Our Sun is just one of billions of stars that fill our galaxy. While it isn't entirely average (it's a little bigger and a lot more rich in heavy elements than average), it isn't so unusual that there are dozens of similar stars in our local celestial neighborhood. With so many similar stars, it is scientifically natural to ask if the processes that lead to our Earth occurred in these other star systems as well. Beginning in the early 1990's scientists started looking for Doppler shifts in the light of stars that could be an indication these stars are getting pulled backward and forward by the gravitational attraction of their planets. In recent years, scientists have also begun to look for planets via the very slight eclipsing they cause if they pass directly in front of their host star. Between these two techniques, astronomers have discovered thousands of planets orbiting nearby stars. This is dramatic progress. Before 1995, we knew of no planet orbiting a star other than the Sun; now we know of thousands. By some estimates, it is believed that most stars in our Galaxy have at least one planet. A new field of science has exploded onto the scene.
The first planets discovered didn't actually orbit a normal star. In 1992, planet sized chunks of rock (of several Earth masses) were found orbiting the pulsar PSR B1257+12. They were detected by a radio telescope. The exquisite accuracy of pulsar timing data allowed the planets to be identified because of their very subtle affects on the pulsar period. These objects are likely second generation objects that coalesced from the debris of the pulsar's parent star and its supernovae blast. While these objects don't help us understand planet formation in general, and while they are extraordinarily unlikely to host life, they did show that planets can form in places other than in our Solar System.
The astronomical community would have to wait three more years, until 1995, for the next planet to be announced: 51 Pegasi b. This planet was like nothing solar system formation models of the time had predicted; it was large (roughly 0.5 Jupiter masses) and extremely close to its host star. Think of it — a gas giant planet orbiting its star twenty times faster than Mercury orbits the Sun! Until that discovery, models based on our Solar System all assumed that gas giants always existed far from their host star and rocky worlds lived close in. This system, and tens of systems that would soon follow, showed that large planets, called hot Jupiters, can have orbits smaller than Mercury’s.
In the years to follow, scientists would learn that planets come in nearly endless orbits and locales, and our prior limits on where planets "should be" underestimated what the universe was capable of forming. Today it is estimated that 30% or more of all stars have planets.We now know the Solar System is not a fluke; planets form as a natural byproduct of star formation and they are ubiquitous.
Even with all the planets so far discovered, many questions remain to be answered: Can planets form around any kind of star? Planets have thus far been found around everything from small red dwarfs to huge yellow supergiants. They've also been found in multiple binary star systems, including multiple planets in binary star systems! The only real limit may be the presence of heavy elements. So far, no planets have been found in globular clusters or other populations of so-called metal poor stars.
How frequent are Earth-like planets? The technology to find Earth-sized planets only came into use in 2009 and it typically takes several years of data to confirm small planets in the types of long orbits expected of Earth-like planets. By late 2011, the Kepler mission found some of the first Earth-sized planets. Around this time it was announced that the first planet orbiting in the habitable zone of a Sun-like star had been discovered!
The bizarre details of the first wave of extrasolar planets should not blind us from the larger truth. Science has taken a major new step in the Copernican revolution. We live on a rocky cinder sheltering in the glow of a nearby star. It's natural to wonder how rare such planets are in the universe and whether the history of other planets involves the creation of life. For the first time we have evidence to take us beyond pure speculation. It will be some time before we know how common planets are. It will be even longer before we know the conditions on these planets and their likelihood of harboring life. But the impilcation of life elsewhere in the universe so profound that researchers are energized to tackle these challenging observations.