Recently a few such planets have been found, but most are Jupiter-sized and none seems suitable for life. The search, however, goes on, and you can find out about its current status (and much more) on the Planet Quest web site of NASA's Jet Propulsion Lab (JPL). One recent discovery (15 April 1999) has been the system of upsilon Andromeda which appears to contain at least 3 planets. For another recent discovery associated with a distant planet, click here.
Planetary Transits (added December 2010)
In recent years a second mode of searching for planets has emerged: observing the transit of an "exoplanet" in front of its star, like the observed transit of Venus, and also its passage behind the star. Such events require the Sun-star line to pass close to the orbital plane of the planet, and since orbital planes are randomly oriented, only 1% or fewer were found with this property. When the planet passes in front of the star, the light output dips slightly, and with relatively big planets (or close orbits), this can be observed.
This method has an important virtue: while the wiggle of an orbit depends on the
mass of the planet (relative to that of the orbited star), the dimming depends on the size of the planet, the radius of the dark disk passing in front of the star. If both wiggle and dimming are observed, one can estimate both mass and radius (and hence volume) of the planet, and from those its density.
That is very important for the search for habitable planets: life based on chemistry similar to life on Earth requires, not only temperature in (or near?) the range of liquid water, it also presumably needs a star with a well-defined surface, solid or maybe watery. Most planets observed so far have masses comparable to those of Jupiter and Saturn, and often are close to their star and too hot. Alien astronomers would have a hard time discovering Earth: because of the one-year orbital period, it would take many years of observation to analyze the orbital wiggle (which would also contain periodicities of other planets) and because Earth is so far from the Sun, the dimming during its transit is very very small.
However, there also exist "M-class stars", dwarf stars maybe 20% the mass of the Sun and redder. Actually, at least close to us, there exist about 10 times such stars than Sun-size ones. Being smaller and dimmer than the Sun may not bar life - it just means that to be in the temperature range of liquid water, a planet must have a smaller orbit and shorter orbital period, factors which help track the wiggle and increase the chance of an orbit transiting the Sun-star line.
The "Kepler" spacecraft, a 1-ton instrument sharing the Earth's orbit around the Sun, was launched March 7, 2009 to help detect and observe extra-solar planets, especially those with observable transits. It has a 95-cm telescope and extremely sensitive detectors, which can scan and resolve a section of the color spectrum. By observing the star while the planet passes behind it, one can get the pure spectrum of the star itself, and subtracting that from light with the planet offside gives (within limits) the spectral signature of the planet. The "Eearth" array, a battery of automated telescopes atop Mt.Hopkins, Arizona, scans a selections of M-type stars, searching for transiting planets.
So far a small number has been found, including a relatively small planet which may be covered by ocean. Stay tuned.