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September 2001

Tim Brown shares frustrations and inspirations along the trail to newfound planets

Tim Brown. (Photo by Carlye Calvin.)

The folks who packed the Boulder Public Library auditorium on 20 August to hear HAO's Tim Brown give the fourth Walter Orr Roberts Distinguished Lecture got what they came for—the latest on searching for extrasolar planets—plus a birthday cake, a juggling demonstration, and some reflections on the pursuit of basic science.

UCAR president Rick Anthes opened the evening by noting that this year's lecture fell on what would have been Walt's 86th birthday—hence the cake. Rick also introduced the three prior Roberts lecture honorees, Warren Washington (CGD), John Firor (NCAR director emeritus), and Susan Solomon (NOAA Aeronomy Lab).

Peter Gilman (HAO) introduced Tim by noting that Walt's continually expanding vision of NCAR's research purview, "from the bottom of the ocean to the center of the farthest star," was finally being realized. Peter suggested that Tim's measurements of a planet transiting its star were "truly historic" and predicted that "long after we are gone and our work has been forgotten, the textbooks of the year 2100 in astronomy are all going to have [the graph of that transit] in it."

Tim's subject was the pursuit of planets of other stars and "how I didn't really get here intentionally." Tim wondered about the seemingly unplanned nature of his journey, interweaving explanations of the methods behind his work with reflections on influences, mentors, setbacks, and serendipity.

Looking for the wobble

Using Doppler spectroscopy, the radial velocity method looks for tiny shifts in spectral lines to detect oscillations—motion toward (red) or away (blue) from the observer. While stars exert gravitational pull on planets, the reverse is also true. The detected radial velocity is a faint wobble in a star created by the gravitational pull of its orbiting planet. This method yields three kinds of data: how close the planet is to its star, how massive the planet is, and the shape of its orbit.

From stellar oscillations to finding planets

For a sense of history, Tim shared a quote from Bruno, Galileo's 16th-century contemporary, who suggested that countless suns and planets exist but are invisible because the planets are dark and too close to their very bright suns. As prescient as Bruno was, "he got burned at the stake for his troubles," Tim noted.

Tim's planet-finding work began in 1992, when he, with colleagues at NCAR and the Smithsonian Astrophysical Observatory (SAO), mounted a spectrograph they'd designed and built on a 60-inch (1.5- meter) telescope at the Whipple Observatory, operated by SAO on Mt. Hopkins in Arizona. They'd created the spectrograph to study subtle oscillations in the light coming from the Sun and other stars. Tim soon realized it could be used for planet-hunting as well, and he began his search, just as others were launching similar quests.

It wasn't until 1995 that the first extrasolar planet was actually found, circling the star 51 Pegasi, by Michel Mayor and Didier Queloz of the Geneva Observatory. Tim's team, which had also been investigating the star, was one of the first to confirm the ground- breaking observations.

In his talk, Tim used a scale model of the one-planet 51 Peg system to help explain radial velocity and the Doppler shift spectrometry used to detect it (see sidebar). About 60 additional planets orbiting sun-like stars have since been found by a handful of teams around the world using the technique.

The planets found thus far tend to be quite massive, with orbits close in to their stars. The more massive planets orbiting the Sun are much further out in our solar system. Another difference: The orbits of extrasolar planets tend to be highly elliptical, in contrast to the nearly circular orbits around the Sun. As to whether newfound planets tell us anything about our solar system, Tim's answer is, "Nobody knows. It's just not clear whether we're looking at the same or a different process." At least not yet. For one thing, none of the planets discovered thus far has a mass much lower than Jupiter's because "our techniques aren't good enough to detect [smaller ones]." As for planets at greater distances from their stars, which therefore take more time to complete their orbits, "we haven't been looking long enough."

Influence and inspiration

"Nothing that I had started to do is like what I ended up doing," Tim mused. But, "if you want to know why a jazz musician sounds the way he does, you need to know: Who were his influences? Who did he listen to when he was young, who were his teachers?" So Tim took time to reflect on some of the people that played an influential role in his career.

