The Goldilocks Zone refers to the habitable zone around a star where the temperature is just right - not too hot and not too cold - for liquid water to exist on an planet.
Liquid water is essential for life as we know it. Where we find liquid water on Earth we also find life.
"The only life we know about is our carbon-based life, and water plays a crucial part in our own existence, and so it's only natural that we direct our attention to planets in locations capable of having liquid water," Professor John Webb of the University of New South Wales said.
There are at least a dozen or so potentially habitable exoplanets, planets which are in varying degrees similar to Earth.
"There's plenty of life on Earth and there's plenty of water, but we've yet to find life on other planets even in our own solar system."
Looking for planets in the Goldilocks Zone is a way that allows scientists to hone in their search for Earth-like planets that could contain life.
Basically, the assumption is that if it's possible there may be liquid water on the planet, then it's also possible that the planet may be habitable.
Goldilocks Zones in other star systems
"The location of a Goldilocks Zone around another star depends on the type of star," Professor Webb said.
Bigger hotter stars have their Goldilocks Zones further out, while smaller cooler stars such as M-type red dwarf stars have habitable zones much closer in.
Red dwarfs are the most common type of star in the Milky Way galaxy, and have very long life expectancies.
"This means life should have lots of time to evolve and develop around such as star," he said.
Observations by the European Southern Observatory's High Accuracy Radial velocity Planet Searcher, has concluded about 40 per cent of red dwarfs have super-Earth class planets orbiting in their habitable zone.
Alternatively, NASA's planet hunting Kepler space telescope searches for planets orbiting in the habitable zones of Sun-like stars by looking for planets with an average 365-day orbit.
More than just temperature
Just because a planet or moon is in the Goldilocks Zone of a star, doesn't mean it's going to have life or even liquid water.
After all, Earth isn't the only planet in the Sun's Goldilocks Zone - Venus and Mars are also in this habitable zone, but aren't currently habitable.
"Venus is Earth's sister planet, both are about the same size and in the same region of the solar system, and Venus once also had water," Professor Webb said.
"However, Venus now has a runaway greenhouse effect going on, with a surface temperature of over 460 degrees Celsius, which has boiled away all its liquid water."
At the other end of the Sun's Goldilocks Zone is Mars which also once had liquid water flowing across its surface in rivers, lakes and oceans.
"However, the Red Planet is now a freeze-dried desert, with a thin carbon dioxide atmosphere, and only one 99th the atmospheric pressure of sea level on Earth," Professor Webb said.
"The lack of both a significant atmosphere and a global magnetic field - thanks to its mostly solidified core - means the Martian surface is constantly being irradiated by the Sun.
"Any water still on Mars, which hasn't degassed into space and been blown away by the solar wind, or irradiated into hydroxyls on the surface, is either frozen in the planet's ice caps and permafrost, or quickly subducts directly from ice to gas during the local Martian summer."
While there is some evidence pointing to the possible existence of subsurface salt water brines which can seep to the surface, we're yet to find any life on the Red Planet.
"Finally, and much closer to home, we have a third terrestrial world, the Moon, which has virtually no atmosphere, just the hint of a dusty exosphere above its surface, and with the only water being either locked up as ice on the shaded floors of deep craters, or as hydroxyls on the irradiated lunar surface, and definitely no life," Professor Webb said.
Getting it right is hard
"If you want to calculate the average temperature that some exoplanet has, given its distance from its host star, you actually need to know a lot about that exoplanet, including the kind of atmosphere it has, the reflectivity of its clouds, and whether it has any kind of greenhouse effect," he said.
"And the trouble is you actually don't know those things, so the calculations can give you the wrong answers."
Professor Webb said Earth and Venus could be good examples of getting it wrong.
"If you perform a simple calculation for Earth, taking into account the apparent reflectivity of our clouds, that is the sunlight that's being reflected back into space without heating the surface, and you ignore the effect of greenhouse gases, you can actually get the wrong answer and conclude that Earth is not habitable," he said.
"And if you calculate the mean surface temperature of Venus based only on the reflectivity of its cloud cover, then one would expect a surface temperature of minus 10 Celsius, over 470 Celsius less than its actual surface temperature.
"There are at least a dozen or so potentially habitable exoplanets, planets which are in varying degrees similar to Earth," Professor Webb said.
For these reasons, he said we should relax our definition of the Goldilocks or the habitable zone around stars somewhat, or we could miss a major discovery.
"Ultimately when the technology and methodology improves, we will be able to measure any atmosphere around these planets, and that might give us some clue to what's really going on there, but right now these things are very hard to do."
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