June 9, 1998
The discovery of planets orbiting other stars has long been a dream of astronomers. It has been only within the last few years that solid evidence for planets beyond our own solar system has been produced. Careful analysis of the light from a few selected stars has resulted in the discovery of a dozen extra-solar planetary systems, with Jupiter-sized planets. According to Dr. Nordgren, "Right now the hunt for extra-solar planets is like when Lewis and Clark returned from the Pacific Ocean almost two hundred years ago. Now that we know it can be done, the rush to find what else is out there begins."
Though extra-solar planets cannot generally be imaged directly because they do not emit their own light, they can be inferred from the light from their parent stars using the "Doppler effect". By spreading starlight out into its component colors and examining the unique dark lines produced by different elements in the star's spectrum, astronomers measure the speed at which a star is moving relative to the Earth. If these dark lines are seen to shift to redder colors, the star is moving away from us; if they shift to bluer colors, the star is moving toward us. A periodic pattern of redshifts and blueshifts, produced by the star's motion about the center-of-mass of the star-planet system, is the sign that a planet may be present. The larger the wavelength shift, the larger the planet (and/or the smaller the orbit of the planet about the parent star). An instrument with the ability to precisely measure tiny shifts can be valuable for detecting tiny (i.e. Earth-mass) planets. Currently the instruments in use by at least a half-dozen teams are only capable of discovering very large gaseous planets, like Jupiter, and are most sensitive to those planets found very close to the parent star. The discovery of a planet like our own Earth, with a similar mass and a similar distance from its sun, is the ultimate goal for many astronomers engaged in the search for other worlds.
The new interferometry device of Hajian and Nordgren is based on a concept developed almost a hundred years ago by America's first Nobel laureate, Albert A. Michelson. The system is based upon the idea of splitting light from a star into two beams and then recombining it, but only after first introducing a variable path-length delay in one beam. The resulting interference pattern between the two beams can be analyzed by a computer to reconstruct the spectrum of the initial starlight. Fire a laser beam of a known precise wavelength through the system and the star's spectrum can be calibrated to a precision determined by the frequency stability of the laser (modern lasers used in the laboratory have very high stability). This procedure will extend the precision with which astronomers can measure very small velocities.
This technique of measuring the spectrum is called "Fourier Transform Spectroscopy", and has long been used by astronomers to determine very precise wavelengths. Traditionally, however, this method has not been very efficient, and only very bright sources such as the Sun could be observed. Dr. Hajian overcame this limitation by coupling an FTS with a traditional spectrograph, the device normally used for measuring velocities of astronomical sources. With this hybrid mFTS system, the velocity precision of an FTS is retained, while the sensitivity to faint sources characteristic of normal spectroscopy is achieved.
Preliminary observations of Betelgeuse, presented in San Diego today, confirm the theoretical predictions of the mFTS's performance. Although these test-bed observations, using a collecting area only 12cm (5 inches) in diameter, are not sensitive enough to detect Earth-sized planets around this particular bright star, the results from the prototype instrument suggest that, for a telescope 3 meters (10 feet) in diameter, planets a few times the size of the Earth could be detected around many of the stars visible to the naked eye in the night sky.
All of the equipment used for the FTS, the spectrograph, the optics for variable delay paths, the laser system, and the telescope, are components of the Navy Prototype Optical Interferometer (NPOI), an array of telescopes being built by the U.S. Naval Observatory (USNO), the Naval Research Laboratory (NRL), and Lowell Observatory, for the purpose of making ever-sharper images of astronomical objects. Dr. Hajian, who works for the USNO, realized that the equipment could also be used for increasing the accuracy of velocity measurements. Current spectrometers used by most planet-hunting teams have a minimum velocity precision of 3 meters (10 feet) per second, which is equivalent to the speed of an average person running. The new hybrid FTS is projected to achieve a velocity resolution of only 30 centimeters (one foot) per second, which is the equivalent speed of a child's crawl.
Strangely, the biggest impediment Dr. Hajian and Dr. Nordgren foresee in the discovery of Earth-sized planets with their instrument may come from the stars themselves. The surfaces of stars are boiling cauldrons of super-hot gas. This gas is constantly moving. It is possible that, at the small velocity scales needed to detect planets like the Earth, the motion of the gas itself may be the dominant effect, and detecting an Earth-size planet by the change in velocity of its star may be impossible, no matter how accurate the measuring device. But, even if this is the case, according to Dr. Hajian, we will learn valuable information about the surfaces of stars. Says Hajian, "The planet-search people may be upset, but the stellar astrophysicists will be thrilled. One person's garbage is another person's Ph.D. thesis."
The new multichannel Fourier Transform Spectrometer (mFTS) developed by Dr. Arsen R. Hajian and Dr. Tyler E. Nordgren of the United States Naval Observatory, is soon expected to be able to detect the subtle motions of Earth sized planets around nearby stars. In this figure, laser light, which follows the same path as the star light, enters the laboratory from the telescope located outside Flagstaff Arizona. The FTS splits the light into two beams and adds a varibale path length to one beam before the two are recombined. Computer analysis of the resulting interference pattern indicates that the velocities of stars can be determined to higher precision than by conventional spectrometers. Preliminary observations of the bright star Betelgeuse were presented at the American Astronomical Society meeting in San Diego, CA on June 9, 1998.
PHOTO CREDIT: Dr. Arsen R. Hajian and Dr. Tyler E. Nordgren.
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