Continuing the Search
In 2000, astronomers were planning more than a dozen such missions, centered on advances in three of the basic observational methods. One advance was improved sensitivity to transits. Extreme sensitivity is essential for the transit technique because terrestrial planets are so small in comparison with their star. Thus, the dimming of the star's light when such planets transit it is barely perceptible. Another advance involves greater precision in detecting Doppler shifts in a star's spectrum caused by the gravitational pull of planets. Terrestrial planets have such small masses that they create only minute changes in the motion of their parent stars and thus only tiny shifts in their spectra. And finally, the most intriguing possibility was the development of better methods to observe planets directly.
To improve the transit method, a small NASA spacecraft called Kepler was scheduled to be launched into Earth orbit in 2003. Kepler's mission was to monitor some 100,000 stars from its position above Earth's atmosphere. There, Kepler would avoid the blurring effect that the atmosphere causes in Earth-based telescopes, the interruptions caused by seasonal weather, and the cycle of days and nights to make highly accurate brightness measurements.
Astronomers were also hoping to measure the actual shifts in a star's position instead of just the spectral shifts because of its motion. To do so, they were developing space-based interferometers. An interferometer is an instrument that combines light from two or more telescopes, creating the resolving power of a much larger telescope. With this high resolution, astronomers can measure the positions of stars with unprecedented accuracy. Interferometry has long been widely used with radio telescopes on Earth. However, since stars are not strong radio-wave emitters, the search for tiny stellar wobbles caused by planets required interferometry using visible light, which is much more difficult. NASA's Space Interferometry Mission (SIM), which was to be launched around 2006, and the European Space Agency's Darwin mission, planned for launch after 2008, were to be the first to test this new technology.
In theory, interferometry can also be used to obtain direct images of planets. An instrument can be set up in such a way that unwanted light from a star is cancelled out, or “nulled,” allowing light from a planet to be seen. Astronomers led by Roger Angel at the University of Arizona in Tucson reported in 1998 on an experiment that they had conducted using a nulling interferometer. The astronomers bounced light waves from two mirrors in the Multiple Mirror Telescope with such precise positioning that the peaks from one light wave arrived at a detector at the same time that the troughs of the other light wave arrived, causing the waves to cancel each other out.
Astronomers were also hopeful that computer analysis of light from distant stars could provide a way of detecting planets more directly. In December 1999, astronomer Andrew Cameron and his colleagues at the University of St. Andrews in Scotland isolated a portion of the spectrum from the star Tau Bootis, located about 50 light-years from Earth. They used computer analysis to separate Tau Bootis's light from the light of what they said was a planet orbiting the star. As of early 2000, however, other astronomers had not been able to duplicate their efforts, so the finding remained unconfirmed.
As the year 2000 began, many astronomers were hopeful that within a decade, Earthlike planets would be found oribiting other suns. The new technologies for planet hunting may not tell astronomers whether the human race is alone in the universe, but astronomers hoped at least to answer the question of whether Earth is unique.
