Exoplanets are planets beyond our solar system, usually orbiting some other star or stellar remnant. The first to be discovered was Gamma Cephei Ab, about one and a half times the size of Jupiter, within the Errai binary star system in the northern constellation of Cepheus (the King), approximately 45 light-years from Earth. It was spotted in 1988 by Canadian astronomers at Victoria and British Columbia Universities.
Since that time, over 1700 more were confirmed by 2014. A little less than a third are parts of multiple planetary systems, while some are free floating, outside of any stellar orbit. The nearest so far discovered is about 12 light-years away.
NASA's Kepler space telescope spent four years scouring our galaxy for potentially habitable planets, tracking 156,000 stars with snapshots every 30 minutes. In its four-year mission, Kepler detected 4,229 possible exoplanet candidates, and, though not yet confirmed, astronomers are confident that at least 90% of them are genuine.
Kepler went offline in 2013 when its stabilizing system failed, after having searched only a tiny fraction of our galaxy - a patch of sky which includes part of the constellation Cygnus (the Swan), also called the Northern Cross. The planets it revealed were limited to those which transited (crossed in front of) their host stars from Kepler's vantage point. Vastly more exoplanets are likely to be found in the future.
Astronomers estimate the Milky Way's star population by measuring relative mass or luminosity (brightness), and most set the number at 100 to 200 billion stars. However, our instruments aren't sensitive enough to measure many smaller dwarf stars, so the number could be as high as one trillion in our galaxy alone.
What's truly exciting is that Earth-like planets are "relatively common throughout the Milky Way," according to the University of Hawaii's Dr. Andrew Howard: each star hosts at least 1.6 planets on average. A little over one out of five stars is of the same class as our sun and hosts an Earth-sized planet within the "habitable" zone - a distance at which liquid water can exist, potentially hosting life.
The research team based their calculations by focusing on 42,000 G type yellow stars similar to our sun in size and heat production.
The most conservative estimate of 100 billion stars (with 1.6 planets, one-fifth in the habitable zone), means our galaxy contains over 160 billion exoplanets, 32 billion of which orbit sunlike stars within the water-sustainable zone.
There could, however, could ten times as many, depending again upon the number of as-yet-undetectable smaller stars. The estimates also don't account for free-floating rogue planets, found outside any stellar orbit; these may outnumber stellar-bound exoplanets by as much as 50 percent, or two rogue planets for every star in the Milky Way.
That said, Earth-sized planets within the habitable zone may not necessarily be able to support life as we know it. Exoplanet diversity is, say researchers, "stunning"; some are much bigger than Earth, solid rocky giants with thick atmospheres like Neptune, dense gas giants like Jupiter, fantastically light, airy gas giants like Saturn (which could float in water, given a large enough ocean), or perhaps even dead stars that hardened into solid diamond planets (though maybe not). Some may also have dense atmospheres that cook the planetary surfaces like our irascible twin Venus. However, it's likely that many are rocky and capable of supporting liquid water.
We inhabit, it seems, a very crowded galaxy indeed.
Sources: Number Of Alien Planets Confirmed Beyond Our Solar System Nears 1,000, Data Shows, September 29,2013, Mike Wall, Space.com
Astronomers answer key question: How common are habitable planets?, Press release, November 4, 2013, Robert Sanders, Berkeley University
One in Five Stars Has Earth-sized Planet in Habitable Zone, News Release, November 4, 2013, Erik Petigura, W. M. Keck Observatory