Combing the Universe for Planets
Tuesday, April 8, 2008
Science Update - A look at CfA discoveries from recent journals

There are currently 228 known planets around stars other than our sun, and all of them were discovered in one of only three kinds of observations: the periodic wobble of the "color" of the light from the parent star as the planet orbits around it (the Doppler method), the periodic dimming of the parent star's light as the planet crosses between it and our view of the star (the transit method), or via its chance passing near our line of sight to a more distant star whereby it bends that star's light (the gravitational lensing method). The large majority of the discoveries were made with the Doppler method because it demands very little from the geometry or alignment of distant solar systems. All of the newly discovered planets help to shed light on the earth, and how it and our solar system were formed. Unfortunately, no earth-sized planets have ever been discovered, in part because the wobble produced by an earth-sized planet orbiting a solar-type star is minuscule - only centimeters per second, ten times smaller than the meter per second (a few miles per hour) precision now attainable. Thanks to research at the CfA, however, this is about to change dramatically.

The current issue of the journal Nature devotes a remarkable three articles to a revolutionary new laser device to enable greatly improved measurements of minute velocities, down to one centimeter per second, in astronomical sources. The main paper is the research report of CfA scientists Chih-Hao Li, Alexander Glenday, David Phillips, Dimitar Sasselov, Andrew Szentgyorgyi, and Ron Walsworth, along with three colleagues. (The other two articles are about the great significance of the new work.)

The CfA has long been renowned for its work in laboratory astrophysics, and in particular for producing some of the best timekeeping devices in the world: hydrogen-maser clocks, used by NASA (for example) to track its satellites, as well as by radio astronomers around the world to make precision measurements of cosmic phenomena. The CfA maser group has continued to develop advanced technologies over the years, and to turn them into new tools to probe the heavens. The new laser device -- which they dub an "astro-comb" -- was designed in consort with observational astronomers and physicists, typical of the powerful, interdisciplinary collaborations made possible at the CfA. The instrument produces an extremely fine, extremely stable comb of many reference wavelengths of light that can be used to compare against single (or multiple) wavelengths from astronomical sources... like the light from a slightly wobbling star. The device uses a laser flashing brief bursts of light every billionth of a second to generate this comb of wavelengths, and sophisticated atomic clocks reliable to about one second in ten thousand years to anchor those wavelengths to a solid reference.

The new device will travel to the MMT observatory at Mt. Hopkins, Arizona, in May for demonstration tests. If it works on the telescope as well as it does in the laboratory, it will be able for the first time to spot an earth-sized planet in an earth-like orbit around a sun-like star. Not only that -- the comb will be able to make significant contributions to the study of dark matter, and might even measure directly the slowing down of the expanding universe. Not least, this new instrument demonstrates the dramatic advances possible when combining technical savvy with physical insight and astronomical skill, and the continuing leadership of the CfA in all three fundamental areas.