Synopsis: Goldilocks zone can be a misleading name.
The first problem with the concept of a “Goldilocks Zone”, is that the term does not mean what most people I have asked answer that they think it means. The most common answer is “the distance right distance from a star to have a planet to have the right temperature and/or conditions for us to live”.
The first problem is that there is no fixed distance from a star for the right temperature, as every star continually increases in temperature throughout its life.
The second problem is the ambiguity as to who or what would find the temperature in “the zone” as being not to hot or not too cold, and the zone does not apply to us humans.
It sounds like it is “not too hot or too cold” from the perspective of Goldilocks, a human who even rejects the porridge assumedly preferred by papa bear as being too hot. Instead “not to hot or too cold” of from the perspective of Goldilocks, or even just any one of the three bears, it is instead neither too hot or too cold of the most temperature tolerant life possible which can include life able to survive below zero or beyond 100°C (212°F).
As shown in these diagrams, the Earth is currently in what most astronomers and astrobiologists would call the “Goldilocks zone”, but as the zone moves, from around 1.5 billion years from now, the zone will have moved so that the Earth is no longer in the Sun’s “Goldilocks zone”.
Perhaps a bigger problem is that fact that even Earth will become inhospitable to humans before the Earth leaves the Sun’s “Goldilocks zone”, because we only leave the zone when it is too hot for any life at all anywhere on the planet, even including extremophiles that can thrive when the temperature is over 100°C (212°F).
There is only one planet we know of so far that is teeming with life––Earth. And on our planet, water is a critical ingredient for life as we know it. While astronomers still don’t know whether there’s life on other planets, they narrow the search for potentially habitable worlds using a handful of criteria. Because our blueprint for life is Earth, astronomers look for planets with Earth-like characteristics, like liquid water. But a celestial object can only orbit so close (like Mercury) or so far (like Pluto) from its star before water on its surface boils away or freezes. The ‘Goldilocks Zone,’ or habitable zone, is the range of distance with the right temperatures for water to remain liquid. Discoveries in the Goldilocks Zone, like Earth-size planet Kepler-186f, are what scientists hope will lead us to water––and one day life.
Nasa: Goldilocks zone
“Fully Goldilocks” and the “fully Goldilocks window”.
Goldilocks window: Time when conditions potentially support life.
I use the term “Goldilocks window” to address the confusion arising from the fact that Goldilocks zones move over time. Because the zone moves, a planet, as even the case with Earth which will leave the Goldilocks zone in around 1.2 billion years when Earth becomes too hot for even extremophiles, will typically be only in the zone only for a specific limited time, rather than being permanently included or excluded.
Fully Goldilocks window: Time when conditions support complex life.
As many people assume the “Goldilocks zone” is where we could find a potential “Earth 2.0”, I think it is useful to have a term that means just that, a zone that meets the stricter conditions that means it where humans could potentially humans live. I have adopted the term “fully Goldilocks” for “just right enough for even humans to survive”.
So, while “Goldilocks conditions” allow for some form of life, even if only extremophiles, this site uses “fully Goldilocks” for conditions allow for complex life like us.
Astronomers and astrobiologists using the term “Goldilocks zone” are not expecting to find Earth like conditions and complex life, but the simplest procaryote single-celled life that we suspect might be common in the universe. The more we learn the more it seems that it is not life itself that is particularly special, but it is complex life that is the miracle, and which requires a series of incredibly unlikely steps to evolve from simple life.
This means while the “Goldilocks zone” is part of the search for where some form of life could possibly exist, “Goldilocks window” is more applicable in determining if humans could possibly survive. Just like the porridge Goldilocks found too hot was assumedly quite acceptable to Papa Bear, a planet in the Goldilocks zone could be totally acceptable to more extreme forms of life, but totally uninhabitable for humans.
Thus, the term “fully Goldilocks window” is designed to describe the time period for a planet within a “Goldilocks zone” when an actual human could survive at least somewhere on the planet, potentially needing clothing as worn on places on the Earth, but without needing a spacesuit.
Note the “habitable zone” used in astronomy and astrobiology is far more flexible as it allows for orbits “within which a planetary surface can support liquid water given sufficient atmospheric pressure“. Liquid water does not even restrict temperatures to 100°C as it usually does on Earth, because with enough atmospheric pressure on a planet, liquid water could reach 374 °C (705 °F)!
Life can thrive in environments that to us seem extreme.
There is a huge potential difference between an environment suitable for humans and one suitable for other forms of life. For example, even complex life such as tardigrades, can survive the vacuum of space and some can survive extremely cold temperatures down to 0.01 K (−460 °F; −273 °C) (close to absolute zero), while others can withstand extremely hot temperatures up to 420 K (300 °F; 150 °C). Tardigrades cannot spend their whole lives at such temperatures, but they can survive in an environment where such extremes occur.
Other more basic forms of life are designated “extremophilic”, because not only can they survive in extreme conditions, but they can also even spend their entire lives and thrive in what we consider extreme conditions, which include temperatures from 110 °C (230 °F) to 121 °C (250 °F), and yes, with the right pressure, water can be liquid at these temperatures.
The range of what is considered “life” ranges from organisms so simple they could almost be considered replicating chemical structures through to us, our pets, and the creatures we can see with our eyes and admire in the wild.
The search for environments where life such as extremophiles can exist is very different than the search for places where people or most forms of life as we know it could exist.
The need for a temperature stabilisation system.
As explained below, and in more detail in “The Earth, the Sun and CO2“, the Sun continually and slowly dials up the temperature. Too slowly to even detect within thousands of years, but over billions or years, enough to have added well over 100 °C to the temperature, and over time taking things from too cold for life to too hot for life. So how has life survived? Through a dramatic change to the atmosphere, with CO2 levels on Earth initially thousands of times the current levels!
