What do you suppose is going to happen first ... aliens visiting Earth or finding a Goldilocks planet? Astronomy buzz ... We have located a planet that could sustain life as we know it say astronomers.

A "Goldilocks planet" is a planet that falls within a star's habitable zone, often specifically used for planets close to the size of Earth. The name comes from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one that is neither too close nor too far from a star to rule out life (as humans understand it) on the planet.

Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race. The Drake equation, which attempts to estimate the likelihood of non-terrestrial intelligent life, incorporates a factor (ne) for the average number of life-supporting planets in a star system with planets.

The discovery of extrasolar Goldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earth hypothesis, which posits that a series of extremely unlikely events and conditions led to the rise of life on Earth. High estimates would reinforce the Copernican mediocrity principle, in that large numbers of Goldilocks planets would imply that Earth is not especially exceptional.

Finding Earth-sized Goldilocks planets is a key part of the Kepler Mission, which uses a space telescope (launched on 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets.

Although the extrasolar planet 70 Virginis b was initially nicknamed "Goldilocks" because it was thought to be within the star's habitable zone, it is now believed to be far too warm to be "just right" for life, and is thus not a Goldilocks planet. Gliese 581 d and Gliese 581 g are currently believed to be rocky planets lying within the habitable zone of their star.

  Alien planet looks 'just right' for life   MSNBC - September 30, 2010
Astronomers say they've found the first planet beyond our solar system that could have the right size and setting to sustain life as we know it, only 20 light-years from Earth.

  Alien life certain to exist on Earth-like planet, scientists say   Telegraph.co.uk - September 30, 2010

Alien life certain to exist on Earth-like planet, scientists say. The chances of alien life existing on a newly-discovered Earth-like planet are 100 per cent, an astronomer has claimed.

Scientists find potentially habitable planet near Earth   PhysOrg - September 29, 2010
If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one. To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

Zarmina's World   NASA - October 1, 2010

A mere 20 light-years away in the constellation Libra, red dwarf star Gliese 581 has received much scrutiny by astronomers in recent years. Earthbound telescopes had detected the signatures of multiple planets orbiting the cool sun, two at least close to the system's habitable zone -- the region where an Earth-like planet can have liquid water on its surface.

Now a team headed by Steven Vogt (UCO Lick), and Paul Butler (DTM Carnagie Inst.) has announced the detection of another planet, this one squarely in the system's habitable zone. Based on 11 years of data, their work offers a very compelling case for the first potentially habitable planet found around a very nearby star. Shown in this artist's illustration of the inner part of the exoplanetary system, the planet is designated Gliese 581g, but Vogt's more personal name is Zarmina's world, after his wife. The best fit to the data indicate the planet has a circular 37 day orbit, an orbital radius of only 0.15 AU, and a mass 3.1 times the Earth's. Modeling includes estimates of a planet radius of 1.5, and gravity at the planet's surface of 1.1 to 1.7 in Earth units. Finding a habitable planet so close by suggests there are many others in our Milky Way galaxy.