One of the first planets discovered outside of the Solar System, Tau Bootis b, has eluded numerous attempts to measure the light coming from its atmosphere and so has remained something of a mystery. Now, for the first time, an international team has used an innovative technique to unravel direct light from the exoplanet itself to reveal its mass and orbit. Their results will be reported in Nature on June 28.

"The problem with exoplanets is that in general we do not know the orientation of their orbit as we see them from Earth," says team member Ernst de Mooij, a postdoctoral fellow in the University of Toronto's Department of Astronomy and Astrophysics. "This prevents us from getting a good determination of their masses. However, with our new technique we were able to measure the motion of Tau Bootis b, from which we have determined that it orbits its host star at an angle of 44 degrees as seen from the Earth, and that the planet is six times the mass of Jupiter."

Discovered in 1996, the exoplanet Tau Bootis b is located in the constellation Bootis. It orbits its host-star at a distance of less than five per cent of the distance between the Earth and the Sun.

Although the star is easily visible with the naked eye, the planet itself is not, and several attempts to catch light from it over 15 years have failed.

"Eventually it took three nights of observations with the Very Large Telescope, located at the European Southern Observatory on Cerro Paranal, Chile to see the planet," says lead author Matteo Brogi of Leiden University.

"By very precisely studying the spectrum of the system, we conclude that 99.99 per cent of the light comes from the star, and only about 0.01 per cent from the exoplanet."

The team had first applied their unique approach in the context of a transiting planet - a planet that periodically passes between the Earth and its star. They soon realized they could apply the same method to the larger population of non-transiting planets if they searched for the light emitted by the planet itself.

Along with De Mooij and Brogi, the team included Jayne Birkby, Ignas Snellen of Leiden University, The Netherlands, Remco de Kok of the Netherlands Institute for Space Research (SRON) and Simon Albrecht of Massachusetts Institute of Technology.