By Erin Sherbert
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Astronomers have a technique for detecting how fast stars are receding from or approaching the Earth. That method involves what is known as the Doppler effect, a phenomenon first described by 19th-century Austrian scientist Christian Doppler.
When the source of a sound moves toward, then away from a person, the pitch of the sound seems to change because of the perceived compression, then stretching of sound waves. (The change in pitch of the sound of a freight train passing is the commonly used example.)
The kinds of changes that occur when a speeding object creates sound waves also occur with light, which has some wave characteristics. When a star moves toward the Earth, the light it emits appears to be shifted slightly toward the blue end of the visible light spectrum. When a star recedes, its light shifts to appear toward the low -- or red -- end of that spectrum. A jiggling telescope may create phantom changes in a star's position, but not its color, the reasoning went.
To search for this effect, Marcy and Butler decided to tease stars' light through a spectrograph -- an astronomer's high-tech prism. The prism casts starlight in rows of "spectral lines" on an electronic plate linked to a computer. If those lines move from blue to red, the San Francisco astronomers figured, they could prove a star is wobbling -- perhaps as a result of the gravitational tug of a planet orbiting nearby.
When Marcy and Butler first decided to use this type of planet search 10 years ago, even the highly sophisticated Doppler technique was too crude to positively detect the subtle nudging effect planets have on stars.
Nearly anything -- the expansion and contraction of a spectrograph's metal and glass with the changing of seasons, the sagging of the spectrograph under its own weight -- can distort spectral lines and fool an astronomer looking for such subtle effects in light from stars that are light-years distant from Earth.
"It's like taking a wooden ruler and trying to measure to within a billionth of an inch of accuracy," says Steve Vogt, the UC Santa Cruz astronomer who designed the powerful Hamilton Spectrograph through which Butler and Marcy made their observations at Lick Observatory. "If you stand the ruler on its edge, it will become shorter because of the weight of the ruler. You can't pick up the ruler -- the heat from your hand will expand it.
"We need an accuracy of one part in 100 million to see these planets. It's a horrendous calibration problem."
It was a problem that astrophysics -- the blending of astronomical observations with laboratory physics -- had been unable to solve. But a third science changed the planet-finding equation, thanks to Butler's chemistry degree.
Butler knew that the element iodine, like all matter, casts an unchanging, signature "shadow" when light is shined through its gas form. On Butler and Marcy's spectrograph, the shadows are cast in the form of a series of tiny, rulerlike lines.
Previously, astronomers had included guesswork calculations in their research to account for the shifting, shrinking, distorting effects created by changes in their spectrographs. But Marcy and Butler were able to measure the constant they were searching for -- the rocking back and forth of distant stars -- against another constant: the immutable telltale lines cast by illuminated iodine gas.
While it seems self-evident now, Butler had to spend a year puttering in a chemical lab to come up with their iodine measuring stick. Now, even competitors say the trademark blown-glass iodine cell that Butler and Marcy cast their starlight through makes the San Francisco astronomers' observing techniques the most precise in the world. NASA's planet-search program recently paid Marcy and Butler $200,000 for a version of the cell, which cost only $400 to build.
But for all the technical wizardry involved in this type of search, peers credit the success of Butler and Marcy to the dogged determination that led them to spend thousands of hours in chemistry labs, university computer rooms, and telescope observation rooms. That their peers saw them as the scientific equivalent of UFO buffs didn't make things easier.
"You'd go to the dining room for dinner, and other astronomers would say, 'We're looking for high red-shift galaxies,' and we'd say, 'We're looking for planets,' and they'd sort of look back at their food," says Chris McCarthy, a UCLA doctoral candidate who did graduate work with Butler and Marcy at S.F. State. "At the time, that was only slightly better than the people looking for signals from life in outer space."
Butler recalls reading a planet-search paper at one conference where attendees literally laughed out loud at him.
"I'd been laughed at, but it didn't bother me because I knew I was right, and I was quite sure that if I continued on this track, I would be shown to be right. And in fact that I would win the race. So yeah, we stuck our neck out quite a lot, but we were fairly confident.
"It's not that we were so smart. There are a lot of people out there smarter than us. But we don't like to lose. We are quite confident that we will do what it takes to win."