The first truly Earth-like alien planet is likely to be spotted next year, an epic discovery that would cause humanity to reassess its place in the universe.
While astronomers have found a number of exoplanets over the last few years that share one or two key traits with our own world — such as size or inferred surface temperature — they have yet to bag a bona fide "alien Earth." But that should change in 2013, scientists say.
"I'm very positive that the first Earth twin will be discovered next year," said Abel Mendez, who runs the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.
Artist's concept of the potentially habitable planet candidate Tau
Ceti f, which was detected in December 2012. The possible planet, which
is found just 11.9 light-years from Earth, is at least 6.6 times as
massive as Earth.Planets piling up
Astronomers discovered the first exoplanet orbiting a sunlike star in 1995. Since they, they've spotted more than 800 worlds beyond our own solar system, and many more candidates await confirmation by follow-up observations. [The Strangest Alien Planets (Gallery)]
NASA's prolific Kepler Space Telescope, for example, has flagged more than 2,300 potential planets since its March 2009 launch. Only 100 or so have been confirmed to date, but mission scientists estimate that at least 80 percent will end up being the real deal.
The first exoplanet finds were scorching-hot Jupiter-like worlds that orbit close to their parent stars, because they were the easiest to detect. But over time, new instruments came online and planet hunters honed their techniques, enabling the discovery of smaller and more distantly orbiting planets — places more like Earth.
Last December, for instance, Kepler found a planet 2.4 times larger than Earth orbiting in its star's habitable zone — that just-right range of distances where liquid water, and perhaps life as we know it, can exist.
The Kepler team and other research groups have detected several other worlds like that one (which is known as Kepler-22b), bringing the current tally of potentially habitable exoplanets to nine by Mendez' reckoning.
Zeroing in on Earth's twin
None of the worlds in Mendez' Habitable Exoplanets Catalog are small enough to be true Earth twins. The handful of Earth-size planets spotted to date all orbit too close to their stars to be suitable for life. [Gallery: 9 Potentially Habitable Exoplanets]
But it's only a matter of time before a small, rocky planet is spotted in the habitable zone — and Mendez isn't the only researcher who thinks that time is coming soon.
"The first planet with a measured size, orbit and incident stellar flux that is suitable for life is likely to be announced in 2013," said Geoff Marcy, a veteran planet hunter at the University of California, Berkeley, and a member of the Kepler team.
Mendez and Marcy both think this watershed find will be made by Kepler, which spots planets by flagging the telltale brightness dips caused when they pass in front of their parent stars from the instrument's perspective.
Kepler needs to witness three of these"transits" to detect a planet, so its early discoveries were tilted toward close-orbiting worlds (which transit more frequently). But over time, the telescope has been spotting more and more distantly orbiting planets — including some in the habitable zone.
Habitable zones for different stars. An intelligent civilization could allow a planet outside the zone to still be habitable.
An instrument called HARPS (short for High Accuracy Radial velocity Planet Searcher) is also a top contender, having already spotted a number of potentially habitable worlds. HARPS, which sits on the European Southern Observatory's 3.6-meter telescope in Chile, allows researchers to detect the tiny gravitational wobbles that orbiting planets induce in their parent stars.
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