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Astronomers use an old trick to open new window on extrasolar planets

Two teams of astronomers used a technique for finding extrasolar planets to directly measure one such planet. The approach could allow the study of more exoplanets' atmospheres than ever before.

By / June 27, 2012

In this artist's conception Tau Bo├Âtis b looks at its sun.

NASA

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A method astronomers have used for nearly 20 years to indirectly find planets orbiting other stars has for the first time been used to take the direct measure of an extrasolar planet itself.

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The approach not only yields precise estimates of a planet's mass, a critical piece of information that can help determine a planet's bulk composition. It also can reveal its orbital inclination. And perhaps most important, it could allow astronomers to study atmospheres of more extrasolar planets than ever before.

The reason: The technique doesn't require a star to backlight a planet's atmosphere as the planet passes in front of its star. Until now, this kind of backlighting, during what's known as a transit, has provided the only window astronomers have had on the make-up of an extrasolar planet's atmosphere. And the number of transiting planets so far is only about one-third the number of planets discovered by two international teams of astronomers who used this approach and whose findings are published this month.

The orb the teams observed, known as tau Bootis b, was discovered in 1996 by a group led by astronomer Geoffrey Marcy at the University of California at Berkeley. It orbits a star nearly 51 light-years from Earth in the Northern-Hemisphere constellation Bootes.

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.

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