The grand tally of planets we now know exist beyond our solar system—also known as exoplanets—has climbed past a thousand, according to the online Extrasolar Planets Encyclopaedia.
After just two decades of hunting, this symbolic milestone was surpassed with this week's announcement of the discovery of 11 new worlds by the British Wide Angle Search for Planets (WASP) detection program. (Related: "Bumper Crop of Habitable Worlds Discovered?")
While the total number of planets known to orbit distant stars has skyrocketed to an impressive 1,010, thousands of potential worlds are still on the waiting list to be confirmed, and billions more remain undetected across the Milky Way galaxy (map).
"One thousand confirmed exoplanets in 21 years is a small number considering conservative estimates on the expected number of exoplanets per star," said Abel Mendez, an astronomer at the Planetary Habitability Laboratory at the University of Puerto Rico.
"It is a great achievement of science and technology nonetheless, but just the tip of the galaxy."
The Road to a Thousand
In 1992, the presence of three planet-size masses—the first exoplanets discovered—was reported 2,000 light-years away around a dead star, pulsar PSR1257+12.
Three years later astronomers detected the first planet—a gas giant orbiting closer to its star than Mercury orbits our sun—around the sunlike star 51 Pegasi, 50 light-years from Earth. (Related: "Tons of Tatooines: Planets With Two Suns Common?")
The rate of discovery has rapidly climbed since, thanks to the development of indirect detection techniques that rely on measuring the effects orbiting planets have on their host star.
The two most successful hunting methods have been measuring a star's wobble—known as radial velocity—caused by the gravitational tug of orbiting planets, or measuring the tiny dips in starlight as planets pass in front of them, known as transit.
With the use of these techniques, in the last two decades the average rate of alien-world discoveries has shot up from about three per year to between 50 and 100 per year.
NASA's Kepler Space Telescope—which went offline this summer—has 156 confirmed planets to its credit and has close to 3,000 potential planetary candidates awaiting further analysis and confirmation.
"This is just the beginning of the era of exoplanet hunting," said Mendez. "In the coming years the number of exoplanet detections should continue to increase dramatically."
Past finds, many of which seem right out of science fiction, continue to surprise their discoverers. They include worlds with two suns, exoplanets orbiting a dead star, and even stellar systems with many rocky, terrestrial-type planets, said Mendez. (Related: "Newly Discovered Pink Exoplanet on the Lighter Side.")
Search for Earth 2.0
However, the hunt is still on for the holy grail of astronomy—a truly Earth-like world with the right size and temperature to support liquid water on its surface, and possibly to support life. (Related: "Most Earthlike Planets Found Yet: A 'Breakthrough.'")
Jaymie Matthews, an astronomer at the University of British Columbia in Canada, believes that elusive Earth-like worlds may very well be found in the existing Kepler data set.
"At this point we may be close to finding an Earth-like world, and it could just be a matter of trolling through the data sets and doing follow-up observations with ground-based observatories," he added.
According to the latest calculations based on Kepler surveys, astronomers believe that a hundred billion planets populate the Milky Way galaxy.
Of those, as many as 17 billion may be rocky, Earth-size planets with conditions suitable for liquid water on their surface. (Related: "MacArthur Genius Searching for Signs of Life on Exoplanets.")
The next step will be the use of improved monitoring technology to focus on worlds in habitable zones, said Matthews.
NASA's TESS (Transiting Exoplanet Survey Satellite) mission, scheduled for launch in 2017, will most likely take over exoplanet-hunting duties from Kepler.
"This exoplanet mission, in combination with the James Webb Space Telescope, may help not only reveal ever smaller exoplanets, ones that orbit at larger distances from their parent star, but even gas giants and their moons," he explained.
By combining information obtained from radial velocity and transit measurements, astronomers will be able to determine the composition of atmospheres, including the presence of oxygen and methane. Those two gases are the telltale signs of alien life. (Related: "For First Time, Astronomers Read Exoplanet's Color.")
"Director James Cameron's vision of habitable moons like Pandora may very well move from Hollywood screens into reality," said Matthews.
Golden Age of Exoplanet Research?
For exoplanet researchers like William Welsh, the pace of discovery has been breathtaking and truly represents just the beginning of the golden age of exoplanet hunting.
"We've clearly only scratched the surface," said Welsh, a member of the Kepler science team and an astronomer at San Diego State University. "I'm sure we'll hit 10,000 planets in much less time than it took to find the first thousand."
The detection of Earth-like planets that can support life, along with exomoons in the habitable zone, will be historic, he said.
Measuring the atmospheric composition of such planets and moons would be a giant step closer to the ultimate dream of many exoplanet hunters, which, according to Welsh, is the detection of gases with bio-signatures indicating the presence of life.
"In some ways, [this week's] quiet benchmark is a milestone in the history of humankind," he said.
"In the great sea of space, we have found some islands. Most are too treacherous to harbor our ships, but we are on our way to finding worlds that beckon us to visit and stay."