The search for worlds outside our solar system has come a long way since the first exoplanets were confirmed in the early 1990s. Since then, the average rate of alien-world discoveries has shot from about three per year to between fifty and a hundred per year in the last five years. As of the end of 2012, with the tally standing at 854 newfound worlds and reports of new detections being announced nearly every week, thanks in large measure to NASA's Kepler space telescope, astronomers are calling this the golden age of exoplanet discovery.
Now the race is on to find Earth's twin, the elusive Earth-size planet in the habitable, or "Goldilocks," zone around a star where liquid water can exist—and experts believe we may hit the cosmic jackpot soon.
"I think we are very close to finding a potential 'Earth 2.0'—possibly next year," said Abel Méndez, a researcher with the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.
In 2012 astronomers came closer than ever to zeroing in on an earthly doppelganger, or what Méndez refers to as "terran" planets—or at the very least a planet considered potentially habitable.
"However, we are far from confirming the habitability of any of these planets until we have the capability to observe their atmosphere, but that will take many years," Méndez said. "The big goal now is to find an Earth-size planet in [its] star's habitable zone—something more similar to Earth."
But until then, here are five of the most interesting exoplanetary discoveries of this past year:
Tau Ceti e and f. At less than 12 light-years away, the Tau Ceti planetary system, announced in December, is arguably one of the most exciting and important exoplanet discoveries of 2012. Tau Ceti is only the fourth such system known with five planets. Two of the outer planets, four and six times the mass of Earth, may have just the right conditions to support liquid water and even life.
While all five planets circle their star closer than Mars does our own, since their parent star's brightness is only about 55 percent of our sun, both Tau Ceti e and Tau Ceti f fall within the habitable zone. These alien worlds now stand as the closest potentially habitable exoplanets to Earth that also happen to call a sunlike star home. (Read about other stars that may support life.)
Alpha Centauri Bb. The October detection of an Earth-size planet orbiting Alpha Centauri B, only 4.3 light-years from Earth, makes this the closest exoplanet to our solar system. Hugging its star at only a few million miles away—ten times closer than Mercury is to our sun—this rocky world is too hot to be considered habitable, but it opens the possibility that the neighboring Alpha Centauri triple-star system may be home to other planets at the right distance to support liquid water. (Read more about Earth-size planets.)
Gliese 667Cc. In February astronomers announced the discovery of the first exoplanet confirmed to orbit within the so-called habitable zone of a star—only 22 light-years from Earth. With a mass 4.5 times that of Earth, the rocky exoplanet is classified as a super-Earth and orbits its parent star in only 28 days. Despite its proximity, the planet is bathed in warming infrared radiation and receives only 10 percent less light than we receive from the sun, which may mean liquid water can exist on its surface.
Kepler-42 b, c, and d (aka KOI-961 b, c, and d). Sitting 126 light-years away in the constellation Cygnus, these are the smallest confirmed exoplanets found to date. Discovered in January, the Kepler 42 planetary system consists of three rocky worlds that come in sizes comparable to Venus and Mars, orbiting a tiny star only 70 percent bigger than Jupiter. These tiny, record-setting worlds are all smaller than Earth, but they hug their dim star so closely that it takes them only two days to circle it. Their tight orbits make them far too hot for even liquid water to survive on their surfaces. (Read more about these diminutive planets.)
Kepler-34b and Kepler-35b. A number of planets with double suns, reminiscent of the fictional world of Star Wars' Tatooine, were found in 2012. But two Saturn-size worlds—each orbiting their own binary star system—are real standouts.
Kepler-34b is a gas giant with almost 22 percent the mass of Jupiter and orbits its host stars in 289 days. Its orbit takes Kepler-34b as far away from its stars as Earth is from our sun. The exoplanet lies 4,900 light-years from Earth. Meanwhile Kepler-35b is about 13 percent of Jupiter's mass, has a year 131 days long, and orbits a pair of sunlike stars 5,400 light-years distant.
Astronomers once believed that environmental conditions around binary stars would be too chaotic for stable planetary orbits, but these two discoveries now demonstrate that "double sun" worlds may in fact be commonplace. (Related: "Tons of Tatooines: Planets With Two Suns Common?")