Image: NASA's Kepler mission has discovered a world where two suns set over the horizon instead of just one.
NASA/JPL-Caltech
NASA's Kepler mission has discovered a world where two suns set over the horizon instead of just one. The planet, called Kepler-16b, is not thought to be habitable. It is a cold world, with a gaseous surface, and it circles two stars, just like "Star Wars" Tatooine.
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updated 11/21/2011 2:19:02 PM ET 2011-11-21T19:19:02

Less than 20 years after discovering the first world beyond our solar system, astronomers have bagged alien planet number 700.

As of Nov. 18, the tally stands at 702 exoplanets, according to the Extrasolar Planets Encyclopedia, a database compiled by astrobiologist Jean Schneider of the Paris-Meudon Observatory.

Schneider's list is one of two main alien-world trackers out there. The other is called "PlanetQuest: New Worlds Atlas," run by NASA's Jet Propulsion Laboratory. PlanetQuest's current count stands at 687; the difference between the two databases highlights the uncertainties involved in exoplanet detection and confirmation.

Regardless, both tallies have been growing quickly lately. And both will add hundreds, or perhaps thousands, of new alien planets in the near future.

Hunting down alien planets
Astronomers first discovered an alien world in 1992, when researchers detected two planets orbiting a rotating neutron star, or pulsar, about 1,000 light-years from Earth. Confirmation of a planet circling a "normal" main-sequence star other than our sun did not come until 1995. [ Gallery: The Strangest Alien Planets ]

But the finds have been piling up recently. The count topped 500 in November 2010, and it passed 600 just two months ago when scientists with the European Southern Observatory announced 50 newfound planets, including one "super-Earth" that might be a good candidate for hosting life.

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And the pace of discovery is only going to keep accelerating, as scientists continue to hone their planet-hunting techniques and the data continue to pour in.

The finds of one instrument in particular could more than double the number of known exoplanets. Since its launch in 2009, NASA's Kepler space telescope has identified 1,235 planet candidates. To date, just 25 of them have been confirmed, but Kepler scientists have estimated that around 80 percent will end up being the real deal.

A diverse array of alien worlds
The numbers by themselves are exciting, but the search for alien planets isn't just about increasing the tally. Rather, it's a quest to better understand the nature and diversity of alien worlds in our galaxy and beyond, researchers say.

And that diversity appears to be huge. Astronomers have found one planet as light and airy as styrofoam, for example, and another as dense as iron. They've discovered an alien world that orbits two suns, like Luke Skywalker's home planet of Tatooine in the "Star Wars" films.

And, perhaps most intriguingly, they've confirmed a number of planets that appear to orbit in their stars' habitable zone, that just-right range of distances where liquid water — and maybe life as we know it — could exist.

"There's zero indication that the surprises are petering out," astronomer Greg Laughlin, of the University of California, Santa Cruz, told SPACE.com recently. "Just looking at all the new discoveries on the preprint server [where many studies are posted before being published] is close to a full-time job."

You can follow SPACE.com senior writer Mike Wall on Twitter:@michaeldwall. Follow SPACE.com for the latest in space science and exploration news on Twitter@Spacedotcomand on Facebook.

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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