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 March 7, 2009 UTC) to survey and compile the characteristics of habitable-zone planets. On November 4, 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. Read more ...
'Earth 2.0' found in Nasa Kepler telescope haul BBC - July 24, 2015
Kepler-452b orbits at a very similar distance from its star, though its radius is 60% larger. Mission scientists said they believed it was the most Earth-like planet yet. Such worlds are of interest to astronomers because they might be small and cool enough to host liquid water on their surface - and might therefore be hospitable to life.
Kepler-452b: NASA Mission Discovers 'Older, Bigger Cousin' to Earth NBC - July 23, 2015
NASA on Thursday announced the discovery of Kepler-452b, the most Earth-like planet ever found. Located 1,400 light-years from our planet, NASA called it "Earth 2.0," because it's the first small, rocky planet discovered in the habitable zone of a G star similar to our sun.
Eight new planets found in 'Goldilocks' zone: Two are most similar to Earth of any known exoplanets Science Daily - January 6, 2015
Astronomers announced today that they have found eight new planets in the 'Goldilocks' zone of their stars, orbiting at a distance where liquid water can exist on the planet's surface. This doubles the number of small planets (less than twice the diameter of Earth) believed to be in the habitable zone of their parent stars. Among these eight, the team identified two that are the most similar to Earth of any known exoplanets to date.
Earth-size Kepler-186f NASA - April 18, 2014
Planet Kepler-186f is the first known Earth-size planet to lie within the habitable zone of a star beyond the Sun. Discovered using data from the prolific planet-hunting Kepler spacecraft, the distant world orbits its parent star, a cool, dim, M dwarf star about half the size and mass of the Sun, some 500 light-years away in the constellation Cygnus. M dwarfs are common, making up about 70 percent of the stars in our Milky Way galaxy. To be within the habitable zone, where surface temperatures allowing liquid water are possible, Kepler-186f orbits close, within 53 million kilometers (about the Mercury-Sun distance) of the M dwarf star, once every 130 days. Four other planets are known in the distant system. All four are only a little larger than Earth and in much closer orbits, also illustrated in the tantalizing artist's vision. While the size and orbit of Kepler-186f are known, its mass and composition are not, and can't be determined by Kepler's transit technique. Still, models suggest that it could be rocky and have an atmosphere, making it potentially the most Earth-like exoplanet discovered so far ...
First potentially habitable Earth-sized planet confirmed by Gemini and Keck observatories Science Daily - April 18, 2014
> The first Earth-sized exoplanet orbiting within the habitable zone of another star has been confirmed by observations with both the W. M. Keck Observatory and the Gemini Observatory. The initial discovery, made by NASA's Kepler Space Telescope, is one of a handful of smaller planets found by Kepler and verified using large ground-based telescopes. It also confirms that Earth-sized planets do exist in the habitable zone of other stars.
Kepler telescope bags huge haul of planets BBC - February 27, 2014
The science team sifting data from the US space agency's (Nasa) Kepler telescope says it has identified 715 new planets beyond our Solar System. This is a huge new haul. In the nearly two decades since the first so-called exoplanet was discovered, researchers had claimed the detection of just over 1,000 new worlds. Kepler's latest bounty are all in multi-planet systems; they orbit only 305 stars. The vast majority, 95%, are smaller than our Neptune, which is four times the radius of the Earth. Four of the new planets are less than 2.5 times the radius of Earth, and they orbit their host suns in the "habitable zone" - the region around a star where water can keep a liquid state.
Goldilocks moons PhysOrg - January 16, 2012
The search for extraterrestrial life outside our Solar System is currently focused on extrasolar planets within the habitable zones of exoplanetary systems around stars similar to the Sun. Finding Earth-like planets around other stars is the primary goal of NASA's Kepler Mission. The habitable zone (HZ) around a star is defined as the range of distances over which liquid water could exist on the surface of a terrestrial planet, given a dense enough atmosphere. Terrestrial planets are generally defined as rocky and similar to Earth in size and mass. A visualization of the habitable zones around stars of different diameters and brightness and temperature is shown here.
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