A Goldilocks planet is a planet that falls within a star's habitable zone, and the name is often specifically used for planets close to the size of Earth. The name comes from the children's fairy tale 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 liquid water on its surface and thus life (as humans understand it) on the planet. However, planets within a habitable zone that are unlikely to host life (e.g., gas giants) may also be called Goldilocks planets. The best example of a Goldilocks planet is the Earth itself.
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. On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists.
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 closer to its sun, making it far too warm to be "just right" for life. Thus it is not a Goldilocks planet.
The Gliese 581 system has a set of slightly oversized terrestrial planets mirroring our own solar system's. It is currently believed that the third planet, planet c, is analogous to Venus's position (slightly too close), the fourth planet g (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifth planet d to the Mars position. Planet d may be too cold, but unlike Mars, it is several times more massive than Earth and may have a dense atmosphere to retain heat. One caveat with this system is that it orbits a red dwarf, probably resulting in most of the issues regarding habitability of red dwarf systems, such as all the planets likely being tidally locked to the star.
More recently, on February 2, 2011, the Kepler Space Observatory Mission team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the "habitable zone." Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."
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