets – even down to the mass of the Earth – to be detected with relatively small diameter telescopes. For the first time, using the technique known as Transit Timing Variation (TTV), a team of astronomers from Germany, Bulgaria and Poland have discovered an exotic extrasolar planet with 15 times the mass of Earth in the system WASP-3, 700 light years from the Sun in the constellation of Lyra.

The vast majority of extrasolar planets have been discovered using the radial-velocity and transit methods. After originally being discovered with the radial-velocity method, in 1999 HD 209458 b became the first exoplanet to be seen with the transit method. The transit method involves observing the drop in the brightness of a star as a planet crosses (or transits) in front of it. The amount the star dims will depend on the size of the star and the size of the planet. This method is being deployed by the Kepler and Corot space missions in its search for planets similar to the Earth.


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TTV was suggested as a technique for discovering planets years ago. Like the transit method, the new TTV approach is an indirect method. If a (typically large) planet is found, then the gravity of additional smaller planets will tug on the larger object, causing deviations in the regular cycle of transits. The TTV technique compares the deviations with predictions made by extensive computer-based calculations, allowing astronomers to deduce the makeup of the planetary system.

Discovering WASP-3c

In their search of the WASP-3 system the team used the 90cm telescopes of the University Observatory Jena and the 60cm telescope of the Rozhen National Astronomical Observatory in Bulgaria to study transits of WASP-3b, a large planet with 630 times the mass of the Earth.

“We detected periodic variations in the transit timing of WASP-3b. These variations can be explained by an additional planet in the system, with a mass of 15 Earth-mass (i.e. one Uranus mass) and a period of 3.75 days”, said Dr Maciejewski.

“In line with international rules, we called this new planet WASP-3c”. This newly discovered planet is among the least massive planets known to date and also the least massive planet known orbiting a star which is more massive than our Sun.

This is the first time that a new extra-solar planet has been discovered using this method.

The new planet appears to be trapped in an external orbit, twice as long as the orbit of the more massive planet. Such a configuration is probably a result of the early evolution of the system.

The TTV method is very attractive, because it is particularly sensitive to small planets, even down to the mass of the Earth. For example, an Earth-mass planet will pull on a typical gas giant planet orbiting close to its star and cause deviations in the timing of the larger objects’ transits of up to one minute.

This is a big enough effect to be detected with relatively small 1-m diameter telescopes and discoveries can be followed up with larger instruments. The team are now using the 10 meter Hobby-Eberly Telescope in Texas to study WASP-3c in more detail.