Planet Hunting, Down to Earth

Planet-Hunting / by Lee Billings /

The emerging technology of laser frequency combs may usher in a new golden era of ground-based astronomy.

Credit: European Southern Observatory

Two teams of researchers are now competing to develop a device that could profoundly change our understanding of the universe…but you’d be forgiven if you mistook it for a vaguely menacing hair-restoration product. Called a “laser frequency comb,” these are special laser systems that rapidly emit pulses of light across a wide range of frequencies or colors. In a plot of the emitted light, each distinct frequency appears as a peak; collectively, all the frequencies resemble a fine-toothed comb. And by examining starlight through the teeth of a laser comb, astronomers could begin finding Earth-like extrasolar planets on the cheap using ground-based observatories rather than expensive space telescopes.

A star’s spectrum, its component colors of light, can reveal whether or not it has planets circling it. Every planet gently tugs on its star as it orbits, pulling the star toward and away from us in a regular pattern that we can detect via subtle periodic changes in the star’s color. Similar to how a train whistle rises in pitch as it approaches or falls in pitch as it departs, a star’s light becomes bluer as it draws near us, and redder as it recedes. This is called a “Doppler shift.”

Doppler shifts are how the majority of planets known beyond our solar system have been discovered, but these worlds resemble Jupiter or Neptune—giant planets whose large gravitational tugs are correspondingly easier to see. Planets like Earth are far more difficult to detect via Doppler shift. Due to this difficulty, experts have believed for years that the only sure way to find extrasolar “Earths” in the near future would be via expensive space-based telescopes that would detect the dips in starlight caused by planets transiting the faces of their suns. This is how the recently launched Kepler satellite looks for planets.

This method only reveals the discovered planets’ diameters and orbits—to find other vital information about them, like their mass, astronomers must rely on Doppler shifts, which rely in turn on precisely calibrated light-measuring devices called spectrographs. But even the best current calibration methods unavoidably introduce inconsistencies into spectrographic measurements. Further, each spectrograph is unique and custom-built, causing problems for sharing and comparing data between telescopes.

Laser combs could change all that by providing a standardized—and exceedingly precise—way of measuring Doppler shifts from stars’ spectra. Not only would this make it possible to study planets Kepler finds in more detail, it could potentially allow searches for Earth-like worlds to take place from the ground, at a much lower cost.

“We’re actually starved of spectrographs right now,” says Dimitar Sasselov, an astronomer leading the laser comb team based at the Harvard-Smithsonian Center for Astrophysics. “With Kepler, we’ll have more planet candidates than we can actually confirm. The more spectrographs equipped with a laser comb, the better, because then you can easily combine and compare observations from different telescopes.”

Much work remains to be done before laser combs are ready for prime-time astronomy, though. To ensure it properly calibrates a spectrograph, a laser comb must itself be calibrated. This means syncing its ultra-fast laser bursts with the time signature from an atomic clock, a task made less daunting thanks to the US Air Force’s public network of GPS satellites. More problematically, the typical output of a laser comb doesn’t cover enough of the spectrum to be very useful to astronomers, and also has frequency “teeth” so close together they’re incompatible with existing spectrographs. Both the Harvard-Smithsonian team and their German competitors at the Max Planck Institute for Quantum Optics have painstakingly overcome these obstacles in the lab, but the resulting devices are fragile.

“These aren’t yet ‘turn-key’ products—they’re the subjects of laboratory development,” says Bill Cochran, a veteran planet-hunter at the University of Texas, Austin. “They require a small army of technicians to make them work, but I need something where I can just flip a switch, and it must work consistently and reliably for ten years!”

Consequently, both teams have begun testing their laser combs on actual observational equipment—the Harvard-Smithsonian team is using a telescope on Mount Hopkins in Arizona, while the Max Planck researchers are using a European telescope in La Silla, Chile. Next summer, just in time for Kepler’s first batch of Earth-like planet candidates, the Harvard-Smithsonian team plans to link their laser comb to a state-of-the-art spectrograph at the giant William Herschel Telescope in the Canary Islands. The Max Planck team also plans to bring its laser comb technology to the next generation of very large telescopes.

If either team succeeds in making laser combs practical for astronomy, the increased precision they would bring to observations will be nothing short of revolutionary. In addition to enabling the ability to detect Earth-like planets from the ground, laser combs would also allow much more in-depth studies of stellar activity and a better knowledge of our galaxy’s structure. They could even unlock the secrets of dark matter and dark energy, the elusive components of our universe that represent one of the greatest unsolved mysteries of modern science.

“It will affect not only our understanding of extrasolar planets and stars and galactic dynamics, but also cosmology.” Sasselov says. “Imagine being immersed in an ocean not of water but of light, surrounded by light waves that are all traveling in different directions and have different wavelengths, and trying to measure them all. Astronomers do this all the time—the waves of light that come from objects are all we really have to study them. And suddenly, you can see clearly through it, as opposed to having a blurry picture. This is what being able to calibrate using laser combs means.”

Originally published May 26, 2009

Tags innovation space technology

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