Tim's hidden talent helped him illustrate the ever-enlarging orbit of a planet picking up energy from its companion in a multiple-planet system. (Photo by Carlye Calvin.)

A key formative force was his father, James Brown, a chemist and meteorologist who switched to a career in English literature after World War II and went on to serve as chancellor of the Southern Illinois University system. Tim figured his father, who's now retired, and Walt would have enjoyed meeting because of their uncommonly broad interests in "absolutely everything: science, art, literature, music, sailing, aviation, you name it."

Tim also discussed his undergraduate and graduate advisors, James Faller (Wesleyan University) and Henry Hill (University of Arizona). "If Jim's reaction when confronted with a new problem was 'build me a model,' Henry's was always 'show me the equations.'" Tim's doctoral thesis examined changes in the diameter of the Sun, and his principal result was a power spectrum of diameter oscillations. "As it turned out, none of it was real, and within a couple of years I had to retract most of the observation results from my Ph.D. thesis. This is a chastening thing."

But "these previous results, even though totally incorrect, provoked an awful lot of useful theoretical work for the oscillations in the Sun, which, it turns out, we could observe; they were real." The work examining oscillations on the Sun by measuring radial velocities occupied Tim through the 1980s. During some of that time Tim worked with Jack Evans, Walt's first HAO employee, who had just retired from directing the Sacramento Peak Observatory. "His energy and his enthusiasm were just amazing."

By the early 1990s, when it was suggested that an extrasolar planet could be discovered by observing the subtle oscillations produced by its gravitational tug on its star, Tim had developed the precise measurement techniques to do it. "So that's one line into planetary studies from solar physics. You know how to do the measurement," Tim said.

In April 1999 Tim's team provided radial velocity observations which, when combined with those from teams at Harvard-Smithsonian Center for Astrophysics and San Francisco State University, revealed that not one, but three planets were orbiting the star Upsilon Andromedae.

The telescope in the parking lot

In late summer 1999 Tim and graduate student David Charbonneau set up a small telescope in the parking lot of the Foothills Lab and pointed it skyward. Tim ground the lenses himself for the STARE (Stellar Astrophysics and Research on Exoplanets) telescope. It uses precise time-series photometry to detect the dimming caused when a planet crosses in front of its parent star. David, an HAO Newkirk Graduate Fellow from Harvard University, had information on star HD 209458 that suggested it was a good candidate for planet hunting. They found what they were looking for on two nights in September. The data plot of their observations created the illustration Peter predicts will still be in textbooks 100 years from now.

The new information the transit method offers is based on learning the relative cross-sectional areas of the star and its planet. For planets that are 10% of the diameter of their star, there's a drop in the star's brightness of about 1%. By combining the data on diameter with data on a planet's mass from radial velocity measurements, "you can determine its density, and if you know its density, then—eureka!—you know what it's made of."

STARE is now ensconced in a new dome on Tenerife, one of the Canary Islands, where it will look at many stars at once. "We'll see how many planets we can sweep up" looking at several large fields of stars per year.

So was there a plan?

After acknowledging his planet-finding collaborators around the world, Tim returned to the question of whether there was a conspiracy or plan that led him to planet finding. Tim said he'd come to realize that the plan lay in the people who have influenced and inspired him, including Walt Roberts, who had the foresight to create an institution where basic research could flourish.

"Walt couldn't know what the important scientific problems were going to be . . . , and he knew that." So he brought together people and resources in an organization capable of tackling new problems as they arose. "And during the time that I was growing up, it was sending out its feelers and tentacles, and I got snared."

HAO director Michael Knölker echoed Tim with some observations on the administrative challenge research "outside the box" can sometimes present. Of Tim's achievements, Michael said, "I am a little proud that we succeeded in continuing this entrepreneurial spirit of HAO as it was founded by Walt Roberts."

• Zhenya Gallon

On the Web:
Planet-finding history, methodology, and the STARE telescope

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Last revised: Mon Sep 17 15:38:00 MDT 2001