Of all of the subdisciplines of astronomy and astrophysics, the field of exoplanet discovery has hit grown the most rapidly over the past few years. Exoplanets, which are planets that have been detected orbiting other stars, were first discovered around 1991. By 1995, astronomers had identified a dozen confirmed exoplanets, and by the year 2000 this number had more than quadrupled. As of late October 2013, more than 1000 confirmed exoplanets had been discovered with more than half of those discovered in just the last four years.

So how did we get to this point? One of the primary reasons for the enormous successes in exoplanet research is the Kepler Telescope, a NASA space-based observatory tasked with observing a section of the sky in the band of the Milky Way and monitoring the brightness of the over 150,000 stars continuously for four years. Kepler, like many other planet-hunting telescopes, searches for variations in the brightnesses of the stars it observes—variations that could be the result of a planet or planets passing (or "transiting" in astronomy-speak) between the parent stars and the Earth. Observing multiple transits and obtaining precise measurements of the transit durations as well as other transit characteristics allow astronomers to glean properties of a planet and its orbit.

As with all fields of science, the more precise the data the more definitive the conclusions. As of early November 2013, astronomers had used ground-based telescopes to confirm the planetary status of 167 of Kepler's planetary candidates. Astronomers have only 3,538 more known candidates to go. Unfortunately for exoplanet scientists, hardware malfunctions that occurred over the last 12 months have ended the main mission of the Kepler telescope; however, mission specialists are scrambling to see what other science can be done with the craft even though Kepler is no longer able to point precisely enough to continue the original mission of identifying planet transits. From now forward, astronomers will use other current and future projects to fill Kepler's shoes.

As exoplanet candidates are confirmed and physical characteristics of these bodies are determined, astronomers are constantly dazzled by the diversity of worlds they are finding. For example, the double-star system known as Kepler-47 has not one but two planets orbiting around the central pair of stars, and recent data analysis shows that there is almost certainly a third planet. In addition, it has been found that the triple-star system that goes by the lavish name HD 188753 may harbor a planet in orbit around its largest star. Ten years ago, astronomers would have dismissed the notion that a planet could form, let alone survive, in a multiple star system in which the gravitational tug-of-war from all of the stellar components was expected to disrupt the formation of a small body such as a planet. But as we continue our exploration, we find more and more examples of planets orbiting in multiple-star systems.

Still other planet discoveries have surprised us in further ways. Before any exoplanets had been discovered, theories for the formation of planetary systems all produced ones like our own: theorists widely assumed that gas giant planets like Jupiter and Saturn would form only in the outer portion of the solar system and that rocky planets like Earth and Venus would constitute the inner planets. This line of thinking was decisively refuted when some of the first exoplanets discovered turned out to be massive Jupiter-like planets orbiting very close to their parent stars. A perfect example of this is the Jupiter-like exoplanet WASP-12b. It orbits so close in to its parent star that not only does it take the planet only one Earth-day to orbit its star but it is also heated to over 2,200°C (4,000°F) on the side of the planet that always faces the star, just as one side of the Moon always faces the Earth. Astronomers believe that these "hot Jupiters" likely formed much farther out from their parent stars and migrated inward over time.

One of the most surprising (and more recent) results in the field of exoplanet hunting was the announcement that planets like our Earth, those made of rock and at the right distance from their stars to support liquid water, are likely very common throughout our Milky Way. Recently, a team of astronomers analyzing Kepler data set out to determine just how common a planet like Earth might be. In their analysis of the light curves (how the brightnesses of stars vary with time) of 42,000 Sun-like stars, they found signatures of dimming in the light curves indicating the presence of 10 Earth-like planets orbiting within the habitable zones of these stars. Ten out of 42,000 may not seem like many but once the team accounted for how many of these types of planets were almost certainly missed because of systematic biases in the observations, they determined that 22% of Sun-like stars in the Milky Way are statistically likely to host Earth-like planets. Whether you estimate that the Milky Way is host to only 10 billion or as many as 100 billion Sun-like stars, these results mean that billions of Earth-like planets exist in our own galaxy in orbits with distances such that they are heated by their host stars to temperatures at which liquid water (and possibly life as we know it) could exist on their surfaces.

Whether 2 or 20 billion, that's a lot of Earth-like planets, and when you then think about how many galaxies exist in our universe and what the chances are for life outside our own blue marble, you have to marvel at the prospect. This is an exciting time in which to live, and we can only imagine what future discoveries await us.

Billy Teets is Outreach Astronomer at the Vanderbilt Dyer Observatory.

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