While from less than 1 billion years after being formed the Earth has had temperatures that allow life, those temperatures ranged from 60 °C warmer than current levels, to so cold the whole planet most likely became a “Snowball Earth” during the billions of years before complex life arose just over 500 million years ago.
The Phanerozoic Era, starting 540 million years ago, was the beginning of the “Goldilocks window” on Earth, and temperatures have remained within limits for complex life to exist courtesy of plants acting as a quite effective thermostat, keeping CO2 concentrations at the level necessary for complex life, without which, complex life would have already ended on Earth.
The ‘Goldilocks Zone’ as a place: A dangerous fairy tale.
The zone is always on the move.
You may have heard of the ‘goldilocks zone’. The distance from a star that is ‘not too hot’ and ‘not too cold’ for liquid water, and thus life, to exist on a planet.
Giant stars have very short life spans and build rapidly to a supernova, leaving only smaller main sequence stars likely to provide any planet with “Goldilocks window” conditions for longer enough for life to evolve on the stars’ planets, should life have to time to even begin at all.
Now consider, all main sequence stars, also build up more and more heat throughout their lifetime. This means, as per the diagrams here, any goldilocks zone moves outward during the life of the star, from close to the star when the star is younger, to further out as the star ages.
Fairy tale? Why dangerous?
The ‘fairy tale’ seems to imply that a planet at a fixed distance from any star could continue to be neither too hot, nor too cold for a significant period of the life of a main sequence star. Life persisting requires a mechanism to compensate for the changing heat from the star. The danger of the fairy tale is that it creates a misunderstanding what is needed for a planet to support life over a period of time.
The danger of this fairy tale is the potential false impression that stars are static, and ‘goldilocks zone’ is a fixed distance from any given star. This can lead to complacency and thinking that Earth will always be in the goldilocks zone. It can also give that impression distance from the star alone determines the temperature on a planet. Ignoring the role of the atmosphere is highly dangerous.
The varied range of the ‘Goldilocks’ Or Habitable Zone: Allowance of Atmosphere.
Life as we know it requires liquid water. So, it is expected that life in universe is most likely to must exist in areas where water can be liquid. The name ‘Goldilocks Zone’ comes from the ‘not too hot‘, as water becomes a gas if too hot, and ‘not too cold‘, as water becomes solid ice if too cold. The less frivolous name is the ‘habitable zone’.
It is not heat from a planets star alone that determines the temperature on a planet. Planets orbiting a star will typically have the star as their primary source of heat, but that head can be supplemented by secondary factors such as the planets atmosphere, amplifying or partially blocking the heat from the star.
This makes the Goldilocks zone not just the very narrow band where the exact right amount of heat from the star is present, but instead a wider band that begins at first the point where there is not too much heat from the star to be too hot even with some heat blocked, and ends at the point where no matter how effective the atmosphere at retaining heat, it seems impossible to have sufficient heat to prevent all water freezing.
Goldilocks Lessons From Our Solar System: We are now living on the edge.
The first diagrams I saw of the Goldilocks zone in our solar system, they stated Venus was too close to the Sun and that is why it is too hot, and Mars was too far from the Sun and that is why it was too cold. Those early diagrams place the Earth at the centre of the Goldilocks zone.
Since then, we have learnt that Venus, Earth and Mars all at one time had liquid water, so therefore all were in the Goldilocks zone. Since the Goldilocks zone moves outwards away from the Sun, it is possible that Venus is no longer in the zone even if it was in the past, but since Mars was had liquid water billions of years ago and the band moves outwards, Mars can only more in the zone than ever now, despite now being too cold.
One lesson is that Earth is now right on the edge of the Goldilocks zone, and within the next few million years as the Sun becomes a slightly hotter star, Earth will exit the Goldilocks zone.
Another lesson, this time from the example of Mars, is that a planet being within the Goldilocks zone, is no guarantee it will be neither too hot nor too cold. Mars lost its auxiliary heat from its greenhouse gas-based atmosphere and became colder even though the Sun became hotter.
Life could not start have started on the early Earth if the Sun was at its current temperature.
We know the atmosphere of the early Earth was able to raise the temperature on Earth from -95°C that would have been experienced without any atmosphere to, sufficiently for liquid water to exist. How much colder could the planet be and still have liquid water? Snowball Earth theories suggest perhaps only a –5°C average change could be sufficient to reduce Earth to a snowball.
It seems most likely that the Earth atmosphere, which is known to have at least around 500x more CO2 than our current atmosphere, had to have warming effect of at 90°, which would still mean an Earth more than 25°C colder than it is now, and with average temperature well below zero, even before factoring in the additional cooling from the high albedo of an ice-covered surface. This is 57
It seems most likely that the Earth atmosphere, which is known to have at least around 500x more CO2 than our current atmosphere, had to have warming effect of at 90°, which would still mean an Earth more than 25°C colder than it is now, and with average temperature well below zero, even before factoring in the additional cooling from the high albedo of an ice-covered surface. This means at least 57°C more warming that our current atmosphere, even when there is less radiant heat to retain.
The warming effect of the Earth’s original atmosphere would mean temperatures at least 57°C hotter than we experience now, for an average temperature on Earth of 57°C + 18°C = 75°. However, at 75°C, water vapour levels add even more greenhouse gases, which would mean thermal runaway as on Venus would be inevitable.
Clearly, the original Earth, before plants dramatically lowered CO2 levels, would have been far too hot for liquid water or life to start if it had been faced with a Sun as powerful as our Sun is now.