One of the most effective means employed by astronomers to search for planets around stars other than our sun is to try to detect the velocity shift of the star as the planet moves about the star in its orbit. This well proven technique employs an instrument called a spectrograph to measure the small shifts in wavelength of features in a star's spectrum. The shifts are due to the familiar Doppler effect and are directly related to the changes in the star's velocity. But these velocity shifts are very small, and in order to measure them with great accuracy it is necessary to employ some scheme to calibrate the wavelength of the light in the spectrograph. One relatively new technique is to use interferometry, measuring the "interference" of two beams of light when they come together. At a particular wavelength, the two beams of light will cause fringes to appear which are spaced according to their wavelengths. By measuring the spacing of these fringes, wavelengths can be determined very accurately.
Dr. Jerry Edelstein of the University of California-Berkeley and Dr. James Lloyd of Cornell University are applying this technique to a spectrograph that works not in visual light, but in the infrared. Infrared light is very useful in the study of cooler and smaller stars which are more numerous than hot stars. Finding planets around these stars will give us a much clearer picture than we currently have of how planets are actually formed and evolve. Since these stars are cooler than the massive hot stars that have been most frequently studied, their habitable zones, the region at the distance from the star where conditions may be suitable for life to begin, are much closer to the parent star. Most of the planets discovered so far have been "gas giants" like Jupiter and are located too far from their star for biology as we know it to exist. But with this new instrument, smaller earth-like planets may be detected in the habitable zone of cool stars. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
We have developed an unconventional method of precisely measuring the color within light and also of the changes of color in light. This method lets us hunt for planets around other stars and determine exactly what the stuff in the stars is made of. Physical scientists often use color to determine the composition and condition of things. For example, astronomers use color to tell what type of gases are in stars and how hot or cold these gases are. Different gases glow in different colors and these colors and their purity change with temperature. Furthermore, astronomers can tell if a star is moving toward or away from us by seeing whether its colors change, becoming more red or blue, in the same way our ears can tell whether a train is approaching us or zooming away from the distorted sound the train’s whistle. In fact, by measuring very tiny shifts in color, astronomers can tell if a star is wobbling because a circling planet is tugging on its star. Many hundreds of planets have been discovered around other stars this way. Because small planets, like our Earth, only tug a small amount on their host star, we need instruments that can detect very tiny changes in color. In our research, we built an optical instrument that sends light through an echo chamber and then through a convention spectrograph, like a prism that spreads out the colors in a rainbow on a sensor. The echo chamber adds a thin striped pattern to the rainbow, mixing with the thin lines of color from the star. The mixed lines make a wide and easy to see Moire pattern, like the patterns we see driving by picket fences next to the road. The Moire pattern can be seen even if the color lines are too narrow to be detected. That means more precise color measures can be made with smaller, less expensive instruments. We tested our instrument at the Palomar Observatory by looking at stars and proved that this method increases color precision by an important factor of ten. Our increased precision can now be applied to a host of astronomical observations. Our project was also used as a training ground for new scientists, engineers and technicians. Three post-doctoral workers, one graduate student, and eight undergraduate students have worked on the project to design, build, test and operate the new instrument and then analyze its data and report the results. These workers were trained in the fields of astronomy, optics, mechanics, electronics, computer science, programming, and data analysis.
