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Planet found tugging
on transits

KEITH COOPER
ASTRONOMY NOW
Posted: 9 July 2010


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A brand new technique for finding exoplanets, which piggybacks on an already established method, has turned up its first discovery, a planet 15 times the mass of Earth orbiting the star WASP-3. It also promises to be able to find Earth-sized planets.

An artist’s impression of a planet transiting its star. We can now use deviations in the transits to detect unseen worlds. Image: NASA/JPL–Caltech/R Hurt (SSC).

The Transit Timing Variation (TTV) method has been pioneered by a team of astronomers from Germany, Bulgaria and Poland, led by Dr Gracjan Maciejewski of Jena University, Germany. Using the 0.9m telescope at University Observatory Jena and the 0.6m telescope at the Rohzen National Astronomical Observatory in Bulgaria, they observed transits (i.e. as a planet passes in front of its star, dimming some of its light) of the gas giant planet WASP-3b 700 light years away in the constellation of Lyra. WASP-3b was discovered in 2007 by the SuperWASP project and has twice the mass of Jupiter, but Maciejewski’s team found deviations within the regularity of WASP-3b’s transits, pointing to the gravitational influence of another planet with 15 times the mass of the Earth (about Uranus’ mass) in an external orbit with a period of 3.75 days. This equates to a distance from its star of 0.0507 astronomical units (7.58 million kilometres). For comparison WASP-3b is 4.7 million kilometres from its star. This second planet has been called WASP-3c.

Schematic showing the orbits of the two planets in the WASP-3 system. Image: Maciejewski et al.

The TTV method is so sensitive, says Maciejewski, that it could detect deviations in the timing of the transit of a gas giant as small as a minute, caused by planets the same size and mass as Earth. “Such a planet must be very close to the giant planet,” says Maciejewski, adding that so long as the orbital period is no greater than 2–3 times longer it can create a significant degree of deviation, particularly if they are in orbital resonance, i.e. if the smaller planet orbits three times for every two orbits of the transiting gas giant, for example. The more massive the perturbing planet, the farther it can be from the transiting planet and still be detected.

Although the method only works for systems in which we have already identified a transiting planet, the results, which are being published in the Monthly Notices of the Royal Society, suggest that the Kepler spacecraft is no longer our only means of discovering Earth-sized planets. Intriguingly, a similar method has also been suggested for detecting large exo-moons as they orbit gas giants transiting in front of stars.