Marcy's team found tau Bootis b via the slight wobble it imparted to its host star's spectrum as it orbits. As the planet swings around its star and its gravity tugs the star away from the viewer, it slides the star's bar-code-like chemical signature toward the red end of the spectrum. As the planet continues its orbit and tugs the star toward the viewer, the spectrum shifts toward the blue end. Over multiple orbits, the star's spectrum "wobbles" from blue to red and back.
The idea to use this approach to detect a planet directly is an old one, notes Simon Albrecht, an astronomer at the Massachusetts Institute of Technology in Cambridge, Mass., and a member of one of the two teams. His group was led by Matteo Brogi with the Leiden Observatory in the Netherlands. The other team was led by Florian Rodler, with the Institute of Space Science in Barcelona.
All it needed was the right combination of technology and a target close enough and bright enough to pursue. Tau Bootis b fit the latter requirement. For hardware, both teams used a new, exquisitely sensitive near-infrared spectrometer bolted to the back end of one of four eight-meter telescopes that make up the Very Large Telescope array. The facility, run by the European Southern Observatory, is located on a mountaintop in the Atacama Desert of northern Chile.