While there's great excitement surrounding these new findings, this isn't the first time we've spotted a potentially habitable planet.
ENGADGET: NASA's Kepler discovers three potentially habitable planets
The reason why that's exciting, is one of the properties we'd like to have on a habitable planet is liquid water on the surface.
And a distant world discovered in February called GJ 667Cc, orbiting a star 22 light years away from Earth, has an ESI value of 0.85, the highest ranking so far of any confirmed potentially habitable planet.
"Assuming that 1:10, 000 are similar to the Earth would give us 5, 000, 000 such planets, " added Tuomi, who led teams reporting the discovery of several potentially habitable planet candidates this year, including an exoplanet orbiting the star Tau Ceti just 11.9 light-years from Earth.
At NASA, Hubbard's goals included understanding Mars as a system, looking for potentially habitable environments, examining whether the planet was habitable in the past and preparing to return samples from Mars to Earth.
This phenomenon makes the planet habitable, by trapping heat that would otherwise be lost into space.
"The take-away message is, regardless of who eventually gets bragging rights to the discovery of the first truly confirmed habitable Earth-sized planet, we are starting to find them in unexpected numbers, and unexpectedly nearby, " Vogt said.
But Yellowknife seems to be the next best thing: the first truly habitable site outside of our planet.
The nearest habitable, Earth-sized planet could be just 13 light-years away, research suggests.
BBC: Exoplanets near red dwarfs suggest another Earth nearer
"The planet may be habitable, but I wouldn't expect to see any intelligent life forms, " says Martin Griffiths, senior lecturer at the Centre for Astronomy and Science Education at the University of Glamorgan.
This past December, for example, NASA's Kepler space telescope confirmed its first planet in the habitable zone, a "super Earth" known as Kepler-22b that's thought to be 2.4 times as wide as our planet.
But if Curiosity could make the definitive identification of organics in Gale Crater, it would be a eureka moment and go a long way towards demonstrating that the Red Planet did indeed have habitable environments in its ancient past.
Researchers at the U.K.'s University of East Anglia now are working on a scale they call the "dwell time index" to calculate how much time a planet has orbited in a habitable zone, compared with the time it took life to evolve on Earth.
The most basic of the indexes rates a world's suitability for life based on its place within a habitable zone, which is calculated by a planet's distance from its parent star, the star's luminosity and temperature.
It includes a science lab that will collect mineral samples from the Red Planet, and try to determine if Mars is habitable.
The stakes have just been raised a notch: Not only is there another planet right next door in our own solar system that had habitable environments on its surface long ago, but there appear to have been common and recognizable -- perhaps even Earthlike -- environments on Mars.
Micro-organisms on Earth are much more common in fresh water than acidic water, and so rather than Mars just having been a potential haven for some extreme forms of life, Curiosity's discovery means that Mars had -- and still preserves evidence of -- environments that were habitable for an enormous variety of micro-organisms that thrive on our own planet.
"We have found a habitable environment which is so benign and supportive of life that if you had been on the planet, the water would have been pure enough to drink, " said planetary scientist John Grotzinger, at the California Institute of Technology, who is the mission's project scientist.
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.
In their paper, the co-authors argue that gravitational tidal heating caused by the gravitational friction of an earth-like exo-moon on an eccentric orbit around a Jupiter-like planet lying at a Jupiter-like distance from its parent star, could, in fact, create habitable conditions.
FORBES: Earth-Like Exo-Moons Can Lurk Well Beyond Planetary Habitable Zones, Say Astronomers
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