Tim Brown

Spying the secrets of planets beyond our solar system

Tim Brown with the STARE telescope (Photo by Carlye Calvin.)

Tim Brown has been interested in stars ever since he was a child reading about the launch of Sputnik and other satellites in the 1950s. "I can't remember wanting to be anything but an astronomer," he says. Now, he's an astronomer and senior scientist at NCAR's High Altitude Observatory.

Tim and his father built a telescope when he was 14 years old. Then he built a small observatory for the telescope so he and his friends could be out of the wind as they watched the stars.

After earning a doctorate in astrophysics from the University of Colorado, Tim came to NCAR to pursue research into the Sun. In the early 1980s, he designed and built an instrument known as the Fourier Tachometer, which produced the surprising finding that the solar interior is rotating at a sharply different rate than the outer third of the Sun. This work has shed light on solar dynamics, ultimately leading to breakthrough research into the behavior of sunspots that can affect Earth's atmosphere.

His work took a dramatic turn in 1992, when he and a team of scientists created a spectrograph (also known as a spectrometer) to study subtle oscillations in the light coming from the Sun and other stars. Tim realized the spectrograph, which "sees" motion as changes in the wave shapes of light-absorbing particles, could also be used for an exciting new scientific field: planet hunting.

Planets that orbit distant stars are too far away and too small to be observed directly by even the most powerful telescopes. But Doppler spectroscopy can detect tiny shifts in starlight that indicate motion toward or away from the observer. Such motion, known as radial velocity, is actually a faint wobble in a star--a sign it is being tugged by the gravitational field of an orbiting planet.

Tim and his colleagues used spectroscopy to confirm the existence of a planet that another team of scientists had found in 1995, and they went on to discover a few planets of their own.

He also ground lenses for a specialized telescope, called STARE, designed to detect the dimming caused when a planet transits, or crosses in front of, its parent star. When he and a colleague in 1999 trained the instrument on a star some 150 light years from Earth, they found the characteristic dimming pattern that indicated the existence of a planet.

"It was quite an exciting moment for me," Tim recalls. "When I was a child, no one knew if the galaxy even harbored planets outside our solar system."

Breaking new scientific ground, Tim and his colleagues then used the transit technique to determine whether the planet had an atmosphere. He worked closely with David Charbonneau, who was then a graduate student fellow at NCAR and is now on the faculty of Harvard University. Together they measured the starlight passing by the planet to see if it is distorted in certain telltale ways. In 2001, the team observed several transits using the imaging spectrograph on NASA's Hubble Space Telescope. The results showed traces of sodium, which could only have come from an atmosphere around the planet.

The discovery made headlines because it marked the first time scientists had detected an atmosphere around a planet orbiting another star. It pointed the way to observing atmospheres of other distant planets, thereby learning more about the formation of other solar systems--and potentially even gathering clues about life elsewhere in the galaxy.

This June, Tim used spectroscopy to observe a rare moment in astronomy: the first transit of Venus across the surface of the Sun since 1882. He is now analyzing his observations to see whether he can use color shifts in spectral lines to infer the movement of winds in the upper atmosphere of Venus. The same analysis, in theory, could also enable scientists to gather information about winds on planets orbiting other stars.

For Tim, finding and studying extrasolar planets represents quite a scientific departure from his original aim of investigating the Sun. But, as he said after receiving an NCAR award for distinguished achievement, "Sometimes there are side trips we just have to make. This side trip worked out really well."

• by David Hosansky
August 2004

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