The discovery was made by astronomers Geoffrey Marcy, a professor of physics and astronomy at San Francisco State University, and Paul Butler, a postdoctoral researcher with a joint appointment at the University of California at Berkeley and San Francisco State University. They were the same astronomers who confirmed the first discovery of a planet outside our solar system, at 51 Pegasi.
The finding is significant for several reasons. After the planet orbiting 51 Pegasi was discovered, scientists couldn't be sure that planets were commonplace around stars, as they only had a single observation to go on. With the new discoveries, planets do not appear to be rare objects in the universe.
Secondly, the fact that the planets have temperatures at which liquid water is likely to occur is exciting to those looking for support of life existing elsewhere than just on our own Earth.
Using the 120-inch Shane reflector telescope at the University of California's Lick Observatory the astronomers used a highly accurate spectrometer to detect Doppler shifts in the stars' spectral lines, enabling them to determine the stars' radial motions to an accuracy of three metres per second. This accuracy allowed them to detect the tiny wobbles in the star's motion caused by the orbiting planets. They achieved this extraordinary accuracy by placing a glass chamber filled with iodine gas in front of the telescope so that light from the star passes through and is partly absorbed. The known absorption spectrum of iodine is then superimposed on the star's true spectrum, allowing them to determine the Doppler shift. By averaging 500 such measurements over time, a very precise determination of the star's motion can be found.
More new planets are expected to be announced in the near future, now that the instrument to detect them has been developed. Marcy and Butler started the program in 1987 and have been monitoring 120 stars between 10 and 100 light years from the Earth.